Password Strength and Entropy Calculator
Check how strong a password is, see how long it could take to crack, or make a strong random password or passphrase. Your password never leaves this page.
Your password never leaves this page. It is checked by code running in your browser and is never sent, saved or put in a share link. No ads or analytics scripts run on this page. The example password is Sunshine2024! Replace it with yours.
Adds a row to the crack-time table. You can type 1e9 for a billion.
Some sites refuse certain symbols. Type them here and they won't be used.
For sites that insist on a number and a symbol.
How your password strength is worked out
Entropy = length × log2(characters to choose from), minus what patterns give away.
- 1
Count the characters
13 characters26 + 26 + 10 + 32= 94 each
- 2
If it were random
13 × log2(94)= 13 × 6.555= 85.2 bits
- 3
Spot the patterns
common word, year+ 1 random character= 19.0 bits
- 4
Time to crack
2^19.0 ÷ 2 guessesat 10 billion/s= Instantly
Read more: the formula, a worked example, crack-time assumptions and tips
Entropy counts how many guesses a password could need, in bits. One bit means 2 possibilities, 10 bits means 1,024, and every extra bit doubles the work. A password is only as strong as the way it was made: random characters keep all their entropy, while words, names, dates and keyboard patterns give a lot of it away.
The formula
naive entropy (bits) = length × log2(characters to choose from)
passphrase entropy = words × log2(words in the list)
realistic entropy = cheapest mix of patterns and random characters that builds the password
common password = log2(its rank in the list)
dictionary word = log2(2,048) = 11 bits, +1 for a capital, +1 per l33t swap
year = log2(140), date = log2(366 × 140)
random character = log2(pool)
average guesses = 2^bits ÷ 2
time to crack = average guesses ÷ guesses per second
length needed = bits wanted ÷ log2(pool), rounded up
Worked example
Take the example Sunshine2024!. It has 13 characters and uses lowercase (26), uppercase (26), digits (10) and symbols (32), so each character could be one of 26 + 26 + 10 + 32 = 94. If it were random it would have 13 × log2(94) = 13 × 6.5546 = 85.2 bits.
But it isn't random. "sunshine" is number 20 on our list of the most common passwords, worth log2(20) = 4.32 bits, plus 1 bit for the capital S. "2024" is a year between 1900 and 2039, worth log2(140) = 7.13 bits. Only the "!" counts as a random character: log2(94) = 6.55 bits. Total: 4.32 + 1 + 7.13 + 6.55 = 19.0 bits, which is very weak.
Average guesses = 2^19.0 ÷ 2 ≈ 263,000. At 10 billion guesses a second (a stolen database with a fast hash) that takes 0.00003 seconds, which is instant. Even at 10 guesses a second against a throttled login it is about 7 hours.
Crack-time assumptions
These speeds are illustrative, rounded assumptions for comparison, not measurements of any real system. Times are averages (half of all possibilities).
| Attack | Guesses/s | 40 bits | 60 bits | 80 bits |
|---|---|---|---|---|
| Online, throttled A website that slows or locks repeated logins | 10 | 1.7 thousand years | 1.8 billion years | 1.9 × 10^15 years |
| Online, no limit A poorly protected login page or API | 1,000 | 17 years | 18 million years | 19 trillion years |
| Offline, slow hash Stolen database using bcrypt, scrypt or Argon2 | 10,000 | 1.7 years | 1.8 million years | 1.9 trillion years |
| Offline, fast hash Stolen database using MD5, SHA-1 or NTLM, one GPU rig | 10 billion | 55 seconds | 1.8 years | 1.9 million years |
| Massive cracking Large GPU cluster or nation-state, fast hash | 1,000 billion | Instantly | 6.7 days | 19 thousand years |
Entropy by length
| Length | Digits (10) | Lowercase (26) | Letters + digits (62) | All keyboard (94) |
|---|---|---|---|---|
| 8 | 26.6 bits | 37.6 bits | 47.6 bits | 52.4 bits |
| 10 | 33.2 bits | 47.0 bits | 59.5 bits | 65.5 bits |
| 12 | 39.9 bits | 56.4 bits | 71.5 bits | 78.7 bits |
| 16 | 53.2 bits | 75.2 bits | 95.3 bits | 104.9 bits |
| 20 | 66.4 bits | 94.0 bits | 119.1 bits | 131.1 bits |
| Random words (2,048 list) | Entropy | Same as random keyboard characters |
|---|---|---|
| 3 | 33 bits | 6 |
| 4 | 44 bits | 7 |
| 5 | 55 bits | 9 |
| 6 | 66 bits | 11 |
| 7 | 77 bits | 12 |
| 8 | 88 bits | 14 |
Strength bands
Under 28 bits very weak, 28–35 weak, 36–59 fair, 60–79 strong, 80 and over very strong. Any password on a common-password list counts as very weak whatever its length.
How the generator picks
Passwords and passphrases are made on your device with crypto.getRandomValues, the browser's cryptographic random number source. Picking one of n items with "random number mod n" would make some items slightly more likely, so the calculator throws away the few top random values that would cause that bias (rejection sampling) and draws again. With "At least one of each", whole passwords are redrawn until every kind appears, which keeps every allowed password equally likely; the entropy shown is the exact count of allowed passwords, slightly below length × log2(pool).
The passphrase list is our own list of 2,048 common, easy-to-spell English words, so each word adds exactly log2(2,048) = 11 bits. The dice list option in Entropy maths (7,776 words, 12.9 bits a word) matches the well-known five-dice word lists.
Tips
- Use a different password for every account. A strong password reused on a site that gets breached is no longer strong.
- Let a password manager make and remember them; then you only need one strong passphrase.
- Turn on two-step login (an app or security key) for email, banking and anything important.
- Length beats complexity. NIST's guidance for websites (SP 800-63B) favours longer passwords and screening against known-breached lists over forced symbol rules.
The pattern check is a simplified estimate in the style of the open-source zxcvbn library, with a list of about 200 very common passwords and our 2,048-word list. Real attackers use lists of billions of leaked passwords, so treat the realistic figure as a best case. Everything runs in your browser; nothing you type is sent, saved or shared.
Common questions
Is it safe to type my password here?
Yes. The check runs entirely in your browser: the password is not sent to our server or anyone else, not saved on your device and never put in a share link. You can even load the page, switch off your internet and it still works. If you are cautious, test a password that is similar to yours rather than the real one.
What is password entropy?
Entropy measures how many guesses a password could take, in bits. Each bit doubles the number of possibilities. For a password picked at random it is length × log2(characters to choose from): 12 random characters from all 94 keyboard characters give 12 × 6.55 = 78.7 bits. Passwords people make up have far less, because they use words, names, dates and patterns.
How many bits of entropy is a strong password?
As a rough guide, under 28 bits is very weak, 28–35 weak, 36–59 fair, 60–79 strong and 80 or more very strong. For a password that protects something important, such as your email or password manager, aim for 80 bits or more: about 13 random characters from all keyboard characters, or 8 words from a 2,048-word list.
Is a passphrase better than a password?
A passphrase of random words is easier to remember and type, and just as strong if it has enough words. Each word from a 2,048-word list adds 11 bits, so 6 random words give 66 bits and 7 give 77. The words must be picked at random, not chosen by you: a line from a song or film is guessed quickly.
Does adding a symbol or capital letter make a password strong?
Less than people think. Password1! has a capital, a digit and a symbol but is one of the first things attackers try, because they know people add them at the start and end. Swapping @ for a or 0 for o adds only about one bit per swap. Length and randomness matter far more.
How long would it take to crack my password?
It depends on how the attacker can guess. A login page that locks you out after a few tries allows about 10 guesses a second, while a stolen database of fast MD5 or NTLM hashes can be tested at around 10 billion guesses a second on one GPU rig. The calculator shows the average time (half of all possibilities) at five such speeds; they are illustrative assumptions, not measurements.
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Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.