Password Entropy Calculator
Estimate how strong a password is — in bits of entropy and approximate brute-force crack time — from its length and character set. This free online developer calculator runs entirely in your browser — no signup, no data sent anywhere.
Inputs
Results
How It Works (Formula & Method)
Entropy in bits is length × log₂(pool size). A 12-character password drawn from the 94 printable ASCII characters has 12 × log₂(94) ≈ 78.7 bits. The number of possible passwords is 2 raised to that power. Crack time assumes an attacker testing 10 billion guesses per second — a realistic rate for offline attacks on fast hardware.
Worked Example
Below is a worked example using the calculator's default values. The same numbers are pre-filled in the form above so you can press Calculate and see the result without typing anything.
Inputs used:
- Password Length (characters): 12
- Character Set: 94
With these inputs, the calculator computes the metrics shown in the Results panel. Change any value and press Calculate again to see how the result responds — the live widget and the chart both update instantly.
About the Password Entropy Calculator
Password entropy measures unpredictability in bits — how many guesses, on average, an attacker would need to find the password by brute force. It depends on two things: how long the password is and how large the pool of possible characters is. This calculator estimates entropy and roughly how long an offline cracking attempt would take.
Tips & Considerations
- Length beats complexity. Adding characters raises entropy faster than adding symbol types to a short password.
- This estimate assumes a truly random password. Real passwords based on words, names, or patterns have far less effective entropy.
- Aim for 75+ bits for important accounts; a passphrase of several random words is an easy way to get there.
Frequently Asked Questions
What is a good number of entropy bits?
Below 40 bits is weak. Around 60 bits is reasonable for low-value accounts. 75 bits or more is strong and recommended for anything important.
Why does the calculator assume a random password?
The entropy formula only holds if every character is chosen independently and uniformly at random. Passwords built from real words or predictable patterns are far weaker than their length suggests.
Does a longer password really help that much?
Yes. Entropy grows linearly with length, so each extra character multiplies the number of possibilities by the full pool size — a dramatic gain.
What does the Password Entropy Calculator compute?
The Password Entropy Calculator takes 2 input values and returns 3 results. Estimate password strength in bits of entropy and approximate brute-force crack time from length and character set.