What password entropy measures
Entropy measures how unpredictable a password is, expressed in bits. A higher bit count means an attacker guessing randomly would need to try exponentially more combinations before finding the right one, which is the standard way to compare password strength independent of any specific attack tool.
The formula
Entropy in bits equals the password length multiplied by the base-2 logarithm of the character pool size. The pool size depends on which character types are used: 26 for lowercase letters, 26 for uppercase, 10 for digits, and roughly 32 for common symbols. The calculator also estimates a rough brute-force crack time by dividing half the total possible combinations (the average case) by an assumed guess rate.
This is a theoretical estimate assuming a fully random password and a brute-force attack. Real-world cracking often exploits patterns, dictionary words, or reused passwords, which can make a password far weaker than its raw entropy suggests. Use a password manager to generate genuinely random passwords for real security.