Password Entropy Calculator
Entropy Measures How Many Guesses a Password Actually Costs
"Strong password" is a vague instruction until it's converted into a number. Entropy, measured in bits, expresses exactly how large the search space is that an attacker has to work through — each additional bit doubles the number of possible passwords, which is why a small increase in length or character variety produces an outsized increase in resistance to guessing.
The Formula
Charset size is the sum of every character class enabled: 26 for lowercase, 26 for uppercase, 10 for digits, and 32 for symbols. A 12-character password using all four classes draws from a charset of 94 characters.
Where This Matters
- Password policy design — setting minimum length and complexity rules that guarantee a target entropy floor.
- Comparing password choices — deciding whether a longer password with fewer character types beats a shorter one with more variety.
- Security audits — quantifying how much margin a password policy actually provides against offline guessing.
Entropy by Length and Character Set
| Length | Character classes | Charset size | Entropy |
|---|---|---|---|
| 8 | Upper + lower + digits | 62 | 47.63 bits |
| 12 | Upper + lower + digits | 62 | 71.45 bits |
| 12 | Upper + lower + digits + symbols | 94 | 78.66 bits |
| 16 | Upper + lower + digits + symbols | 94 | 104.87 bits |
Values computed directly from Entropy = Length × log2(Charset Size).
How to Use This Calculator
- Enter the Password Length in characters.
- Check the character classes included: lowercase, uppercase, digits, and/or symbols.
- Select Calculate to get the entropy in bits.
Related Calculations
Pair entropy with a plain-language rating using the Password Strength Calculator, or see how entropy translates into real crack time with the Brute Force Attack Time Calculator.