How to Calculate Percentages: The Three That Cover Almost Everything
Almost every percentage question in real life is one of three calculations. Learn all three and you will rarely be stuck again.
Percentages turn up everywhere — discounts, tips, exam scores, interest rates, price rises. Yet many people freeze when a percentage question appears, not because it is hard, but because it was never broken into clear pieces. Here is the simple truth: nearly every everyday percentage problem is one of just three calculations. This guide covers all three.
First, What "Per Cent" Means
The word percent comes from "per hundred." A percentage is simply a fraction with 100 on the bottom. So 25% means 25 out of 100, which is the same as the fraction 25/100, or the decimal 0.25. Converting between the two is the foundation of everything below: to turn a percentage into a decimal, divide by 100; to go the other way, multiply by 100.
Calculation 1: Finding a Percentage of a Number
This is the "what is 20% of 80?" type of question — the one you need for discounts, tips, and taxes.
To find 20% of 80: convert 20% to 0.20, then multiply — 0.20 × 80 = 16. A jacket priced at 80 with 20% off is reduced by 16, so it costs 64.
Calculation 2: Finding What Percentage One Number Is of Another
This answers "32 is what percent of 50?" — useful for test scores, progress toward a goal, or what share of a budget an expense represents.
If you scored 32 out of 50 on a test: 32 ÷ 50 = 0.64, then × 100 = 64%. The trick is identifying which number is the "part" and which is the "whole" — the whole is always the total you are comparing against.
Run any of these calculations instantly.
Try the Plantrino Percentage CalculatorCalculation 3: Percentage Increase or Decrease
This is the "the price went from 50 to 60 — what percentage rise is that?" question. It is everywhere in news and money: pay rises, price changes, growth figures.
For a price moving from 50 to 60: the change is 60 − 50 = 10. Then 10 ÷ 50 = 0.20, and × 100 = a 20% increase. If the number had fallen, the result would be negative, indicating a decrease. The crucial rule: always divide by the old value, because you are measuring change relative to the starting point.
| Question type | Example | Formula |
|---|---|---|
| Percentage of a number | 20% of 80 | (% ÷ 100) × number |
| What percent is X of Y | 32 out of 50 | (part ÷ whole) × 100 |
| Percentage change | 50 rising to 60 | (change ÷ old) × 100 |
A Handy Shortcut
For calculation 1, remember that "X% of Y" always equals "Y% of X." That sounds like a curiosity, but it is genuinely useful. Working out 4% of 75 in your head is awkward; flipping it to 75% of 4 — which is just three-quarters of 4, or 3 — is effortless. The answer is the same either way.
Reversing a Percentage
One more situation worth knowing: undoing a percentage. If an item costs 90 after a 10% discount, you cannot simply add 10% back, because the 10% was taken off the original, larger price. Instead, divide by the remaining fraction: 90 ÷ 0.90 = 100. The original price was 100. The same logic applies to removing a tax that was added to a total.
Frequently Asked Questions
Why divide by the old value for percentage change?
Because a change is always measured relative to where it started. The old value is your reference point, so it goes on the bottom.
Can a percentage be more than 100?
Yes. If something triples, that is a 200% increase. Percentages above 100 simply mean more than the whole of the original.
How do I add a percentage quickly?
To add 15%, multiply by 1.15. To take 15% off, multiply by 0.85. Turning the percentage into a single multiplier saves a step.
Percentages stop being intimidating once you see that they collapse into three core questions: a percentage of a number, what share one number is of another, and how much something changed. Learn the three formulas, watch for the percentage-point trap, and you will handle almost any percentage life throws at you.