How to Calculate Percentage: 3 Easy Methods
4 min read · Last reviewed July 1, 2026
Percentages come up constantly — discounts, tips, tax, exam marks, interest, statistics. The good news is that almost every percentage question is one of just three types, and once you know them, you can do most of the maths in your head.
This guide walks through all three, with worked examples and a shortcut for estimating percentages quickly.
1. What is X% of Y?
This is the most common one — used for discounts, tips, and tax. Multiply the number by the percentage divided by 100. So 20% of 150 is 0.20 × 150 = 30.
Shortcut: 10% of any number is just that number with the decimal point moved one place left (10% of 150 = 15). Once you have 10%, you can build others — 20% is double it, 5% is half of it, 15% is 10% + 5%.
2. X is what percent of Y?
Use this for scores, shares, and proportions — like turning exam marks into a percentage. Divide the part by the whole, then multiply by 100. So 30 out of 150 is (30 ÷ 150) × 100 = 20%.
This is exactly how marks-to-percentage works: marks obtained ÷ total marks × 100.
3. Percentage increase or decrease
Use this to compare an old value with a new one — a price change, a pay rise, a population shift. Subtract the old value from the new, divide by the old value, and multiply by 100.
From 150 to 180 is (180 − 150) ÷ 150 × 100 = a 20% increase. From 150 to 120 is a 20% decrease. Note the base is always the original (old) value.
Frequently asked questions
▶What is the easiest way to calculate a percentage?
Find 10% first by moving the decimal one place left, then scale up or down. For 15% of 80: 10% is 8, 5% is 4, so 15% is 12. For exact answers, use the percentage calculator.
▶How do I calculate percentage increase?
Subtract the original value from the new value, divide by the original value, and multiply by 100. A rise from 200 to 250 is (250 − 200) ÷ 200 × 100 = 25%.
Try the calculators
Work out a percentage of a number, what percent one number is of another, and percentage increase or decrease.
Open calculator →Convert your exam marks into a percentage — enter marks obtained and total marks to get your result and grade.
Open calculator →Work out the sale price and how much you save from a percentage discount.
Open calculator →