Probability is a type of ratio where we compare how many times an outcome can When we determine the probability of two independent events we multiply the . and Box-and-Whiskers Plot Pre-Algebra Probability and statistic: Calculating. Usually, this is expressed as a ratio (like 1: 3 or 1/3). Calculating odds is central to the strategy of many games of chance, like roulette, horse racing and poker. How to Calculate Probability. Probability is the measure of how likely an event is to occur out of the number of possible outcomes. This wikiHow will show you.