Percentage Calculator: how to use it (and the formulas behind it)
A short guide to the three percentage questions people ask most — with the formulas, worked examples, and how to use the calculator.
Open the Percentage Calculator →What this tool does
The Percentage Calculator covers the three everyday percentage questions in one place, each updating live as you type. No button to press, and nothing is sent anywhere.
The three modes
1. What is X% of Y?
Divide the percentage by 100 and multiply by the number:
(X ÷ 100) × Y. For a 20% tip on a $150 bill: 0.20 × 150 = 30.
2. X is what percent of Y?
Divide the part by the whole and multiply by 100: (X ÷ Y) × 100. If 30 of 150
seats are taken: 30 ÷ 150 × 100 = 20%.
3. Percentage change from X to Y
Subtract, divide by the starting value, and multiply by 100:
((Y − X) ÷ X) × 100. From 150 to 180 is +20%; from 150 to 120 is
−20%.
Everyday uses
- Discounts and sale prices ("30% off $80").
- Tips, taxes, and service charges.
- Test scores and grades.
- Growth and decline — prices, audiences, or metrics over time.
Your numbers stay private
All three calculators run locally in your browser. The figures you enter are never uploaded, so it's fine to use with sensitive financial numbers.
FAQ
How do I find a percentage of a number?
Divide the percentage by 100 and multiply by the number — 20% of 150 is 30.
How do I calculate percentage change?
(new − old) ÷ old × 100. From 150 to 180 is +20%.
Is my data sent anywhere?
No — it all runs in your browser.
Ready to try it? Open the Percentage Calculator →