Interactive probability calculator showing how all probabilities sum to 1
Every chance you skip is a certainty someone else claims
Probabilities aren't just numbers — they're pieces of a puzzle that must always fit together perfectly. When you calculate the chance of one event happening, you're simultaneously defining what's left over for everything else. Drag the slider to see how changing one probability forces its complement to adjust, keeping the total locked at exactly 1.
PROBABILITY BALANCE
P(A) = 0.50
Drag to change P(event A) — watch its complement adjust
P(A) + P(NOT A) = 1.00
0.00.250.50.751.0
EVENT A
0.50
NOT A
0.50
The probability formula P(event) = favorable outcomes / total outcomes is the foundation of all chance calculations. If you're rolling a standard six-sided die, the probability of rolling a 4 is 1/6 ≈ 0.167, because there's one favorable outcome out of six total possibilities. This same ratio applies whether you're drawing cards, tossing coins, or analyzing weather forecasts. Every probability value falls between 0 (impossible) and 1 (certain), with 0.5 representing equally likely outcomes. Understanding this ratio transforms vague notions of "chance" into precise, calculable values you can use for decision-making.
Know This
All probabilities in a complete set of outcomes must sum to exactly 1, which means P(not A) = 1 - P(A) — every chance you measure automatically defines what's left over.