What is password entropy — strength in bits

Password entropy is a measure of unpredictability in bits. Each bit of entropy doubles the number of guesses needed for brute force: 40 bits — about a trillion combinations, 80 bits — practically infeasible to crack.

Entropy depends on length and alphabet size under random character selection. The formula is simplified: it assumes every character is chosen uniformly at random — passwords built from dictionary words or patterns are weaker in practice.

The key takeaway: length matters more than “tricky” symbols. A long random string from a large alphabet yields more entropy than a short one with special characters. The ToolsHelper password generator shows the entropy estimate instantly.

Entropy of a password is the log base 2 of the number of possible strings, measured in bits. 80 bits is a large space if the string was chosen at random. It collapses if the string is a word plus a year, because attackers try those patterns first. The figure does not include whether you reused it. This page defines the word. The generator shows a number next to a password it just built.