Brute force time calculator
Choose a character set and length — or a key size such as 128 bits — and the number of guesses an attacker can make per second. The calculator shows how many combinations there are and how long trying them takes, on average and in the worst case, with optional key stretching.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose Password and pick the characters used and the length, or choose Key (bits) for a cryptographic key.
- Enter the attacker’s guesses per second. Numbers like 1e10 are accepted.
- If passwords are stored with PBKDF2 or a similar slow hash, enter its iterations per guess.
- Read the average and worst-case times; the keyspace and entropy are below.
- Combinations N = alphabet size ^ length, or N = 2^bits
- Entropy H = log₂ N = length × log₂(alphabet size)
- Effective rate r = guesses per second ÷ iterations
- Worst case N ÷ r, average N ÷ (2r)
- A year is counted as 365.25 days (31,557,600 seconds).
Worked example
A random 8-character password from all 95 printable ASCII characters, against 10¹⁰ guesses per second:
- combinations: 95⁸ = 6.6342E15 (6,634,204,312,890,625), entropy 8 × log₂ 95 = 52.56 bits;
- worst case: 6.6342 × 10¹⁵ ÷ 10¹⁰ = 663,420 seconds = 0.021 years;
- average: half of that, 331,710.2156 s, which is 3.8392 days.
The same password stored with 600,000 PBKDF2 iterations would take 600,000 times longer: about 6,300 years on average.
Tips
- Length beats complexity. Each extra printable character multiplies the time by 95; switching from 62 to 95 characters at the same length adds far less.
- 128-bit keys are out of reach. Even at 10¹² guesses per second, 2¹²⁸ takes about 5.4 × 10¹⁸ years on average.
- Check an actual password with the password strength checker or create a random one with the password generator.
Frequently asked questions
How is the brute-force time calculated?
The number of combinations is the alphabet size to the power of the length (95⁸ for 8 printable characters), or 2ⁿ for an n-bit key. Trying them all takes combinations ÷ guesses per second. On average the right one turns up halfway, so the average time is half of that.
What guess rate should I enter?
It depends entirely on the attacker and the hash. A fast unsalted hash on GPUs can be tried billions of times per second, a slow password hash only thousands, and an online login form a few times per second before rate limits. Try several rates to see the range.
What does key stretching change?
PBKDF2, bcrypt, scrypt and Argon2 make each guess cost many hash computations. If one guess needs 600,000 PBKDF2 iterations, an attacker who can compute 10⁹ hashes per second makes only about 1,667 guesses per second, so every time here grows 600,000 times. For bcrypt, cost 12 means 2¹² = 4096 rounds.
Does this apply to real passwords?
Only to random ones. People pick words, names and patterns that attackers try first with dictionaries and rules, so a human-chosen 8-character password usually falls much faster than this calculator shows. Use the result for generated passwords and keys.