How to Calculate a Percentage: The 3 Formulas You Actually Need
A percentage is just a part out of 100. That one idea sits behind every percentage question you'll ever face — a tip, a test score, a price cut, a raise. Once you can write a situation as part ÷ whole × 100, you can solve it. Below are the three formulas that cover almost everything, each with a worked example. And if you'd rather skip the arithmetic, you can drop the numbers straight into the percentage calculator and read the answer.
The one formula behind every percentage
Every percentage is a part compared to a whole, scaled to 100:
percentage = (part ÷ whole) × 100
Say you got 18 questions right out of 20 on a quiz. The part is 18, the whole is 20:
- 18 ÷ 20 = 0.9
- 0.9 × 100 = 90%
That's it. The other formulas below are the same idea, rearranged for the number you're missing.
1. How to find a percentage of a number
This is the most common one: you know the percentage and the whole, and you want the part. Turn the percent into a decimal (divide by 100), then multiply.
part = (percentage ÷ 100) × whole
What's 15% of 80?
- 15 ÷ 100 = 0.15
- 0.15 × 80 = 12
You use this every time you leave a tip or read a sale sign. A "25% off" tag on a $60 jacket means 0.25 × 60 = $15 off, so you pay $45. If you're standing in the store and don't feel like doing it in your head, the discount calculator will figure out the sale price for you.
2. How to calculate percentage change
Percentage change tells you how much something went up or down, relative to where it started. This is the one people get wrong most often, because the "whole" is always the original number.
percentage change = ((new − old) ÷ old) × 100
Your rent went from $1,200 to $1,320:
- 1,320 − 1,200 = 120
- 120 ÷ 1,200 = 0.1
- 0.1 × 100 = a 10% increase
If the new number is smaller, you'll get a negative result — that's a decrease. For a stock, a salary, or a price you're tracking over time, the percent change calculator handles the increase-or-decrease sign automatically.
3. What percent one number is of another
Sometimes you have two raw numbers and want to know the relationship — this is the base formula from the top, used directly.
You spent $340 of your $2,000 monthly budget on groceries. What percent is that?
- 340 ÷ 2,000 = 0.17
- 0.17 × 100 = 17%
The same move answers "what's my score?", "what's my conversion rate?", and "what share of the total is this?" — it's always the smaller number divided by the total.
Turning decimals and fractions into percentages
A percentage, a decimal, and a fraction are three ways of writing the same value:
| Fraction | Decimal | Percentage |
|---|---|---|
| 1/2 | 0.5 | 50% |
| 1/4 | 0.25 | 25% |
| 3/4 | 0.75 | 75% |
| 1/5 | 0.2 | 20% |
To go from a decimal to a percent, multiply by 100 (0.35 → 35%). To go the other way, divide by 100. If you're converting a lot of them, the decimal to percent converter does it instantly.
Two percentage mistakes worth avoiding
The reversal trap. A 50% drop followed by a 50% rise does not bring you back to where you started. Start at $100, drop 50% to $50, then rise 50% of $50 — that's only $25 back, landing at $75. Percentage change always measures against the current number, not the original.
Percentage points vs. percent. If a rate goes from 5% to 6%, that's a rise of one percentage point — but a 20% increase in relative terms (1 ÷ 5). News headlines mix these up constantly. When the difference matters, say which one you mean.
Percentages in the real world: margins and money
Most business math is percentages wearing a costume. Profit margin is just profit as a percent of revenue; markup is the increase from cost to price. If that's what brought you here, we go deeper in how to calculate profit margin and what counts as a good profit margin — both are the formulas above applied to a real P&L. For tips, discounts, and increases, see our percentage and everyday math calculators.
Frequently asked questions
How do I calculate a percentage without a calculator?
Use the anchor points you already know. 10% of any number is just that number with the decimal moved one place left (10% of 240 is 24). 1% moves it two places (1% of 240 is 2.4). From there you can build almost anything in your head: 20% is double the 10%, 5% is half of it, 15% is the 10% plus the 5%. So 15% of 240 = 24 + 12 = 36.
How do I find what percentage one number is of another?
Divide the part by the whole, then multiply by 100. If 7 of your 28 emails got a reply, that's 7 ÷ 28 = 0.25, or 25%. The smaller number goes on top.
How do I add a percentage to a number?
Find the percentage of the number, then add it back. To add 8% tax to a $50 order: 0.08 × 50 = $4, so the total is $54. A shortcut: multiply by 1.08 directly and you get $54 in one step. To take a percentage off, multiply by (1 − the rate) — 20% off is × 0.8.
What's the difference between percentage and percentage points?
A percentage point is the raw gap between two percentages; percent is the relative change. Going from 10% to 15% is a rise of 5 percentage points, but a 50% increase relative to the starting 10%. Use "points" when you're comparing two rates directly.
Is percentage change the same as percentage difference?
No. Percentage change has a direction — it measures a new value against an old one, so it can be an increase or a decrease. Percentage difference compares two values without treating either as the starting point, using their average as the base. If order matters (before and after), you want percentage change.
The fastest way to check your work
The formulas are worth knowing — they're the same three moves for the rest of your life. But when you just need the number and you need it right, type the values into the percentage calculator. It's free, runs in your browser, and shows the result the moment you type. Do the math by hand to understand it; use the tool to confirm it.
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