Hash Collision Calculator
Birthday-problem collision odds.
Enter your values
ⓘ Live results
Results
- Collision probability
- 6.776e-7
- Hash space
- 2^64
- 50% collision at
- 5.057e+9 items
- Risk
- Negligible
Quick answer
Collision risk follows the birthday problem, rising far faster than intuition suggests.
P ≈ 1 − e^(−n² ÷ 2N)
How to use the Hash Collision Calculator
- 1Enter the hash or ID length in bits.
- 2Enter how many values you will generate.
- 3Compare against the 50% threshold.
Worked example
Using the values the calculator loads with:
Inputs
- Hash bits64
- Items hashed5000000
Results
- Collision probability6.776e-7
- Hash space2^64
- 50% collision at5.057e+9 items
- RiskNegligible
Frequently asked questions
Is 64 bits enough?
For under a million items usually yes; for billions use 128.
Are UUIDv4 collisions realistic?
No — 122 random bits make collisions effectively impossible.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.