How Percentage Calculations Work
Percentages appear everywhere. Learn the core formulas and common mistakes.
Published 2026-03-05
In this guide
Short answer
A percentage is a fraction of 100. The core formulas are: X% of Y = (X/100)×Y, A as a percent of B = (A/B)×100, and percentage change = (new−old)/|old|×100.
Common formulas
Increase A by B%: A×(1+B/100). Decrease A by B%: A×(1−B/100). Percentage difference uses the average as the base.
Common mistakes
Adding a percentage instead of multiplying, confusing the base value, and mishandling percentage difference are frequent errors. A percentage calculator avoids these.