What is probability?

Probability measures how likely an event is. It runs from 0 (impossible) to 1 (certain), and can be written as a fraction, decimal or percentage. You find it by comparing the number of ways an event can happen with the total number of possible outcomes.

Examples

• The probability of a tossed coin landing heads is 1/2.
• The probability of rolling a 3 on a normal die is 1/6.
• The probability of rolling a number from 1 to 6 on a die is 1 (certain).

Quick facts

• A probability of 0 means impossible; a probability of 1 means certain.
• The probabilities of all possible outcomes add up to 1.