Password Strength Analyzer

What the entropy number means

Entropy measures how many guesses an attacker needs, expressed in bits. Each bit doubles the work. The estimate here is log2(charset_size) * length: a 10-character password drawn from lowercase plus digits has a 36-character pool and roughly 51.7 bits, meaning about 2^51.7 candidates if the attacker knows the pool and the length.

That formula describes a password chosen at random from that pool. It is an upper bound, and human-chosen passwords rarely reach it. Password1! scores well on paper — mixed case, digit, symbol, 10 characters — but it is in every cracking dictionary, so its real strength is a handful of guesses. This is why the checklist here is not just a scorecard; failing the common-password check matters more than passing the symbol check.

Why crack-time estimates vary so wildly

Two things change the answer by orders of magnitude. First, the attacker's hardware: a single modern GPU manages billions of guesses per second against a fast hash like MD5 or SHA-1, and a rented cluster multiplies that. Second, how the target stored the password. A properly configured bcrypt, scrypt or Argon2 hash is deliberately slow and memory-hard, cutting throughput from billions per second to thousands.

So the same password can be trivially broken on one site and comfortably safe on another. The figure shown here assumes offline attack against a fast hash, which is the pessimistic case and the right one to plan around, since you cannot know how any given site stores your password.

What actually helps

Length beats complexity. Adding one character to a lowercase password multiplies the search space by 26; adding a symbol class to a short password multiplies it far less. A four or five word passphrase drawn randomly from a large word list reaches 50-70 bits while staying typeable and memorable, which is the reasoning behind the diceware approach.

The bigger point is that per-password strength matters less than reuse. A strong password used on ten sites is only as safe as the weakest of them, because one breach exposes all ten. A password manager generating a distinct random string per site solves both problems at once and is a bigger practical win than any amount of tuning a password you type by hand.

Note that everything on this page runs locally in the browser. Nothing you type is transmitted anywhere.