Hash collision probability calculator
Estimate collision probability for uniformly random identifiers, or solve the item count for a chosen probability.
Uniform-independent birthday approximation only; not a cryptographic audit, attack-cost estimate, checksum guarantee or algorithm assessment.
Included
- Every visible input and selected unit
- Reconciled intermediate values and result breakdown
- Reproducible URL state
Not included
- Uniform-independent birthday approximation only; not a cryptographic audit, attack-cost estimate, checksum guarantee or algorithm assessment.
What this means
Estimate collision probability for uniformly random identifiers, or solve the item count for a chosen probability.
Uniform-independent birthday approximation only; not a cryptographic audit, attack-cost estimate, checksum guarantee or algorithm assessment.
Intermediate values are not rounded; only the localized display formats the result.
Formula & worked example
p ≈ 1 − exp(−n(n−1)/(2 × 2^bits))
One million uniformly distributed values in a 128-bit space
- Inputs
- n = 1,000,000 and space size = 2^128
- Calculation
- λ = n(n−1) ÷ (2 × 2^128); p = 1 − e^(−λ)
The approximate probability is about 1.47 × 10^−27 under the uniform independent model.
How this calculation works
The calculation follows the formulas, definitions and assumptions explained on this page. The references below support the method and any stated boundaries.
Official sources
- NIST — Hash Functions — Public definitions and technical semantics used by this calculation