Password Strength Tester (Entropy Bits)
Estimate password entropy in bits and the time an offline brute-force attacker would need at common cracking rates. Pure client-side — your password is never sent anywhere.
Result
How to use this calculator
- Type or paste the password into the input.
- Choose the attacker rate — 10 G/s is the realistic modern baseline; pick higher for nation-state threat models.
- Read the entropy bits and the mean / worst-case crack-time. Flags below the result indicate why the number may be over-stated.
- Aim for ≥ 80 bits for accounts that matter; ≥ 128 for master passwords or encryption keys.
About this calculator
A password’s strength is best expressed as its entropy in bits — log₂ of the number of equally-likely passwords an attacker has to try. For a random 12-character string mixing upper, lower, digit, and symbol, that is about 78 bits — strong enough to resist a single GPU rig for centuries on a non-cached hash. This tester computes the theoretical entropy from password length and the character classes it contains, then converts to mean crack-time at four common attacker speeds: rate-limited online, slow bcrypt offline, modern GPU, and state-actor cluster. It also flags weaknesses that ruin the entropy estimate: dictionary words, short length, and use of only one character class.
How it works — the formula
entropy_bits = length · log₂(pool_size)
mean_crack_time = (2^entropy_bits / 2) / attacker_rateFor a uniformly random password from a pool of N characters, the number of equally-likely possibilities is N^length, so log₂ gives the entropy in bits. Mean-case search costs half that. Real-world passwords are far less random; entropy bits are an upper bound on resistance to brute force, not on resistance to dictionary attacks.
Worked examples
- Inputs:
- pwd=password
- Output:
- ~37.6 bits but dictionary-flagged → minutes
- Inputs:
- pwd=Tr0u4!H#9Vk2
- Output:
- ~78 bits → ~years on a GPU rig
- Inputs:
- pwd=correct horse battery staple
- Output:
- ~92 bits → centuries on a GPU rig (but only ~52 bits if attacker uses a word-list)
Limitations
- Entropy ≠ real security: dictionary + rule-based crackers find "high-entropy looking" passwords in seconds when they follow common patterns.
- No breach-list check — use haveibeenpwned.com/Passwords for that.
- Attacker-rate selection is a coarse model — real cracking speed depends on the specific hash function and hardware.
Educational estimate only — for compliance use a tool that also checks against breach databases and known dictionaries.
Frequently asked
Why is the actual security usually less than the entropy bits?+
What is NIST’s recommended length?+
Why does the attacker rate matter so much?+
Why are passphrases recommended over complex short passwords?+
Is my password sent anywhere?+
What about pwned-password breach lists?+
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