# Probability of At Least One Calculator

URL: https://speedycalculator.com/calculators/statistics/probability-at-least-one/
Last updated: 2026-10-25

> This calculator finds the probability that an event happens at least once in n independent tries, given its probability per try. It also shows how many tries give a 50% and 95% chance.

## Inputs

- Probability per try (%)
- Number of tries

## Results

- At least once
- Tries for 50% chance
- Tries for 95% chance

## Formula

```
P(at least one) = 1 − (1 − p)ⁿ
```

## Explanation

Rare events become likely with enough attempts. A 1% chance per try gives about a 63% chance over 100 tries.

### How to use the Probability of At Least One Calculator

- **Probability per try**
- **Number of tries**
- Want to work backwards? Type a value into **At least once** and **Number of tries** is calculated for you.

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Find the chance of the event never happening and subtract it from one.

### Inputs explained

FieldWhat to enterProbability per tryIn %Number of triesA number

### What each result means

- **At least once** = (1 - (1 - [Probability per try]÷100)^[Number of tries])×100
- **Tries for 50% chance** = ceil(ln(0.5)÷ln(1 - [Probability per try]÷100))
- **Tries for 95% chance** = ceil(ln(0.05)÷ln(1 - [Probability per try]÷100))

### Example values

How the main result changes when **Number of tries** changes and everything else stays at the example values above:

Number of triesAt least once (%)5039.4997552.94110063.39712571.52915077.85520086.602

## Worked example

Inputs — Probability per try: 1 %; Number of tries: 100.

Results — **At least once = 63.397 %**; Tries for 50% chance = 69; Tries for 95% chance = 299.

## FAQ

**Is this the drop-chance formula used in games?**
Yes, assuming each try is independent.

**Why is it not simply n × p?**
That overcounts overlapping successes and can exceed 100%.

**What formula does the Probability of At Least One Calculator use?**
P(at least one) = 1 − (1 − p)ⁿ.

**Can I work backwards from the result?**
Yes. Type the result you want into At least once and the calculator finds the matching number of tries.