How strong is that password really? This free password strength calculator estimates entropy in bits based on length and character variety, and gives a general strength rating.
This estimates theoretical strength only – never enter your actual real password into any online tool.
Never type a real password into any website’s strength checker, including this one – use it to understand strength concepts with example lengths only.
How the Password Strength Calculator Works
This calculator estimates the theoretical strength of a password pattern using information entropy, based only on how long the password is and which character types it draws from — never on an actual password you type in. Longer length and a wider mix of character types both push entropy up.
Formula: Entropy (in bits) = Length × log2(Pool size). The pool size is the sum of the character sets you select: lowercase a-z adds 26, uppercase A-Z adds 26, numbers 0-9 adds 10, and symbols adds 32. The resulting entropy value is then matched to a strength rating: under 28 bits is Very Weak, 28 to 35 bits is Weak, 36 to 59 bits is Reasonable, 60 to 127 bits is Strong, and 128 bits or higher is Very Strong.
- Password length — the number of characters, entered in the Password length field.
- Character types used — which of a-z, A-Z, 0-9, and Symbols are toggled on, which determines the character pool size used in the entropy formula.
Example Scenarios
| Password Length | Character Types Used | Entropy & Strength |
|---|---|---|
| 7 | lowercase only | 33 bits — Weak |
| 8 | lowercase only | 38 bits — Reasonable |
| 10 | lowercase + uppercase | 57 bits — Reasonable |
| 12 | lowercase + numbers | 62 bits — Strong |
| 14 | lowercase + uppercase + numbers + symbols | 92 bits — Strong |
| 20 | lowercase + uppercase + numbers + symbols | 131 bits — Very Strong |
Password Strength Calculator FAQ
What character pool size does this calculator assume for symbols?
Selecting Symbols adds 32 possible characters to the pool, alongside 26 for lowercase letters, 26 for uppercase letters, and 10 for numbers. Selecting all four types gives a maximum pool of 94 characters, which is used in the log2(pool size) part of the entropy formula.Why does adding character types matter more than just adding length?
Both increase entropy, but they don’t affect it equally per keystroke. Adding one more character type multiplies the pool size for every position in the password, while adding one more character only extends the length by one exponent, so mixing character types is often a bigger multiplier on strength than a single extra character.What entropy in bits is considered “strong” by this calculator?
This tool rates 60 to 127 bits as Strong and 128 bits or higher as Very Strong. Anything under 36 bits is rated Weak or Very Weak, and 36 to 59 bits falls into the middle Reasonable category.Is it safe to type my real password into this tool?
No — the calculator itself warns against it. It’s designed to model strength using a length and a set of character-type checkboxes, not to analyze an actual string, so there’s no reason to ever paste a real password into it or any similar online tool.How is entropy in bits actually calculated from length and character pool?
Entropy equals the password length multiplied by log2 of the character pool size. Log2 (log base 2) measures how many bits of information each character position carries; a bigger pool means more possible values per position, and more positions means more total combinations to guess.Does this calculator check if a password appears in known data breach lists?
No. It only estimates theoretical entropy from length and character variety. A long password made entirely of dictionary words or a known leaked phrase could show high entropy here while still being easy to guess in practice, since breach-list and dictionary-attack exposure isn’t part of this calculation.Related Calculators
Need help generating truly random values instead of estimating strength? Try the random number generator for randomized character seeds. If you work in IT or networking, the subnet calculator for IP address planning and the binary to decimal converter for number-base conversions are also handy technical tools.