Percentage Calculator

About this calculator

This calculator handles six common percentage tasks in one place: finding what X% of a number is, figuring out what percentage one number is of another, finding the original number when you know X is P% of it, calculating the percentage change between two values, measuring the percentage difference between two values, and adding or subtracting a percentage from a base number. Type into any card and the answer updates instantly.

How to read your results

The page is split into six independent mode cards, each solving a different percentage question. Each card shows two input fields and displays its result — or a short note if the inputs are incomplete or mathematically impossible — directly below the fields. All six modes work simultaneously, so you can use as many at once as you like.

How it's calculated

Each mode uses a direct formula. "What is X% of Y" computes (X / 100) × Y. "X is what % of Y" computes (X / Y) × 100, requiring Y ≠ 0. "X is P% of what" solves for the base as X / (P / 100), requiring P ≠ 0. Percentage change is ((new − old) / |old|) × 100, signed, requiring the old value ≠ 0. Percentage difference is |A − B| / ((|A| + |B|) / 2) × 100, order-independent, requiring the two values not both be zero. "Add / subtract a percent" gives base × (1 + p / 100) and base × (1 − p / 100). All formulas follow standard mathematical definitions as documented on MathIsFun.

Worked example

Enter 15 in the percent field and 80 in the number field of the "What is X% of Y" card. In the "Add / subtract a percent" card, enter 80 as the base and 15 as the percent.

15% of 80 is 12. Adding 15% to 80 gives 92, and subtracting 15% from 80 gives 68 — confirming all three results are consistent.

Frequently asked questions

What is the difference between percentage change and percentage difference?

Percentage change measures a directional shift from an old value to a new one: ((new − old) / |old|) × 100. It has a sign — positive for an increase, negative for a decrease. Percentage difference, by contrast, is order-independent: it divides the absolute gap by the average of the two values, giving a symmetric measure that does not imply direction.

Why does the "X is what % of Y" card refuse to calculate when Y is zero?

The formula is (X / Y) × 100. Division by zero is undefined, so the calculator shows a note rather than an error or an infinite result. Make sure the denominator is a non-zero value.

Can I use these formulas for discounts, tips, or tax?

Yes. To find a discount amount, use "What is X% of Y" with X as the discount rate and Y as the original price. To find the price after a percentage off or on, use "Add / subtract a percent." The same logic applies to tips, VAT, or any markup.

How do I calculate X% of Y?

Multiply the whole by the percentage written as a decimal: Y × (X / 100). For example, 20% of 50 is 50 × 0.20 = 10. The "What is X% of Y" card does this for any two numbers.

How do I find a percentage increase or decrease?

Subtract the old value from the new one, divide by the old value, and multiply by 100: ((new − old) / old) × 100. Going from 100 to 125 is a 25% increase; going from 50 to 40 is a 20% decrease. A positive result is an increase, a negative one a decrease.

X is what percent of Y?

Divide the part by the whole and multiply by 100: (X / Y) × 100. For example, 12 is 15% of 80 because 12 / 80 = 0.15. Use the "X is what % of Y" card, keeping the whole (Y) non-zero.

X is P% of what number?

Divide the part by the percentage written as a decimal: X / (P / 100). For example, 25 is 50% of 50 because 25 / 0.50 = 50, and 15 is 30% of 50 because 15 / 0.30 = 50. Use the "X is P% of what" card, keeping the percent (P) non-zero so the base is defined.

Sources

• www.mathsisfun.com/percentage.html
• www.mathsisfun.com/numbers/percentage-change.html
• www.mathsisfun.com/percentage-difference.html

Reviewed by the YouCalc Team · Last reviewedJune 11, 2026