Data Matching

Soundex Algorithm: How It Works, Examples & Free Calculator

Soundex Algorithm: How It Works, Examples & Free Calculator

Definition

Soundex

Soundex is a phonetic matching algorithm that converts names into four-character codes based on how they sound. It helps identify names with different spellings but similar pronunciations, making it useful for name matching, record linkage, deduplication and data quality.

FREE PHONETIC MATCHING TOOL

Soundex Calculator

Compare two names using the Soundex phonetic algorithm.

Try an example
MATCH Soundex result
Phonetic match

These values produce the same Soundex code.

Soundex keeps the first letter of a name and converts the remaining consonant sounds into a four-character phonetic code.

Soundex Code A R163
Soundex Code B R163
Interpretation Likely phonetic similarity
Important: Soundex is designed mainly for English-language names and can produce the same code for unrelated names. It should normally be used as one matching signal rather than as proof that two records represent the same entity.
Phonetic matching within AdaptiveMatch™

WinPure AdaptiveMatch™ can combine Soundex with other phonetic and fuzzy matching algorithms, thresholds and field-level rules to improve matching accuracy.

What Is the Soundex Algorithm?

Soundex is a phonetic matching algorithm designed to identify names that sound similar even when they are spelled differently. Instead of comparing names character by character, Soundex converts each name into a four-character phonetic code based on its pronunciation.

NameSoundex Code
RobertR163
RupertR163
SmithS530
SmythS530

Because Robert and Rupert produce the same Soundex code, the algorithm identifies them as a potential phonetic match. The same happens with Smith and Smyth. This makes Soundex useful when working with datasets containing misspellings, alternative spellings or variations in people’s names. It has been widely used for searching records, identifying possible duplicates and linking information where exact text matching would miss a potential match.

Best Practice

Soundex is particularly suited to English-language names, but it has limitations. Two different names can sometimes generate the same code, while names that sound similar may occasionally produce different codes. For this reason, modern data matching systems often use Soundex alongside other phonetic and fuzzy matching techniques rather than relying on it alone.

How Does Soundex Work?

Soundex works by converting a name into a four-character phonetic code. Names that sound similar can produce the same code even when their spellings are different.

The algorithm follows four main steps:

1. Keep the first letter

The first letter of the name is retained. For example, with Robert, the letter R is kept.

2. Convert consonants into numbers

The remaining consonants are assigned numbers according to their sound groups:

NumberLetters
1B, F, P, V
2C, G, J, K, Q, S, X, Z
3D, T
4L
5M, N
6R

The letters A, E, I, O, U, H, W and Y do not receive a numeric value.

3. Remove duplicate adjacent sound codes

When adjacent letters produce the same numeric code, only one is retained. This prevents similar sounds from being counted more than once.

4. Create a four-character code

The first letter is combined with the first three numeric values. If fewer than three numbers remain, zeros are added until the code contains four characters.

Soundex Example: Robert

soundex example

Although Robert and Rupert are spelled differently, they both produce R163. Soundex therefore considers them a phonetic match.

This is what makes Soundex useful for name matching and data deduplication: it can identify potential matches that an exact text comparison would miss.

Soundex Encoding Rules

Soundex uses a set of simple encoding rules to convert a name into a four-character phonetic code consisting of the first letter followed by three numbers.

1. Keep the First Letter

The first letter of the name is always retained.

For example: Robert → R

2. Convert Consonants into Numbers

The remaining consonants are grouped according to similar sounds:

Soundex CodeLetters
1B, F, P, V
2C, G, J, K, Q, S, X, Z
3D, T
4L
5M, N
6R

The letters A, E, I, O, U, H, W and Y are not assigned a numeric code.

3. Remove Repeated Sound Codes

If adjacent consonants belong to the same Soundex group, the repeated code is normally recorded only once.  For example, Jackson contains consonants that can produce repeated sound codes. Soundex removes these duplicates rather than treating each one as a separate sound.

4. Handle Vowels, H, W and Y

Vowels and the letters H, W and Y do not receive Soundex numbers, but their treatment matters when determining whether consonant codes should be considered consecutive. In the commonly used American Soundex rules, vowels can separate identical consonant codes, while H and W do not necessarily break the sequence. This distinction is important because different simplified implementations of Soundex can occasionally produce different results.

5. Return Three Numbers

After applying the encoding rules, Soundex keeps the first three numeric codes. If there are more than three, the remaining codes are discarded.

6. Add Zeros When Required

If fewer than three numeric codes remain, zeros are added to the end.

For example: Lee → L000

The final Soundex key therefore always follows the same structure:

Best Practice

First letter + three digits

Examples include R163, S530 and L000.

These rules allow differently spelled names to be reduced to a common phonetic representation, making Soundex useful for identifying potential name variations and duplicate records that exact matching may overlook.

Soundex Examples

The easiest way to understand Soundex is to compare names with different spellings and see the phonetic codes they produce.

Name AName BSoundex ASoundex BResult
RobertRupertR163R163Match
SmithSmythS530S530Match
AshcraftAshcroftA261A261Match
RubinRubenR150R150Match
JacksonJaxenJ250J250Match
RobertRobertsR163R163Match
SmithSchmidtS530S530Match
RobertRichardR163R263No Match

Robert vs Rupert

Robert and Rupert look quite different when compared character by character, but both produce:

R163

Soundex recognises that the important consonant sounds are similar, allowing the names to be identified as a potential phonetic match.

Smith vs Smyth

The difference between Smith and Smyth is the vowel used in the middle of the name. Because vowels are not assigned Soundex numbers, both names produce:

S530

This demonstrates why phonetic matching can find name variations that exact matching would miss.

Ashcraft vs Ashcroft

Both Ashcraft and Ashcroft produce:  A261

Despite their spelling difference, Soundex reduces them to the same phonetic representation.

When the Codes Don’t Match

Soundex does not consider every name beginning with the same letter a match. For example:

Robert → R163
Richard → R263

Because the numeric portions differ, Soundex does not identify these names as a phonetic match.

What These Examples Show

Soundex can be particularly useful for finding alternative spellings, transcription differences and name variations within customer, CRM, historical and other datasets. However, sharing the same Soundex code does not mean two names definitely refer to the same person. For example, Smith and Schmidt both encode to S530 under American Soundex despite being distinct surnames. This is why Soundex is often most effective as one signal within a broader data matching process, alongside other attributes and matching algorithms.

How to Calculate a Soundex Code Step by Step

To calculate a Soundex code, start with the name and convert it into a four-character phonetic key.

Step 1: Keep the First Letter

Take the first letter of the name and keep it unchanged.

For example: Robert → R

Step 2: Convert the Remaining Consonants

Use the Soundex number groups:

NumberLetters
1B, F, P, V
2C, G, J, K, Q, S, X, Z
3D, T
4L
5M, N
6R

For Robert, the remaining relevant consonants are:

B → 1
R → 6
T → 3

So the code becomes: R163

Step 3: Remove Duplicate Sound Codes

If neighbouring consonants produce the same Soundex number, keep only one of them. This helps prevent repeated or closely related consonant sounds from being counted twice.

Step 4: Ignore Non-Coded Letters

The letters: A, E, I, O, U, H, W, Y – do not receive Soundex numbers.

They may still affect how duplicate consonant codes are treated, depending on the implementation.

Step 5: Keep the First Three Numbers

Soundex keeps a maximum of three numeric values after the first letter. If more than three codes are generated, any additional ones are ignored.

Step 6: Add Zeros if Necessary

If fewer than three numbers remain, add zeros until the Soundex code contains four characters.

For example: Lee → L000

Worked Example: Rupert

Now calculate the Soundex code for Rupert.

Keep the first letter: R
Ignore u
P → 1
Ignore e
R → 6
T → 3

The final code is: Rupert → R163

This is the same code as:  Robert → R163

That is why Soundex identifies Robert and Rupert as a potential phonetic match. For larger datasets, this calculation is normally performed automatically. You can use the WinPure Soundex Calculator above to compare names instantly and see the phonetic codes produced.

Soundex for Name Matching

Soundex is particularly useful for name matching because names are often recorded differently across databases, CRM systems and historical records. Exact matching may treat two spelling variations as completely different values, even when they sound almost identical.

Soundex approaches the problem by comparing the phonetic representation of each name rather than relying only on its spelling.

Name AName BSoundex CodesExact MatchSoundex Match
SmithSmythS530 / S530NoYes
RobertRupertR163 / R163NoYes
RubinRubenR150 / R150NoYes
AshcraftAshcroftA261 / A261NoYes

An exact comparison would miss all of these potential matches. Soundex provides another way to identify records that may require further comparison.

Where Is Soundex Name Matching Useful?

Soundex can help when datasets contain variations caused by alternative spellings, manual data entry, transcription differences or older records. Common applications include customer deduplication, record linkage, CRM data matching and searching large databases of names. For example, a customer recorded as Smyth in one system might appear as Smith in another. Comparing their Soundex codes can help bring those records into consideration as a potential match.

Soundex Should Be a Matching Signal, Not the Final Decision

A shared Soundex code does not necessarily mean two records represent the same person. Names such as Smith and Schmidt, for example, can produce the same Soundex code even though they are different surnames. Soundex also focuses heavily on English pronunciation and may be less effective for names from other languages. For higher-confidence entity matching, Soundex can therefore be combined with other information such as address, email, telephone number and date of birth, as well as other phonetic and fuzzy matching algorithms. This is the approach taken by WinPure AdaptiveMatch™, where the matching method can be selected according to the type and characteristics of the data rather than relying on a single algorithm for every field.

Soundex vs Fuzzy Matching

Soundex and fuzzy matching can both identify records that are not exact matches, but they solve different types of matching problems. Soundex compares how names sound, while fuzzy matching measures how similar two strings are based on their characters, spelling or structure.

For example, consider:

Smith → Smyth

Soundex can recognise these as a phonetic match because both names generate the same Soundex code. A fuzzy matching algorithm may also identify them as similar because only one character has changed.

However, the differences become clearer with other types of data.

SoundexFuzzy Matching
Primary purposePhonetic similarityString similarity
Best suited toNamesNames, addresses, companies and other text
ReturnsPhonetic codeSimilarity score or distance
Handles spelling errorsSometimesYes
Handles similar pronunciationYesSometimes
Language sensitivityPrimarily EnglishDepends on algorithm
ExampleSmith / SmythMicrosoft / Microsft

When Is Soundex Better?

Soundex can be useful when the main problem is different spellings of names that have similar pronunciations.

For example:

Smith → S530
Smyth → S530

Because both produce the same code, Soundex identifies them as a potential phonetic match.

When Is Fuzzy Matching Better?

Fuzzy matching is generally more flexible when dealing with misspellings, missing characters, transpositions and other textual differences.

For example:

Microsoft
Microsft

The second value is missing a character. Algorithms such as Levenshtein Distance, Jaro-Winkler or Damerau-Levenshtein can measure how similar the two strings are and return a similarity score or edit distance. Soundex is not designed for this general type of string comparison.

Soundex and Fuzzy Matching Can Work Together

In real-world data matching, choosing between Soundex and fuzzy matching does not always need to be an either/or decision. A phonetic algorithm might be appropriate for a surname, while a fuzzy algorithm could be better suited to a company name or address. Other fields may require exact matching or different techniques altogether. This is why WinPure AdaptiveMatch™ supports different matching approaches at the field level, allowing phonetic and fuzzy algorithms to be applied according to the characteristics of the data.

The goal is not to find one algorithm that works for everything, but to use the right matching method for the right data.

Soundex vs Metaphone

Soundex and Metaphone are both phonetic matching algorithms designed to identify words or names that sound similar despite differences in spelling. However, Metaphone was developed as a more sophisticated alternative to Soundex and uses a broader set of pronunciation rules.

SoundexMetaphone
PurposePhonetic name matchingPhonetic word and name matching
OutputLetter + three digitsPhonetic character code
ComplexitySimpleMore sophisticated
Pronunciation rulesBasic sound groupsMore detailed English pronunciation rules
Best suited toEnglish-language surnamesWider range of English words and names
Example useSmith / SmythSteven / Stephen

How Soundex Works

Soundex keeps the first letter of a name and converts the remaining consonants into numeric sound groups.

For example:

Smith → S530
Smyth → S530

Because both names generate the same code, Soundex considers them a potential phonetic match.

Its simplicity makes Soundex fast and easy to understand, but it also means that different names can sometimes receive the same code.

How Metaphone Is Different

Metaphone analyses pronunciation using more detailed rules rather than simply grouping consonants into numbers. It considers letter combinations and how letters are likely to be pronounced. For example, combinations such as PH, TH, SH and CH can be treated according to their phonetic sound rather than processed as unrelated individual letters. This can make Metaphone more selective for many English-language names and words.

Which Should You Use?

Soundex can be useful when you need a simple, established method for finding potential phonetic variations in English-language names. Metaphone may be more appropriate when pronunciation differences are more complex and Soundex’s relatively broad encoding rules produce too many potential matches. Neither algorithm is universally better. Their effectiveness depends on the language, type of name and characteristics of the dataset.

For data matching, it can therefore be useful to test multiple phonetic approaches rather than assume one algorithm will perform best across every field. WinPure AdaptiveMatch™ is designed around this principle, allowing the matching approach to be selected according to the data being compared.

Soundex vs Double Metaphone

Soundex and Double Metaphone are both phonetic matching algorithms used to identify names that may sound alike despite being spelled differently. The main difference is that Soundex uses relatively simple phonetic rules, while Double Metaphone uses a more detailed set of pronunciation rules designed to handle a wider variety of names.

SoundexDouble Metaphone
PurposePhonetic name matchingMore advanced phonetic name matching
OutputLetter + three digitsPhonetic character code
Pronunciation rulesRelatively simpleMore detailed
Language handlingPrimarily EnglishHandles a broader range of name origins
Alternative pronunciationsNoCan generate primary and secondary codes
Best suited toSimple English name matchingMore complex name variations

How Soundex Works

Soundex retains the first letter of a name and converts the remaining consonants into numeric groups based on similar sounds.

For example:

Smith → S530
Smyth → S530

The identical codes indicate a potential phonetic match. This approach is simple and efficient, but compressing names into just four characters means that different names can sometimes receive the same Soundex code.

How Double Metaphone Is Different

Double Metaphone uses more detailed pronunciation rules and considers combinations of letters rather than simply assigning consonants to numeric groups. One of its distinctive features is its ability to generate two phonetic codes for a name when more than one pronunciation is plausible:

Primary code — the most likely pronunciation
Secondary code — an alternative pronunciation

This can help with names influenced by different languages or pronunciation traditions. However, using multiple phonetic codes can also increase the number of potential matches. Depending on the matching application, a more conservative implementation may choose to use only the primary Double Metaphone code.

Is Double Metaphone Better Than Soundex?

Double Metaphone is generally more sophisticated, but that does not mean it will always produce better matching results. For a straightforward dataset of English-language surnames, Soundex may be perfectly adequate. For datasets containing more diverse names and complex pronunciation variations, Double Metaphone may identify phonetic relationships that Soundex misses. The trade-off is between coverage and precision. Expanding the number of phonetic interpretations can find additional legitimate matches, but may also introduce false positives.

Using Soundex and Double Metaphone for Data Matching

Rather than treating one phonetic algorithm as universally superior, the best approach is to choose the algorithm according to the data being matched and the required level of precision. This is the approach used by WinPure AdaptiveMatch™, where different matching algorithms can be selected at field level. For example, Soundex may work well for one surname dataset while Double Metaphone may perform better for another. WinPure’s implementation of Double Metaphone uses the primary phonetic value only, providing the benefits of its more advanced pronunciation rules while taking a more conservative approach to potential phonetic matches.

Advantages and Limitations of Soundex

Soundex has remained useful for name matching because it is simple, fast and easy to implement. However, its simplicity also creates limitations that are important to understand before using it for data matching.

Finds spelling variations that sound similar. Soundex can identify names that would fail an exact text comparison.

For example:

Smith → S530
Smyth → S530

Although the spelling is different, both names produce the same Soundex code.

Simple and fast

Soundex converts each name into a compact four-character code. This makes it computationally inexpensive and suitable for processing large volumes of names.

Easy to understand

Unlike some more complex matching algorithms, the logic behind Soundex is relatively straightforward. Users can see how a name has been converted and understand why two values have been considered a potential phonetic match. Useful alongside other matching techniques. Soundex can provide an additional phonetic signal alongside exact matching, fuzzy matching and other record attributes such as address, telephone number or email.

Limitations of Soundex

Different names can produce the same code. Soundex reduces names to a relatively small number of phonetic representations. As a result, names that are not actually the same can sometimes share a code.

For example:

Smith → S530
Schmidt → S530

A Soundex match should therefore be treated as a potential match rather than proof of identity. Primarily designed for English-language names. Soundex was developed around English pronunciation patterns. Its performance can be less reliable for names originating from other languages and naming traditions. Alternative phonetic algorithms such as Cologne Phonetics, Daitch-Mokotoff, NYSIIS, Caverphone 2 and Double Metaphone may be more appropriate for particular datasets or name types.

Does not measure how similar two values are

Soundex produces a phonetic code rather than a similarity percentage. Two names either share the relevant code or they do not. Fuzzy algorithms such as Jaro-Winkler or Levenshtein Distance provide a more granular measurement of string similarity.

The first letter has significant importance

Traditional Soundex retains the first letter unchanged. Names that sound similar but begin with different letters may therefore receive different codes.

It is not designed for every type of data

Soundex is primarily a name-matching technique. It is generally not the right algorithm for comparing addresses, company names, product descriptions or other complex text.

When Should You Use Soundex?

Soundex is most useful when you need to identify names that may sound alike despite differences in spelling. It works particularly well as an additional matching signal when exact matching alone is too restrictive.

Good Uses for Soundex

Soundex can be a useful choice when:

  • Matching English-language names where spelling variations are common.
  • Finding potential duplicate records containing names such as Smith and Smyth.
  • Linking records across different systems where the same person’s name may have been entered differently.
  • Searching historical or manually entered data containing spelling and transcription variations.
  • Creating candidate matches that can then be evaluated using other fields such as address, email or telephone number.
  • Processing large datasets where a simple and computationally efficient phonetic algorithm is useful.

For example, two customer records might contain:

Record A: Robert Smith
Record B: Rupert Smyth

Exact name matching would not identify these records as a match. Soundex can identify the phonetic similarities:

Robert / Rupert → R163
Smith / Smyth → S530

This does not mean the records belong to the same person, but it provides useful evidence that they may warrant further comparison.

When Should You Avoid Soundex?

Soundex is less suitable when you need to compare addresses, company names, product descriptions or other general text. Fuzzy matching algorithms are usually better suited to these types of values. It may also not be the best choice for names from languages where English pronunciation rules do not work well. Depending on the data, phonetic algorithms such as Double Metaphone, Cologne Phonetics or Daitch-Mokotoff may provide better results.

Soundex Works Best as Part of a Matching Strategy

In most real-world data matching projects, Soundex should not make the final matching decision by itself. A better approach is to use Soundex where it is appropriate and combine it with other fields, matching rules and algorithms. For example, a surname could use Soundex while an address uses fuzzy matching and an email address uses exact matching. This is the principle behind WinPure AdaptiveMatch™: rather than applying one matching algorithm to every field, organisations can use matching techniques appropriate to the characteristics of their data.

Using Soundex for Data Matching and Deduplication

Soundex can help identify potential duplicate records when names are spelled differently but sound similar. This makes it useful in data matching and deduplication projects where exact matching may miss legitimate variations. Consider two customer records:

FieldRecord ARecord B
First NameStevenStephen
Last NameSmithSmyth
Address24 High Street24 High St
Emailsteve@example.comsteve@example.com

An exact comparison would find differences in the first name, surname and address. Phonetic matching can help identify the name variations, while other matching techniques can evaluate the remaining fields.

Using Soundex as Part of a Matching Rule

Rather than relying on Soundex alone, it can be combined with other matching criteria. For example:

using soundex as part of a matching rule

The combined evidence provides much more confidence than simply checking whether two surnames have the same Soundex code. This is particularly useful for customer deduplication, CRM data cleansing, record linkage, master data management and Customer 360 projects, where the same person may appear multiple times with slightly different information. Soundex can be used within both deterministic rules and weighted or probabilistic matching configurations. Our guide to deterministic vs probabilistic matching explains the difference between these approaches.

Soundex Can Also Help Reduce the Search Space

For large datasets, phonetic codes can be used to identify candidate records before more detailed comparisons are performed. Instead of comparing every record against every other record, records sharing relevant phonetic characteristics can be brought together for further evaluation. More precise matching rules can then determine whether they represent genuine duplicates.

Choosing the Right Algorithm for Each Field

Soundex is useful for certain types of names, but it is not necessarily the best algorithm for every column. A surname might benefit from Soundex, while another field could perform better with Jaro-Winkler, Levenshtein Distance, Double Metaphone or another matching technique. This is an important distinction in modern data matching: the best algorithm can depend on the field and the data it contains. With WinPure AdaptiveMatch™, different matching algorithms can be applied at field level, allowing organisations to combine phonetic, fuzzy and exact matching techniques within the same matching process. This makes Soundex one part of a broader matching strategy rather than requiring it to make the entire deduplication decision on its own.

Soundex in WinPure AdaptiveMatch™

Soundex is one of the phonetic matching algorithms available within WinPure AdaptiveMatch™, allowing it to be applied specifically to fields where phonetic similarity can improve matching results. Rather than applying the same matching method across every column, AdaptiveMatch™ allows the matching approach to be selected according to the type and characteristics of the data.

For example, a matching rule could use:

FieldMatching Approach
First NameDouble Metaphone
Last NameSoundex
Company NameJaro-Winkler
AddressWinPure Fuzzy
EmailExact

This allows Soundex to do what it is good at — identifying potential phonetic variations in names — while other algorithms handle fields where different types of similarity matter.

More Than a Single Algorithm

Real-world data rarely has one type of matching problem. A dataset can contain misspellings, phonetic variations, transposed characters, abbreviations and formatting differences at the same time. AdaptiveMatch™ brings together multiple matching approaches, including phonetic, fuzzy, edit-distance and exact matching algorithms, so matching rules can be configured around the data rather than forcing the data through a single algorithm. Soundex can therefore be used alongside algorithms such as Double Metaphone, NYSIIS, Caverphone 2, Cologne Phonetics, Daitch-Mokotoff, Jaro-Winkler and Levenshtein Distance.

From Soundex to Trusted Matches

A Soundex match is a useful signal, but it should not automatically mean two records are duplicates. AdaptiveMatch™ allows phonetic similarity to form part of a wider matching rule incorporating multiple fields and matching conditions. This helps organisations find more of the matches they want while reducing the risk of false matches caused by relying too heavily on a single phonetic code.

Soundex is not the matching strategy — it is one of the tools within it. AdaptiveMatch™ gives users the flexibility to choose the right algorithm for the right data.

Match Smarter with AdaptiveMatch™

WinPure AdaptiveMatch™ combines phonetic, fuzzy and exact matching algorithms, allowing you to use the right matching method for the right data.

Explore AdaptiveMatch™

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