Birthday Paradox Calculator
Calculate birthday paradox 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 birthdays, uniform dates, no leap-day adjustment.
Example Calculation
- People: 23
- Possible equally likely birthdays: 365
With 23 people and 365 equally likely dates, probability is about 50.7297%.
Frequently asked questions
How do I use this calculator?
Enter people, possible equally likely birthdays. The result updates automatically. Use Reset to restore the example values.
What method does it use?
P(shared) = 1 − product from i=0 to n−1 of (1−i/d). Independent birthdays, uniform dates, no leap-day adjustment.
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.