Hash Collision Calculator
Birthday-problem collision odds.
Enter your values
Enter the hash bits you are working with.
Enter the items hashed you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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)
At a glance
| What it does | Birthday-problem collision odds. |
|---|---|
| Category | Technology |
| Inputs needed | Hash bits, Items hashed |
| Main output | Collision probability |
| Formula | P ≈ 1 − e^(−n² ÷ 2N) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Hash bits
- Enter the hash bits you are working with.
- Items hashed
- Enter the items hashed you are working with.
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.
What do I need to enter into the Hash Collision Calculator?
Just 2 values: hash bits, items hashed. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Hash Collision Calculator work out the answer?
Collision risk follows the birthday problem, rising far faster than intuition suggests. It applies the formula P ≈ 1 − e^(−n² ÷ 2N) and shows the working so you can check each step by hand.
Is the Hash Collision Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.