Why use this calculator
Strength meters that show a coloured bar are guessing at how a human built a password. Entropy measures something real: the number of possibilities an attacker must work through, expressed in bits, where each additional bit doubles that number. This generator makes every character an independent random draw from a known pool, so the entropy figure is exact rather than estimated, and it uses the browser's cryptographic random source so the sequence cannot be predicted from the clock or from earlier output.
It also shows the number that surprises most people: how much more four extra characters buy you compared with adding a symbol, and how many random words would give you the same protection in a form you could actually remember.
How to use it
- Set the length first, because it is by far the strongest lever you have.
- Twelve characters is a reasonable floor for an account you care about, sixteen is comfortable, and more is free when a password manager is doing the typing.
- Choose whether to include uppercase letters, digits and symbols; each set you add makes the pool larger and every character worth slightly more.
- Turn on the look-alike exclusion only when you will have to read the password aloud or retype it from a screen, since it makes the pool a little smaller in exchange for far fewer mistakes.
- Read the entropy figure rather than the word, copy the password straight into your password manager, and never reuse it anywhere else.
What each result means
- Generated password
- The password produced by this run. It is created in your browser, is never sent anywhere, and is different every time.
- Entropy (bits)
- The true strength measure. Each bit doubles the search space, so 60 bits is a thousand times harder to crack than 50.
- Shortest length for Very strong
- The fewest characters that reach 100 bits of entropy, the Very strong band, using the character sets you have switched on. Length is the cheapest way to add strength.
- Strength
- A plain-language reading of the entropy, from very weak below 40 bits to very strong at 100 bits and above.
- Character pool size
- How many distinct characters each position could hold. A bigger pool raises the value of every character in the password.
- Character sets used
- Which alphabets went into the pool, so you can check the password will satisfy a site's complexity rules.
- Length
- The number of characters generated. Adding characters raises entropy linearly, which beats any change to the character sets.
- Offline cracking time
- Roughly how long a well-funded attacker with a stolen password database would need on average to guess this password.
- Equivalent passphrase
- The number of randomly chosen dictionary words that carry the same entropy, if you need something a human can memorise.
The formula
Entropy in bits = length times log base 2 of the pool size
Twenty-six lowercase letters give 4.70 bits per character, adding uppercase gives 5.70, adding digits gives 5.95 and adding symbols gives 6.46 for an 88-character pool. The average number of guesses required is two to the power of the entropy, divided by two, and the cracking time divides that by the attacker's guess rate. Note the entropy is computed as length times the logarithm, not the logarithm of the pool raised to the length, because that intermediate value overflows floating-point arithmetic long before a 128-character password.
How this number is used in practice
Length beats complexity, and the arithmetic shows why. A twelve-character password drawn from all 88 characters carries about 78 bits. Dropping symbols and using only letters and digits takes it to 71 bits, a real loss but a small one. Extending the letters-and-digits password to sixteen characters instead takes it to 95 bits, far beyond what the symbols recovered.
This is the reasoning behind modern guidance from NIST and the UK National Cyber Security Centre: require length, allow every character including spaces, and stop forcing the composition rules that pushed people into predictable patterns like a capital at the front and an exclamation mark at the end.
The same guidance retired scheduled password changes. Forcing a change every ninety days produced weaker passwords, because people incremented a number on the end, and it did nothing against the real threats, which are credential stuffing from other sites' breaches and phishing. Change a password when there is evidence of compromise, use a different one everywhere so a breach cannot spread, and turn on multi-factor authentication, which defeats a stolen password outright.
For the handful of passwords you genuinely have to memorise, such as the one that unlocks the password manager itself and the one on your device, a passphrase of several randomly chosen words is the better shape. Four words from a 7,776-word list carry about 52 bits, five words about 65, and six words about 78, which puts a six-word passphrase in the same territory as a twelve-character random string while being far easier to type on a phone.
The words must be chosen at random, not picked by you, because a self-chosen phrase carries a fraction of the entropy it appears to.
Worked examples
16 characters with letters, digits and symbols
An 88-character pool and about 103.4 bits of entropy, rated very strong.
12 characters using lowercase letters only
A 26-character pool and about 56.4 bits of entropy, which the strength bands here rate as weak.
20 characters with letters and digits but no symbols
A 62-character pool and about 119.1 bits, stronger than 16 characters with symbols.
8 characters with everything enabled
About 51.7 bits, which a well-funded offline attack clears in well under a day and should not be used.
When people use it
- Creating a unique password for every online account you hold
- Generating a master password or recovery code that has to be strong first and memorable second
- Issuing temporary credentials for a new staff member or a contractor
- Replacing a password that appeared in a breach notification
- Producing API keys, database passwords and service account secrets for infrastructure
Tips
- Length is the cheapest strength there is. Going from 12 to 16 characters adds more entropy than adding symbols to a 12-character password.
- Store every generated password in a password manager immediately. A password you cannot retrieve is a lockout waiting to happen.
- Never reuse a password between sites. Credential stuffing takes one breached site's passwords and tries them everywhere else automatically.
- Turn on multi-factor authentication wherever it is offered; it protects you even if the password itself is exposed.
- Only turn off symbols if the site actually rejects them, and treat a site that limits password length or forbids characters as a warning sign about its security generally.
- Use the look-alike exclusion for anything you will dictate over the phone or type from a screen, such as a router or Wi-Fi password.
- For the few passwords you must memorise, use several randomly chosen words rather than a short scramble of symbols.
What it assumes, and where it stops being accurate
- Every character is drawn independently and uniformly from the pool, which is what makes the entropy figure exact rather than an estimate.
- The password is generated in your browser and is never transmitted, logged or stored by this site.
- Cracking times assume an offline attack at 100 billion guesses a second against a fast hash. A properly salted, slow hash such as Argon2 or bcrypt takes far longer.
- Entropy measures resistance to guessing only. It offers no protection against phishing, keylogging, shoulder surfing or a breach of the site holding the password.
- The strength bands used here (40, 60, 80 and 100 bits) are a practical reading of current guidance, not a formal standard.
- Where the cryptographic random source is unavailable the generator falls back to the ordinary pseudo-random source, which is weaker and should not be relied on for secrets.
Terms used on this page
- Entropy
- A measure of unpredictability in bits, where each bit doubles the number of guesses required.
- Character pool
- The set of distinct characters any single position in the password could contain.
- Credential stuffing
- Automated reuse of usernames and passwords stolen from one site against many others.
- Diceware
- A method of building passphrases by choosing words at random from a numbered list of 7,776 words.
- Rejection sampling
- Discarding and redrawing random values that would otherwise bias the result toward part of the range.
Questions people ask
- What is password entropy in bits?
- It is the base-two logarithm of the number of possible passwords your settings could produce. Each extra bit doubles that number, so 50 bits is about a thousand times harder to guess than 40. It is the only strength measure that can be calculated exactly rather than estimated from patterns.
- How long should a password be?
- Twelve characters is a sensible minimum for anything that matters, and sixteen or more is better when a password manager is doing the typing. Because entropy grows linearly with length, four extra characters from a full pool add roughly twenty-six bits, which is a far bigger gain than any change to the character sets.
- Are symbols necessary for a strong password?
- They help, but less than people assume. Adding 26 symbols to letters and digits raises each character from 5.95 bits to 6.46, about nine per cent. Adding four characters to a twelve-character password raises entropy by a third. Use symbols where they are accepted, but never at the expense of length.
- Should I change my passwords regularly?
- No. NIST and the UK National Cyber Security Centre both advise against scheduled expiry, because it pushes people toward predictable variations of an old password. Change a password when there is evidence it has been exposed, use a unique one on every site, and enable multi-factor authentication.
- Are these passwords generated securely?
- Each character is drawn with the Web Crypto API, the browser's cryptographically strong random source, using rejection sampling so no character is favoured over another. Generation happens entirely in your browser and nothing is sent to a server. Where that API is missing, the generator falls back to a weaker pseudo-random source.
- Is a passphrase better than a random password?
- For the handful of passwords you must type from memory, yes. Six randomly chosen words from a 7,776-word list carry about 78 bits, similar to a twelve-character random string, and are far easier to recall and type. The words must be chosen randomly rather than composed by you, or the entropy collapses.
Use this with
Password Generator answers one part of the question. These pick up where it stops: Random Number Generator, IPv4 Subnet Calculator, Number Base Converter and Data Storage Converter.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- NIST: Digital Identity Guidelines, SP 800-63B. Federal guidance on memorised secrets: length, blocklists and forced rotation
- UK National Cyber Security Centre: Password guidance. Three-random-words passphrase advice and password policy design
- MDN: Crypto.getRandomValues(). The cryptographically strong random source this generator uses in the browser
Password Generator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.