Password Strength (Entropy) Calculator
Type any password and instantly see its entropy in bits (log₂(pool^length)), which character classes it uses, and how long it would take to crack on average under four attacker models — throttled online, unthrottled online, offline bcrypt and an offline GPU rig. Everything runs locally in your browser; your password is never sent anywhere.
Please enter a valid number
Entropy
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bits
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Strength scale
< 28 Weak
28–36 Reasonable
36–60 Strong
60–128 Very strong
≥ 128
Length
—
Character pool
—
Classes detected
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Online, throttled (10/s)
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Online, unthrottled (1k/s)
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Offline bcrypt (10k/s)
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Offline GPU MD5/SHA-1 (10 billion/s)
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Upper-bound estimate — ignores dictionary patterns. Use a password manager for truly random passwords.
Formula
entropy_bits = length × log₂(poolSize) poolSize = lowercase(26) + uppercase(26) + digits(10) + symbols(32) + space(1) + other(100) average crack time = 2^(entropy − 1) ÷ guesses_per_second
- · Entropy in bits is the information-theoretic measure of how many yes/no questions an attacker needs to guess a password — every extra bit doubles the search space.
- · This calculator uses the pool-size model from NIST SP 800-63B Appendix A and Wikipedia "Password strength": lowercase 26, uppercase 26, digits 10, symbols 32, space 1, Unicode/other 100.
- · Strength bands: < 28 bits = very weak; 28–36 = weak; 36–60 = reasonable; 60–128 = strong; ≥ 128 = very strong (AES-128 equivalent).
- · Attack-rate references: throttled online 10/s (typical login rate-limit), offline bcrypt ~10⁴/s, offline GPU MD5/SHA-1 ~10¹⁰/s (hashcat benchmarks on an RTX 4090).
- · This is an upper-bound estimate — real attackers also use dictionary words, leaked password lists and leetspeak rules, so a password like "P@ssw0rd1" is much weaker than its formal ~58-bit entropy suggests.
- · Recommendation: generate 16+ character random passwords with a password manager (Bitwarden, 1Password, KeePass) and add a TOTP / hardware-key second factor. A long passphrase (e.g. 4–6 random words from the EFF list) is also a solid option.
Frequently asked
How many bits of entropy is "enough"?
For everyday online accounts, 60 bits is comfortably above what offline attackers can crack — at 10¹⁰ guesses/s it averages ~600 years. For high-value accounts (online banking, crypto wallets, enterprise admin) aim for 80–100 bits, with 128 bits matching AES-128 strength as a target. Bear in mind that entropy is just one dimension of safety: a phished or logged plaintext password is broken regardless of bit count, so pair strong entropy with 2FA and unique-per-site passwords.
Why does "P@ssw0rd1" show 58 bits but is still considered weak?
"P@ssw0rd1" looks like 58 bits under the pool-size formula because it has 9 mixed-class characters. But this assumes uniform random selection — and "P@ssw0rd1" is obviously "Password" with stock leetspeak. Hashcat's rule-based attacks try those substitutions first, so the effective entropy is more like 13–20 bits and a GPU breaks it in seconds. Genuine 58-bit strength needs 9 truly random ASCII characters such as "K7$mP!2qZ".
Is a long passphrase or a complex password better?
Length beats complexity. A 20-character random all-lowercase passphrase has ~94 bits (20 × log₂(26)) — stronger than 8 mixed-class characters (~52 bits) and far easier to remember. The EFF Diceware list gives ~12.92 bits per word (log₂(7776)); 5 words ≈ 64.6 bits, 6 words ≈ 77.5 bits. That is the math behind XKCD #936's "correct horse battery staple". Try both "correct horse battery staple" and "K7$mP!" in this tool to compare.
Does this tool send my password anywhere?
No. The entire calculation is bundled into front-end JavaScript (src/lib/math/password-entropy.ts) and runs entirely offline once the page loads — no network requests are made when you type. We also deliberately avoid writing the password to the URL search params, so it never enters browser history or the Referer header. Copying the share link gives you a clean URL.
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