Percentage Calculator

How to use the percentage calculator

The percentage calculator answers the six questions people ask most about percentages. Each one is written as a sentence, so you fill in the blanks and the answer appears as you type, with the formula next to it. It runs in your browser.

1. Find the sentence that matches your question, for example “What is 15 % of 240?” or “From 80 to 100 is a change of …”.
2. Type your numbers in the boxes. Decimals and negative numbers work.
3. Read the answer and the formula used, and copy it with one click if you need it elsewhere.

The six percentage calculations

What is X percent of a number?

“Percent” means “per hundred”, so 15% is 15 per 100, or 0.15. To find 15% of 240, multiply 240 by 0.15: the answer is 36. A quick mental shortcut is to find 10% first (move the decimal point one place left: 24) and build from there: 5% is half of that (12), so 15% is 24 + 12 = 36.

Tips and discounts use this calculation. A 20% tip on a bill of 48.50 is 48.50 × 0.2 = 9.70.

Percentage increase and decrease

A percentage change compares a new value with an old one. Subtract the old value from the new one, divide by the old value and multiply by 100. A positive answer is an increase, a negative one a decrease, and the calculator says which.

Worked example: a price goes from 80 to 100. (100 − 80) ÷ 80 × 100 = 25%, an increase of 25%. Going back from 100 to 80 is (80 − 100) ÷ 100 × 100 = −20%, a decrease of 20%. The percentages differ because the starting value differs, which is one of the most common surprises with percentages.

The calculator divides by the absolute value of the old number, so a change from −50 to −25 shows as an increase of 50%, which matches how most people read it. When the old value is 0, there’s no meaningful percentage change, and it tells you so.

Adding or subtracting a percentage

To increase a number by a percentage, multiply it by 1 plus the percentage as a decimal: 200 increased by 10% is 200 × 1.10 = 220. To decrease, use a minus sign: 200 decreased by 10% is 200 × 0.90 = 180. This is how sales tax, price rises and sale prices are worked out.

Be careful with “a 10% rise followed by a 10% fall”: 200 × 1.10 × 0.90 = 198, not 200, because the second percentage is taken from a bigger number.

Finding the original number

If you know a part and its percentage, you can find the whole: 36 is 15% of what? Divide 36 by 0.15 to get 240. This “reverse percentage” is how you find a price before tax or a discount. For example, if a jacket costs 68 after a 15% discount, it was 68 ÷ 0.85 = 80 before the discount, because 68 is 85% of the original.

Percentage difference vs percentage change

Percentage change has a direction: from old to new. Percentage difference compares two values with no “before” and “after”, such as two prices in different shops. It divides the gap by the average of the two values, so the answer is the same whichever order you type them in: 100 and 120 differ by 20 ÷ 110 × 100 = 18.18%.

Use percentage change for growth over time and percentage difference for comparing two things side by side.

Common percentage mistakes

• Using the wrong base. “What percent of the class passed?” divides by the whole class, not by the number who failed. Check which number is the whole before you divide.
• Adding percentages that don’t share a base. A 20% discount followed by another 10% off is 28% off the original price, not 30%, because the second discount comes off the lower price (0.8 × 0.9 = 0.72).
• Reversing a change with the same percentage. After a 25% increase, a 25% decrease doesn’t get you back to the start; a 20% decrease does.
• Dividing by zero. A change from 0 has no percentage: the calculator says “Can’t divide by zero” instead of showing an infinite number.

Percentages vs percentage points

When a rate moves from 4% to 5%, it rises by 1 percentage point, but by 25 percent (because 1 is 25% of 4). News reports and bank offers sometimes mix the two. If you’re comparing interest rates over time, the compound interest calculator and the loan calculator show what the change means in money.

Everyday uses

Ask in plain words

• “What is 18% of 4,250?”
• “My rent went from 1,200 to 1,290. What percentage increase is that?”
• “A coat is 68 after 15% off. What was the original price?”

Type it under the calculator or in the AI workspace. For more complex expressions with powers and roots, use the scientific calculator.

Privacy

Numbers you type stay in your browser; nothing is uploaded, stored or used to train AI. More in Trust and privacy.

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