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Percentage Calculator

Four of the most common percentage questions in one place, each answered with the formula and the full working.

Choose your question

Switch tabs to match the question you are trying to answer.

Apply a percentage to a number.

%

What this calculator does

Percentage problems look varied on the surface — a tip, a test score, a price rise, a survey result — but almost all of them reduce to one of four calculations. This page puts those four on separate tabs so you do not have to work out which formula applies before you begin. Pick the sentence that matches your question and the correct method follows automatically.

The answer appears immediately, along with a step-by-step breakdown. That breakdown is the point: copying a number is useful once, but seeing where it came from means you can do the same calculation in your head next time.

How to use it

  1. Choose the tab that matches your question.
  2. Type your two numbers into the boxes. Decimals and negative values are both accepted.
  3. Press Calculate, or tick Instant to see the answer update as you type.
  4. Use Copy result to take the number, or Copy link to share the whole calculation with someone else.

The formulas

1. What is X% of Y?Result = (X ÷ 100) × Y
2. X is what percentage of Y?Percentage = (X ÷ Y) × 100
3. X is Y% of what number?Whole = X ÷ (Y ÷ 100)
4. Percentage changeChange = ((New − Original) ÷ Original) × 100

Worked example

A shop takes 18% off a 2,750 price tag

  1. Convert the percentage: 18 ÷ 100 = 0.18
  2. Multiply: 0.18 × 2,750 = 495 — that is the discount
  3. Subtract: 2,750 − 495 = 2,255 to pay

Notice that the first two steps are exactly the "what is X% of Y" calculation. The third is ordinary subtraction. Most real-world percentage tasks are one of the four formulas plus one easy extra step, which is why learning the four is worth the effort.

Common mistakes

Dividing by the wrong number. In "X is what percentage of Y", Y is always the whole and goes on the bottom. Swapping them turns 30 out of 150 (20%) into 150 out of 30 (500%). If your answer looks wildly large, check which number you divided by.

Forgetting to convert back. Dividing 30 by 150 gives 0.2, not 20. The multiplication by 100 is what turns a decimal into a percentage, and it is the step most often skipped.

Treating a percentage of a percentage as addition. Taking 10% of a value and then another 10% of the result is not 20% overall — it is 19%, because the second slice is cut from a smaller cake.

Frequently asked questions

How do I calculate 15% of a number quickly in my head?
Find 10% by moving the decimal point one place left, then halve that to get 5%, and add the two together. For 240: 10% is 24, 5% is 12, so 15% is 36. The same trick builds 20% (double the 10%), 25% (quarter) and 30% (triple the 10%).
Can I enter negative numbers?
Yes. Negative values are accepted wherever they make mathematical sense, such as a temperature falling below zero or an account moving into deficit. The calculator uses the size of the starting value when working out a percentage change, so the result stays meaningful.
Why does the calculator refuse a total of zero?
Because dividing by zero has no answer in arithmetic. If the whole is zero there is nothing for a part to be a share of, so the calculator explains the problem instead of showing an error code.
How many decimal places do the answers use?
Up to six for general numbers and four for percentages, with trailing zeros removed so results stay readable. Money answers are always shown to two decimal places.
Does the shareable link contain my numbers?
Yes, and nothing else. Copy link builds a web address with your inputs written into it, so anyone opening it sees the same calculation already filled in. Nothing is uploaded — the link is assembled in your own browser.

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