Poisson Probability Calculator
Calculate poisson probability with explicit inputs and a worked example.
Formula
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
- Independent events at a constant average rate; very small probabilities can underflow to zero.
Example Calculation
- Expected events λ: 3
- Observed events k: 2
With mean 3, the probability of exactly 2 events is about 0.224042.
Method references
Frequently asked questions
How do I use this calculator?
Enter expected events λ, observed events k. The result updates automatically. Use Reset to restore the example values.
What method does it use?
P(X=k) = exp(−λ)λᵏ/k!. Independent events at a constant average rate; very small probabilities can underflow to zero.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.