How to Calculate Percentages: Of, Change, Increase and Decrease

By Amey S · Published 8 November 2026

Percentages show up in discounts, marks, interest rates, taxes, salary changes and almost every report. The maths is not hard, but there are several different questions hiding behind the word “percent”, and using the wrong formula for the question is how most mistakes happen. This guide gives you the formula for each, with a worked example.

What a percentage is

“Percent” means “out of 100”. So 25% is 25 out of 100, which is 25/100 or 0.25. Converting between a percentage and a decimal is the first step in almost every calculation: divide by 100 to go from a percentage to a decimal, and multiply by 100 to go back.

1. What is X% of Y?

result = Y × X ÷ 100

Example: what is 15% of ₹2,400? 2,400 × 15 ÷ 100 = ₹360.

2. X is what percent of Y?

percent = X ÷ Y × 100

Example: you scored 432 out of 600. 432 ÷ 600 × 100 = 72%. Another: 25 is what percent of 125? 25 ÷ 125 × 100 = 20%.

3. Percentage change from A to B

change = (B − A) ÷ A × 100

Example: sales rose from 1,000 to 1,200. (1,200 − 1,000) ÷ 1,000 × 100 = 20% growth. If the answer is negative, it is a fall: from 1,200 to 900 is (900 − 1,200) ÷ 1,200 × 100 = −25%. Always divide by the starting value, A.

4. Increase Y by X%

new value = Y × (1 + X ÷ 100)

Example: a 10% raise on a salary of ₹50,000 is 50,000 × 1.10 = ₹55,000.

5. Decrease Y by X%

new value = Y × (1 − X ÷ 100)

Example: ₹2,400 with a 15% discount is 2,400 × 0.85 = ₹2,040. You save ₹360.

Try the percentage calculator →

Working backwards: the original price

If a price is ₹2,040 after a 15% discount, the original price is not 2,040 plus 15%. The 15% was taken off the original, so divide instead: 2,040 ÷ 0.85 = ₹2,400. The same idea applies to tax. A GST-inclusive price of ₹11,800 at 18% contains a net price of 11,800 ÷ 1.18 = ₹10,000.

Common mistakes

A rise of 20% followed by a fall of 20% does not get you back to where you started. 100 becomes 120, and 20% off 120 is 96. Each percentage applies to a different base.
Percentage points are not percentages. A rate that moves from 10% to 12% has gone up 2 percentage points, but it has risen by 20%.
Dividing by the wrong number. In “what percent”, divide by the whole; in “change”, divide by the starting value.
Adding percentages that apply to different bases. A 10% discount followed by another 10% discount is 19% off in total, not 20%, because the second 10% is taken from the already-reduced price.

Quick mental tricks

10% of a number: move the decimal point one place left. 10% of 2,400 is 240.
5% is half of 10%, and 1% is a tenth of 10%. So 15% is 10% plus 5%, which for 2,400 is 240 + 120 = 360.
50% is half, 25% is a quarter, and 75% is three quarters.
X% of Y equals Y% of X, so 4% of 75 is the same as 75% of 4, which is 3.

Calculators and examples here are for general information. For tax, loans or other financial decisions, check the figures with the relevant official source or a professional.

Tools mentioned in this guide

Percentage calculator: Work out a percentage of a number, what percent one number is of another, percentage change, and increases or decreases.
Tax / GST calculator: Add GST or sales tax to a price, or take it out of a tax-inclusive total, with an optional CGST/SGST split.
Loan / EMI calculator: Calculate your monthly loan EMI, total interest and total repayment, with an optional month-by-month schedule.
Word, character & sentence counter: Count words, characters, sentences and paragraphs, and see reading and speaking time.