The Complete Guide to Percentages: Formulas, Calculations & Real-World Uses

Three formulas handle 90% of percentage problems: (1) What is X% of Y? → Answer = (X/100) × Y. (2) X is what percent of Y? → Answer = (X/Y) × 100. (3) X is Y% of what? → Answer = X / (Y/100). A fourth formula handles percentage change: ((New − Old) / Old) × 100.

You cannot simply add percentages. A 10% increase followed by another 10% increase is not 20% — it's 21% (1.10 × 1.10 = 1.21). This compounding effect is why credit card rates and investment returns must be analyzed carefully. Always convert to decimal multipliers when combining percentage changes.