How to Calculate a Percentage: Formulas & Examples

A percentage is a number out of 100. "45%" means 45 out of every 100. Almost every everyday percentage question, such as marks, discounts, GST, interest and price changes, uses one of the four formulas below.

1. Find a percentage of a number

Result = (percentage ÷ 100) × number

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

Quick mental method: 10% of 2,400 is 240 (move the decimal one place). 5% is half of that, 120. So 15% = 240 + 120 = 360.

2. Find what percentage one number is of another

Percentage = (part ÷ whole) × 100

Example: you scored 45 marks out of 60. 45 ÷ 60 × 100 = 75%.

3. Find a percentage increase or decrease

Change % = (new value − old value) ÷ old value × 100

Example: a price goes from ₹1,000 to ₹1,200. (1,200 − 1,000) ÷ 1,000 × 100 = 20% increase. If the price falls from ₹1,200 to ₹1,000, the change is (1,000 − 1,200) ÷ 1,200 × 100 = about 16.7% decrease. The increase and decrease are different because the starting value is different.

4. Find the original value (reverse percentage)

Original = final value ÷ (1 ± percentage ÷ 100)

Example: after a 20% discount a shirt costs ₹800. The price is 80% of the original, so the original is 800 ÷ 0.80 = ₹1,000.

Adding and removing GST

GST is a percentage added on top of a price. The same formulas apply:

• Add 18% GST to ₹1,000: 1,000 × 1.18 = ₹1,180.
• Remove 18% GST from ₹1,180: 1,180 ÷ 1.18 = ₹1,000.

A common mistake is to subtract 18% of 1,180. That gives ₹967.60, which is wrong, because the 18% was calculated on ₹1,000, not on ₹1,180. Always divide by 1.18 to remove it.

Common mistakes

Two percentage changes do not cancel out

Increase ₹1,000 by 20% and you get ₹1,200. Decrease that by 20% and you get ₹960, not ₹1,000. The second 20% is taken from the larger number, so the result is 4% lower than where you started.

Percent and percentage points are different

If an interest rate moves from 8% to 10%, it has risen by 2 percentage points. In relative terms it has risen by (10 − 8) ÷ 8 × 100 = 25%. News and offers use both phrases, so check which one is meant.

Use the right "whole"

The whole is the number you are comparing against. "Marks as a percentage of the total" and "increase compared with last year" use different wholes, so write down which number goes under the division sign before you calculate.

Quick reference

You want to know	Calculate
10% of a number	Move the decimal one place left
50% of a number	Divide by 2
25% of a number	Divide by 4
Price after a 20% discount	Multiply by 0.80
Price after adding 18% GST	Multiply by 1.18

For quick checks, use the Percentage Calculator. For shopping offers, the Discount Calculator shows the final price and the amount saved, and the GST Calculator adds or removes tax from a price.

Frequently asked questions

How do I calculate 20% of a number quickly?

Find 10% by moving the decimal one place left, then double it. For 20% of 350: 10% is 35, so 20% is 70.

How do I turn a fraction into a percentage?

Divide the top number by the bottom number and multiply by 100. For 3/8: 3 ÷ 8 = 0.375, so 37.5%.

Why is a 50% rise followed by a 50% fall not back to the start?

₹100 rises to ₹150. A 50% fall takes ₹75 off, leaving ₹75. The fall is calculated on the larger number, so it removes more than the rise added.