Percentage Calculator

What this calculator does

This percentage calculator solves the four percentage problems people meet most often, each in its own mode. The first mode finds X% of Y — a straightforward share of a number, like a tip or a discount amount. The second answers X is what percent of Y, turning a part and a whole into a percentage. The third computes the percent change between a starting and an ending value, telling you whether something rose or fell and by how much. The fourth adds or subtracts a percentage to a number, which is how you apply a markup, a tax, or a markdown. Every mode shows the answer and a plain-English line describing the exact arithmetic, and the result updates as you type.

Why you might need it

Percentages appear everywhere in daily life. You might be splitting a restaurant bill and need 18% of the total, or checking whether a “30% off” sale price is correct. A salary that moved from one figure to another is a percent-change question. Comparing this month’s traffic, sales, or expenses against last month’s is the same calculation. Applying sales tax, VAT, or a service charge is an add-percentage problem, while a clearance discount is a subtract-percentage problem. Doing these by hand invites slipped decimal points; a dedicated tool removes that risk and shows its working so you can trust the number.

How to use it

1. Pick a mode with the switcher at the top: X% of Y, X is what % of Y, % change, or Add / subtract %.
2. Fill in the fields the chosen mode shows. They relabel themselves so it is always clear what each box means.
3. For the add/subtract mode, choose whether to add or subtract the percentage from the number.
4. Read the headline result and the explanation line beneath it. Use the copy button to grab the answer.

How it’s calculated

Each mode uses a standard percentage formula. X% of Y is (X ÷ 100) × Y. X is what percent of Y is (X ÷ Y) × 100. Percent change is (new − old) ÷ |old| × 100, using the absolute value of the starting number so the sign of the result reflects direction rather than the sign of the base. A positive result is an increase and a negative result is a decrease. Add or subtract a percentage first finds the percentage amount, (X ÷ 100) × Y, then adds it to or subtracts it from the original number. Results are shown with digit grouping and up to four decimal places.

Common pitfalls

The most frequent mistake is confusing a percentage point with a percent change. If an interest rate moves from 4% to 5%, that is a one percentage-point rise but a 25% increase relative to the starting rate — both descriptions are valid, and they are not the same number. Another trap is reversing a percentage: a price reduced by 20% is not restored by adding 20% back, because the 20% is now taken from a smaller base. Finally, percent change is undefined when the starting value is zero, since you cannot divide by zero; the tool flags this rather than showing a misleading figure.

Tips and related calculations

When you apply a discount and a tax together, order does not matter for the final price — multiplying by 0.8 and by 1.1 gives the same result either way — but it does matter how each percentage is described. To reverse a known percentage, divide instead of subtracting: if a total of 120 already includes 20% tax, the pre-tax figure is 120 ÷ 1.2 = 100. For repeated growth, such as interest compounding over several periods, a single percent-change calculation is not enough; you would multiply the growth factor once per period. Because this calculator runs entirely in your browser, you can try as many figures as you like with nothing ever leaving your device.