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Percentage Calculator

Six everyday percentage calculations in one place. Fill in the numbers and the answer appears as you type.

What is % of ?

is what % of ?

is % of what number?

Percent change from to

Increase by %

Decrease by %

Percentages in plain English

A percentage is simply a fraction out of 100, so 15% means 15 hundredths, or 0.15. Almost every percentage question is a variation on one relationship: part = percent ÷ 100 × whole. If you know any two of the three values, you can find the third. This calculator covers the six forms people meet most often, so you do not have to rearrange the formula in your head.

The six calculations explained

  • What is X% of Y? Multiply Y by X and divide by 100. For example, 15% of 200 is 0.15 × 200 = 30. This is how you work out a tip, a commission or a share of a total.
  • X is what % of Y? Divide X by Y and multiply by 100. If you answered 30 questions correctly out of 40, then 30 ÷ 40 × 100 = 75%.
  • X is Y% of what? Divide X by the percentage as a decimal. If 30 is 15% of a number, that number is 30 ÷ 0.15 = 200. This "reverse percentage" is the one most people find awkward.
  • Percent change: subtract the old value from the new one, divide by the old value and multiply by 100. Going from 50 to 75 is a 50% increase; going from 80 to 60 is a 25% decrease.
  • Increase by a percentage: multiply by 1 plus the decimal. Adding 8% sales tax to a 50 purchase gives 50 × 1.08 = 54.
  • Decrease by a percentage: multiply by 1 minus the decimal. A 20% discount on 80 gives 80 × 0.80 = 64.

Common mistakes to avoid

Percent versus percentage points. If an interest rate rises from 4% to 5%, it has gone up by one percentage point, but by 25% in relative terms. Headlines mix these up constantly, so check which one is meant.

Stacking discounts. A 20% discount followed by a further 10% off is not 30% off. After the first discount you pay 80% of the price, and the second takes 10% off that amount: 0.80 × 0.90 = 0.72, so the total saving is 28%.

Undoing a percentage. If a price rose by 25% and you want to return to the original, you need a decrease of 20%, not 25%, because the base number has changed. The reverse-percentage form above handles this: divide the new price by 1.25 to recover the original.

Starting from zero. Percent change from zero is undefined, because you cannot divide by zero. The calculator will tell you when this happens instead of showing a misleading number.

Where you will use it

Shopping discounts and sales tax, tips, exam scores, interest and savings rates, price changes, recipe scaling, business margins and growth figures all rely on the same few formulas. The results here are rounded to at most four decimal places for readability, and all calculations happen locally in your browser. For money, remember that shops usually round to the nearest cent, so the final amount on a receipt may differ by a cent.

Frequently asked questions

How do I calculate a percentage of a number?

Multiply the number by the percentage and divide by 100. For example, 15% of 200 is 15 × 200 ÷ 100 = 30. You can also convert the percentage to a decimal (0.15) and multiply.

How do I calculate percent increase or decrease?

Subtract the old value from the new value, divide the result by the old value, and multiply by 100. A positive answer is an increase and a negative answer is a decrease.

What is the difference between percent and percentage points?

Percentage points measure the simple difference between two percentages (4% to 5% is one percentage point). Percent change measures the relative difference (4% to 5% is a 25% increase).

Why does a 20% discount followed by 10% not equal 30% off?

Because the second discount applies to the already reduced price. You pay 80% and then 90% of that, which is 72% of the original, so the total saving is 28%.

Why does the calculator say a result is undefined?

Some calculations divide by a number, and division by zero has no valid answer. For example, you cannot express a change from zero as a percentage, and you cannot ask what 0% of a number refers to.

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