Math

Percentage Calculator

Six of the most common percentage questions, each on its own tab. Type the numbers and the answer updates as you go — with the formula and every step underneath.

Everything runs in your browser — nothing you type is sent anywhere.

How each mode works

The six tabs cover the questions people actually ask, and they are not interchangeable — picking the wrong one is the single most common source of a wrong percentage.

Tab	Answers the question	Formula
% of a number	What is 15% of 200?	`part = pct ÷ 100 × total`
X is what % of Y	25 out of 200 is what percent?	`pct = part ÷ total × 100`
X is Y% of what	30 is 15% of which number?	`total = part ÷ (pct ÷ 100)`
Increase by %	Add 15% to 200	`result = total × (1 + pct ÷ 100)`
Decrease by %	Take 15% off 200	`result = total × (1 − pct ÷ 100)`
% difference	How far apart are 40 and 50?	`diff = \|a − b\| ÷ ((a + b) ÷ 2) × 100`

How to calculate a percentage by hand

A percentage is just a fraction with 100 on the bottom. "15%" means 15 per hundred, or 0.15. Once you see it that way, every one of the calculations above is a single multiplication or division.

Finding a percentage of a number

Move the decimal point two places left to turn the percentage into a decimal: 15% → 0.15.
Multiply: 0.15 × 200 = 30.

Turning two numbers into a percentage

Divide the part by the whole: 25 ÷ 200 = 0.125.
Multiply by 100: 0.125 × 100 = 12.5%.

The mental shortcut worth knowing: 10% is the number with the decimal point moved one place left, and every other percentage can be built from it. 15% of 80 is 8 (10%) plus 4 (half of 10%) = 12.

The one formula behind all of itpart ÷ whole × 100 = percentage

Worked examples

A $64 jacket is 25% off. How much comes off the price?

Use % of a number: 25 ÷ 100 × 64 = $16 off, so you pay $48. The discount calculator does the whole thing in one step.

You got 43 out of 50 on a test. What percentage is that?

Use X is what % of Y: 43 ÷ 50 × 100 = 86%.

A tip of $12.60 was 18% of the bill. What was the bill?

Use X is Y% of what: 12.60 ÷ 0.18 = $70.

Two quotes come in at 40 and 50 hours. How different are they?

Use % difference: |40 − 50| ÷ 45 × 100 = 22.22%. Note this is not the same as saying 50 is 25% more than 40 — see the note below.

Percentage difference is not percentage change

This trips up a lot of people, including in published statistics. Percentage change has a direction: it measures how far a number moved from a specific starting point, so going from 40 to 50 is a 25% increase, while going from 50 to 40 is a 20% decrease. Same pair of numbers, two different answers.

Percentage difference has no direction. It compares two values against their average, so 40 and 50 are 22.22% apart no matter which you name first. Use it when neither number is "the original" — comparing two suppliers, two measurements, two machines.

If one of your numbers is a starting point and the other is where you ended up, you want the percentage increase and decrease calculator instead.

Where people use this

Shopping: sale prices, coupon stacking and working out whether "30% off" beats "$20 off".
School and grading: converting scores out of any total into a percentage.
Business: margins, commission, month-on-month growth and budget splits.
Health and cooking: scaling a recipe up by 50%, or working out a percentage of a daily target.
Tax and tips: adding a fixed percentage on top of a total.

Frequently asked questions

How do I calculate a percentage of a number quickly in my head?

Find 10% first by moving the decimal point one place to the left, then build from there. For 35% of 60: 10% is 6, so 30% is 18, and 5% is 3 — that gives 21. It works for any percentage and is usually faster than reaching for a calculator.

Why do 25% off then 10% off not equal 35% off?

Because the second discount is taken from the already-reduced price, not from the original. $100 becomes $75, then 10% off $75 is $67.50 — an effective 32.5% off. The discount calculator has a stacked-discount mode that shows this properly.

Can a percentage be more than 100?

Yes. Anything above 100% simply means more than the whole. If sales went from 40 to 100, the new figure is 250% of the old one — an increase of 150%. This calculator handles values above 100% in every mode.

What about negative numbers?

They work, and the maths stays consistent: 10% of −250 is −25. The only calculation that refuses a value is percentage difference when the two numbers average out to exactly zero, because there is nothing to compare against.

Why does the calculator sometimes show a long decimal?

Because many percentages genuinely do not divide evenly — one third of a total is 33.333…%. Results are shown to twelve significant figures so the answer is honest rather than silently rounded, and you can round it yourself for whatever you are doing.

Is this the same as a percentage change calculator?

Partly. The "increase by" and "decrease by" tabs apply a known percentage to a number. If instead you have an old value and a new value and want to know what percentage moved between them, use the percentage increase calculator.