Probability Calculator

How it works

1. 1Enter the number of favorable outcomes — the specific results you’re interested in — and the total number of equally likely outcomes.
2. 2P(A) = favorable ÷ total gives the probability as a decimal between 0 and 1; multiplied by 100 it becomes a percentage.
3. 3The complement P(not A) = 1 − P(A) is the probability the event does NOT occur; P(A) + P(not A) always equals 1.

Use cases

• Rolling dice: finding the probability of rolling a 3 on a standard die (1 favorable, 6 total = 0.1667 or 16.67%).
• Card games: calculating the chance of drawing an ace from a 52-card deck (4 favorable, 52 total ≈ 7.69%).
• Quality control: estimating the defect rate when 3 defective items are found in a batch of 200.

Frequently asked questions

What does a probability of 0 or 1 mean?

A probability of 0 means the event is impossible — there are no favorable outcomes. A probability of 1 means the event is certain — every outcome is favorable. All real-world probabilities fall strictly between these two values.

Why must the favorable outcomes be ≤ total outcomes?

Probability is defined on [0, 1]. If favorable outcomes exceeded the total, P(A) > 1, which has no meaningful interpretation. The calculator returns — for any input where favorable > total or total ≤ 0.

How is the complement useful?

Sometimes it’s easier to calculate the probability of an event NOT happening and subtract from 1. For example, the probability of rolling at least one six in four rolls is 1 − (5/6)⁴ ≈ 51.8%, simpler than summing each favorable case directly.