How to Calculate a Percentage (The 3 Methods You Actually Need)
Percentages come up constantly — discounts, tips, test scores, interest, statistics — yet plenty of capable people freeze when asked to work one out. The truth is there are really only three percentage calculations you'll ever need, and once you've seen each with a worked example, they stop being scary. Let's walk through all three.
The one idea behind every percentage
"Per cent" literally means "per hundred". So 25% is just 25 out of 100, or the fraction 25/100 = 0.25. Every percentage calculation comes back to that single idea: a percentage is a fraction of something, scaled to 100. Hold onto that and the rest is mechanical.
Method 1: Find X% of a number
This is the most common one — "what is 20% of 80?" Convert the percentage to a decimal (divide by 100) and multiply:
20% of 80 = 0.20 × 80 = 16
So a 20% tip on an $80 meal is $16. Any "X% of Y" question works exactly this way.
Method 2: What percentage is X of Y?
"15 out of 20 — what percentage is that?" Divide the part by the whole, then multiply by 100:
(15 ÷ 20) × 100 = 0.75 × 100 = 75%
This is how you turn a test score, a conversion rate, or any "X of Y" into a percentage.
Method 3: Percentage increase or decrease
"A price went from $50 to $65 — what's the percentage increase?" Find the difference, divide by the original value, then multiply by 100:
(65 − 50) ÷ 50 × 100 = 15 ÷ 50 × 100 = 30% increase
The key mistake to avoid: always divide by the starting number, not the new one. A decrease works identically — you'll just get a smaller new value.
Skip the mental maths
Once you understand the method, there's no shame in letting a tool do the arithmetic — that's what they're for. Our free percentage calculator handles all three cases instantly, so you can check your working or just get the answer. If you're working out money saved in a sale, the discount calculator does the percentage-off maths for you.
Bottom line
Every percentage problem is one of three things: a part of a whole, a ratio expressed per hundred, or a change from a starting value. Convert to a decimal, multiply or divide, and you're done. For a deeper grounding, Maths is Fun has clear visual explanations — but for everyday use, these three methods cover it.
