Three Percentage Formulas That Cover Every Situation

Every percentage problem reduces to one of three questions. What is X percent of Y. What percent is X of Y. Y is X percent of what number. Three formulas. That covers every percentage calculation you will ever encounter. The three formulas What is X% of Y? Result = (X / 100) * Y What is 15% of 200? → (15/100) * 200 = 30 What percent is X of Y? Percent = (X / Y) * 100 What percent is 30 of 200? → (30/200) * 100 = 15% Y is X% of what? Total = Y / (X / 100) 30 is 15% of what? → 30 / (15/100) = 200 function percentOf ( percent , total ) { return ( percent / 100 ) * total ; } function whatPercent ( part , total ) { return ( part / total ) * 100 ; } function percentOfWhat ( part , percent ) { return part / ( percent / 100 ); } Percentage change Change = ((New - Old) / Old) * 100 From 80 to 100: ((100 - 80) / 80) * 100 = 25% increase From 100 to 80: ((80 - 100) / 100) * 100 = 20% decrease Note the asymmetry: a 25% increase followed by a 20% decrease returns to the original value. They are not