Password Entropy Calculator

NIST SP 800-63B dropped mandatory composition rules for a reason: the classic entropy formula is just length times the log2 of how many characters could occupy each slot, and a policy that forces one digit and one symbol barely moves that number. Password Entropy Calculator computes exactly that formula, two ways. Paste a real password and it reads the classes present. Design a policy instead, and it prices the pool from a length and a set of checkboxes before you have typed a single character.

This is deliberately the naive count, not the pattern search that Password Strength Meter runs. Tr0ub4dor&3 comes back as 72.1 bits here, because eleven characters drawn from a 94-symbol pool is what the formula sees, and the formula cannot know that troubador is a dictionary word. That gap between the two tools is the entire point of keeping them separate.

The Controls: Two Ways to Set the Pool

From a Password scans whatever you type for four classes (lowercase, uppercase, digits, symbols) and sizes the pool from whichever ones actually appear, so a digits-only PIN never gets credit for an uppercase pool it does not use.

• From a Policy swaps the password field for a length slider (1 to 128) and four checkboxes, each labelled with its own pool size: a–z (26), A–Z (26), 0–9 (10), symbols (32).
• A custom character set field overrides the checkboxes entirely. Type ACGT for a four-symbol DNA-style alphabet and the checkboxes grey out.
• Generate Example then draws a sample string from that exact pool using crypto.getRandomValues with rejection sampling rather than Math.random, so the illustration itself carries no modulo bias.

How It Calculates the Bit Count

Entropy is length × log2(pool size), full stop. Twelve characters pulled from lowercase, uppercase and digits, no symbols, land at 71.5 bits, because that pool has 62 members. Load Sample types Tr0ub4dor&3 and turns on the reveal toggle. The page shows 5.1 × 10²¹ estimated guesses and calls it equivalent to a 16-character all-lowercase random password.

Four attack rows convert that bit count into time, from a login form throttled to 100 guesses an hour up to an unsalted fast hash working through 100 billion a second. For Tr0ub4dor&3 the fast row lands at roughly 2 thousand years, comfortable against brute force, and exactly why the second tool exists to check the rest.

The Limits of a Bit Count

A high number here means the pool is large and the length is real. It says nothing about whether the specific string sitting in that pool is correcthorsebatterystaple or a line from a breach dump, because the formula treats every character as equally likely by construction.

• Leaving every checkbox unchecked and the custom field empty raises Select at least one character class, or enter a custom set, to compute entropy. There is no pool to compute against.
• The password field caps at 256 characters; the policy slider caps at 128. Both ceilings sit well past the point where the bit count already outruns every attack row below it.
• Characters outside the four tracked classes (accented letters, emoji, CJK) are priced at a flat pool of 100 apiece, a deliberately conservative stand-in rather than a measured value.
• For a verdict that accounts for dictionary words, keyboard walks and dates instead of pool size alone, use Password Strength Meter. This page and that one answer different questions on purpose.