How to Calculate Percentages: A Practical Guide With Examples

Percentages come up every single day — a discount at checkout, a tip at dinner, a tax line on an invoice, a "your spending is up 12% this month" notification. The maths is simple, but it's easy to blank on which calculation you actually need. This guide breaks percentages into the four questions that cover almost every real situation, with worked examples for each.

First, what a percentage is

"Per cent" literally means "per hundred." So 25% is just 25 per 100, or 0.25. That single idea — divide by 100 to turn a percentage into a decimal — is the key that unlocks every percentage problem. Every calculation below is really just multiplying or dividing with that decimal.

The four percentage questions

Almost everything you'll ever need falls into one of these four. Our Percentage Calculator has a dedicated mode for each, and shows the formula it used so you can follow the logic.

1. What is X% of a number?

This is the classic. You want a percentage of some value.

Formula: (X ÷ 100) × number

Example — a discount: A £80 jacket is 25% off. How much do you save? (25 ÷ 100) × 80 = 0.25 × 80 = £20 off.

Example — a tip: A $54 bill, 18% tip. (18 ÷ 100) × 54 = $9.72.

2. A is what percent of B?

Here you have two numbers and want to know what proportion one is of the other.

Formula: (A ÷ B) × 100

Example — a test score: You got 42 out of 50. (42 ÷ 50) × 100 = 84%.

Example — a budget: You've spent £900 of a £1,200 budget. (900 ÷ 1200) × 100 = 75% used.

3. Percentage increase or decrease

You have an old value and a new value, and you want to know how much it changed in percentage terms.

Formula: ((new − old) ÷ old) × 100

Example — a price rise: Rent went from £1,000 to £1,150. ((1150 − 1000) ÷ 1000) × 100 = 15% increase.

Example — a sale: Traffic dropped from 8,000 to 6,000 visits. ((6000 − 8000) ÷ 8000) × 100 = −25%, i.e. a 25% decrease.

A common trap here: the change is always relative to the original (old) value, not the new one. Going from 100 to 150 is a 50% increase, but going from 150 back to 100 is a 33% decrease — different percentages, because the starting points differ.

4. Add or subtract a percentage

You want the final amount after applying a percentage — the total with tax added, or the price after a discount.

Formula (add): number + (X% × number) Formula (subtract): number − (X% × number)

Example — adding tax: A $200 item with 8% sales tax. 200 + (0.08 × 200) = 200 + 16 = $216.

Example — a discount total: That £80 jacket at 25% off costs: 80 − (0.25 × 80) = 80 − 20 = £60.

How to calculate percentages instantly

Rather than remembering which formula is which, you can just pick the right mode:

1. Open the Percentage Calculator.
2. Choose your question: % of a number, A is what % of B, % increase/decrease, or add/subtract %.
3. Type in your two numbers.
4. Read the answer — and the formula it used, so you can double-check the method.

It all runs in your browser, instantly, with nothing uploaded.

Handy mental shortcuts

For quick estimates without any tool:

• 10% of anything is just the number with the decimal point moved one place left. 10% of 240 = 24.
• 1% moves it two places. 1% of 240 = 2.4.
• Build from those. Need 20%? That's two lots of 10%. Need 15%? That's 10% plus half of it (5%). Need 5%? Half of 10%.
• Percentages are reversible. X% of Y always equals Y% of X. 8% of 50 is the same as 50% of 8 = 4 — sometimes the flipped version is easier to do in your head.

Where this comes up

• Shopping: discounts, sale prices, "buy one get 50% off."
• Dining and services: tips and service charges.
• Invoicing and tax: adding VAT/sales tax, calculating margins.
• Analytics: growth and decline between two periods.
• Study: turning scores into percentages and grades.

Related tools

Working with dates or counting down to something? The Age Calculator sits right alongside this one in our calculators collection.

Common questions

Why is a 50% increase not cancelled by a 50% decrease? Because each percentage is taken from a different base. 100 increased by 50% is 150; 150 decreased by 50% is 75, not 100. The decrease is calculated from the larger number, so it removes more. This trips up a lot of people reading sales and stock figures.

How do I work out the original price before a discount? If a price after a 20% discount is £80, the original wasn't £96. It's £80 ÷ 0.80 = £100, because £80 represents 80% of the original. Dividing by the remaining fraction reverses a percentage change.

What's the difference between percentage points and percent? If a rate goes from 5% to 7%, that's a rise of 2 percentage points — but a 40% increase in relative terms. Mixing these up is one of the most common ways statistics get misread.

Can I use it for tips and tax? Yes. Use the add/subtract mode: enter the bill, enter the percentage, and choose "add" for tax or a tip, or "subtract" for a discount.

Does it show the working? Every mode displays the formula with your numbers filled in, so you can check the result and learn the method rather than just trusting a black box.

The bottom line

Nearly every percentage problem is one of four questions: percent of a number, one number as a percent of another, percent change, or a total after adding/subtracting a percentage. Pick the right mode in our free Percentage Calculator and get an instant, clearly-explained answer every time.

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