Count the number of possible outcomes and the number of possible outcomes you consider successful. For example, if you want to find the number of rainy days in a month, count the number days in the month and the number of rainy days in the month.
Divide the number of successful outcomes by the number of total outcomes to find the ratio. In this example, if you had 18 rainy days out of a 30-day month, divide 18 by 30 to get 0.6.
Multiply the ratio by 100 to find the percentage. In this example, multiply 0.6 by 100 to get 60 percent.