How to Calculate Percentage: Three Formulas You Actually Need

Most things people call "a percentage problem" are actually one of three different calculations. Once you know which one applies, the arithmetic is simple — the confusion usually comes from not spotting which formula the question is actually asking for.

1. Finding X% of a number

The most common case — a discount, a tax, a commission:

Result = (Percentage / 100) × Number

Example: 15% of 240 = (15 / 100) × 240 = 36.

2. What percent one number is of another

Used when you have two numbers and want the relationship between them expressed as a percentage:

Percentage = (Part / Whole) × 100

Example: 45 out of 180 → (45 / 180) × 100 = 25%.

3. Percentage increase or decrease

Used when comparing an old value to a new one — a price change, a growth rate:

% Change = [(New value − Old value) / Old value] × 100

Example: a price moves from ₹80 to ₹100 → [(100 − 80) / 80] × 100 = 25% increase. A negative result means a decrease — going from ₹100 to ₹80 gives [(80 − 100) / 100] × 100 = −20%, not the same 25% in reverse.

The mistake in that last example

A 25% increase from 80 to 100 does not undo with a 25% decrease — because the "whole" the percentage is calculated against changes (80 the first time, 100 the second). This asymmetry is the single most common source of percentage-related errors in pricing and reporting.

Which formula for which question

• "What's 20% off ₹500?" → formula 1
• "I scored 42 out of 60 — what percentage is that?" → formula 2
• "Sales went from 1,200 to 1,500 units — what's the growth rate?" → formula 3

Our Percentage Calculator covers all three cases directly, so there's no risk of applying the wrong formula to the wrong question. For pricing specifically, the Discount Calculator handles sale price and amount saved in one step.