Probability Calculator
Last updated: July 2026
Calculate a single-event probability and the chance of at least one success over repeated independent trials.
Single-trial probability
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- Probability of failure: —
- At least one success: —
- Expected successes: —
On this page
Quick summary: Calculate a single-event probability and the chance of at least one success over repeated independent trials. Enter the requested values, calculate, review the result, and compare more than one realistic scenario before making a decision.
Read the complete guide
What this calculator estimates
Calculate a single-event probability and the chance of at least one success over repeated independent trials. It is designed for fast planning and comparison in the United States. The output is an estimate, not a binding quote, approval, tax determination, insurance offer, legal conclusion, medical recommendation or professional advice.
How to use it
Enter the requested values using a single consistent time period and currency. Select Calculate, review the headline result and supporting figures, then change one assumption at a time to compare another scenario.
How the calculation works
Single-event probability is successful outcomes divided by total outcomes. At least one success is one minus the probability of zero successes.
Important limitations
Repeated-trial results assume independent trials with the same probability each time.
Frequently asked questions
Is the result exact?
No. It is a planning estimate based on the assumptions entered.
Can I compare different scenarios?
Yes. Change one input at a time so you can see which assumption changes the result.
Does the calculator store my inputs?
The calculation runs in the browser and does not require an account. Avoid entering unnecessary confidential information.