Compare Two Passwords for Similarity — Free Edit-Distance Tool
Type one password into Password A and another into Password B, and the Password Similarity Comparator works out how close they are using Levenshtein edit-distance, giving you back a similarity percentage and the exact number of character edits apart the two passwords are. It's a fast way to catch a password that's just a slightly reworded version of one you've reused elsewhere, or one that's already turned up in a data breach.
Have you ever reused the same password with just one tiny tweak — a digit swapped, a symbol added — and wondered if that counts as a genuinely different password? A password similarity checker answers exactly that question. This tool, the Password Similarity Comparator, compares two passwords character by character and tells you, in plain numbers, how close they really are — so you can catch a reused or barely-modified password before an attacker does.
What Is a Password Similarity Checker?
A password similarity checker — sometimes called a password comparison tool or a password difference checker — measures how alike two passwords are, rather than just checking whether they're identical. Two passwords can look completely different at a glance and still be dangerously close from a security standpoint: Tiger2023 and Tiger2023Admin share almost their entire structure, even though they're not the same string. That closeness is what a similarity score is built to expose.
How This Tool Measures Similarity
This comparator calculates similarity using the Levenshtein distance algorithm, also known as edit distance — the minimum number of single-character insertions, deletions, or substitutions needed to turn Password A into Password B. A shorter edit distance relative to password length means a higher similarity percentage. Every time you type into either field, the comparator recalculates instantly and returns four pieces of information:
- Levenshtein Distance — the raw number of single-character edits separating the two passwords.
- Similarity Percentage — the edit distance converted into a 0–100% similarity score, so you don't have to interpret a raw distance number yourself.
- Character Edits Apart — the same edit-distance count, shown as a plain figure alongside the percentage.
- Similarity Rating — a plain-language label (Identical, Very Similar, Somewhat Similar, or Not Similar) that turns the percentage into an at-a-glance verdict.
Other Ways Similarity Can Be Measured
Levenshtein distance isn't the only method used in password-similarity research and enterprise credential-auditing software. Longest common substring looks for the biggest unbroken chunk two passwords share; prefix/suffix matching flags passwords that only differ at the very start or end (a common pattern when someone appends a service name or a year); character-class analysis compares how the mix of letters, digits, and symbols shifted between versions. This tool focuses on Levenshtein edit distance specifically because it's the most widely understood, verifiable measure of "how many changes apart" two passwords are — the same underlying concept operating-system and directory-service password-history checks rely on when they reject a new password for being too close to an old one.
A Worked Example
Numbers make similarity easier to reason about than instinct alone. Here's how four different password pairs score when run through this comparator, covering each of the four similarity ratings:
| Password A | Password B | Similarity | Character Edits Apart | Rating |
|---|---|---|---|---|
| Summer2024! | Summer2024! | 100% | 0 | Identical |
| Summer2024! | Summer2025! | 91% | 1 | Very Similar |
| Tiger2023 | Tiger2023Admin | 64% | 5 | Somewhat Similar |
| BlueOcean42 | RedForest17 | 9% | 10 | Not Similar |
Notice how little it takes to land in the "Very Similar" band — changing a single digit in Summer2024! still leaves the password 91% similar, because only one character actually moved. That's exactly the kind of password variation an attacker checks first after any data breach.
How to Use the Password Similarity Comparator
You don't need to click a button or wait for a page to reload — the comparison updates live as you type.
- Type (or paste) the first password into the Password A field.
- Type the second password — the one you're considering, or the one you're worried is too close — into Password B.
- Use the eye icon next to each field if you need to double-check what you typed; both fields are masked by default.
- Read the similarity banner that appears: it shows the similarity percentage, the character edits apart count, and a color-coded rating (Identical, Very Similar, Somewhat Similar, or Not Similar).
- If the rating comes back Somewhat Similar or higher, treat it as a signal to rework the new password rather than just tweaking it further.
Why Similar Passwords Put Your Accounts at Risk
Reusing a password outright is an obvious risk, but a slightly modified password isn't much safer. When one of your passwords turns up in a data breach, attackers don't just try that exact leaked value against your other accounts — automated credential stuffing and password spraying tools also run through the small set of predictable tweaks people make: incrementing a number, swapping a letter for a lookalike symbol, or appending the current year. If your base password is compromised, every near-identical variant of it is effectively compromised too.
Common Patterns That Drive Up a Similarity Score
Most high-similarity password pairs fall into a handful of recognizable habits:
- Reusing a base word across every account, changing only the service name around it.
- Incrementing a digit at the end (Password1 → Password2) each time a site forces a password change.
- Swapping letters for lookalike symbols — a → @, s → $, i → 1 — without changing anything else about the password.
- Appending the current year or season, such as Summer2024 becoming Winter2024 or Summer2025.
- Adding a prefix or suffix tied to the account, like putting "Admin" or a company name on the end of an otherwise unchanged password.
A Closer Look: The "Season + Year" Pattern
The season-plus-year habit is worth calling out on its own because it's so common it shows up on nearly every list of breached-password patterns. A password like Winter2024 scores extremely close to Winter2025, Spring2024, or Summer2024 — often 80%+ similar — because only a handful of characters actually change between them. It feels like a fresh password every few months; to an edit-distance calculation, it's barely a different password at all.
What Counts as "Too Similar"? A Similarity Rating Guide
There's no single official cutoff for password similarity, but this tool's rating bands give you a consistent starting point for judging your own results:
| Similarity Range | Rating | What It Means | Recommended Action |
|---|---|---|---|
| 100% | Identical | The two passwords are exactly the same string. | Treat them as one password, not two — reusing it anywhere is a reuse risk. |
| 70% – 99% | Very Similar | A handful of edits apart; likely a direct variation of the same base password. | Rework it — don't just add another character. |
| 40% – 69% | Somewhat Similar | Shares a noticeable chunk of structure, but isn't a trivial guess-and-check away. | Worth reviewing, especially for high-value accounts. |
| Below 40% | Not Similar | The two passwords share little to no recognizable structure. | No action needed based on similarity alone. |
Building Passwords That Aren't Similar to Anything Else
The most reliable way to score "Not Similar" against every other password you own is to stop generating new passwords from an old one in the first place.
- Generate each new password with a dedicated password generator that draws from a wide character set at random, rather than editing an existing password by hand.
- Avoid anchoring passwords to a memorable base word, a season, or a year — those are exactly the patterns this tool's rating guide flags as high-risk.
- Run a proposed replacement password against the old, possibly leaked one here before you commit to it — a "Not Similar" result confirms it's a genuine change, not a disguised one.
- Store each unique password in a password manager instead of relying on memory, which removes the incentive to make passwords similar so you can remember them.
- Watch for the same predictable transformations — digit increments, symbol swaps, service-name prefixes — across every account, not just the one you're actively changing.
Frequently Asked Questions
- What algorithm calculates the similarity score?
- Levenshtein edit distance -- the minimum number of single-character insertions, deletions, or substitutions needed to turn Password A into Password B. The similarity percentage is derived from that distance relative to the longer password's length.
- Why does similarity between two passwords matter?
- If one of your passwords ever leaks in a breach, attackers don't just try that exact value against your other accounts -- they also try common small variations (appending a digit, changing the last symbol, swapping the year). A high similarity score means your "new" password isn't offering much independent protection.
- What counts as "too similar"?
- There's no universal cutoff, but as a rule of thumb: identical or a handful of edits apart (this tool flags 70%+ similarity as "Very Similar") means the two passwords offer little independent protection against credential-stuffing style guessing.
- Should I use this to check a new password against an old, leaked one?
- Yes -- that's a strong use case. If you know a password was involved in a breach, compare your proposed replacement against it here to confirm you're not just making a minor tweak that an attacker's mutation rules would still catch.
- Can I compare more than two passwords at once?
- No -- this tool compares exactly one pair at a time, Password A against Password B. If you want to audit a whole list of passwords for weakness (not pairwise similarity), the Bulk Password Strength Auditor is the tool for that instead.
- Does this tool work on mobile?
- Yes. The comparator is fully responsive and works the same way -- live, in-browser, no server involved -- on a phone or tablet as it does on desktop.
- Are both passwords sent anywhere?
- No. The comparison happens entirely in your browser -- neither password is transmitted to a server, logged, or stored.