How to Calculate Percentages: The Complete Guide
To calculate a percentage, multiply the number by the percent and divide by 100; to find what percent one number is of another, divide the first by the second and multiply by 100; and for percentage change, take (new − old) ÷ old × 100. A free browser calculator does all three live, showing the worked formula so you can trust the answer.
Percentages come up everywhere — tips, discounts, tax, grades, growth rates, interest — yet the three underlying formulas are easy to mix up under pressure. This guide untangles them and shows how to get each answer in seconds.
Why a dedicated percentage tool helps
Most percentage mistakes come from using the right numbers in the wrong formula, or from rounding too early. A dedicated calculator removes both risks: you pick the question you're answering, type two numbers, and read the result plus the exact calculation behind it. It's built for students checking homework, shoppers working out sale prices and tips, analysts computing growth, and anyone splitting a bill. Because it runs locally in your browser, nothing you type is uploaded, and it keeps working offline once loaded.
How to calculate percentages in your browser
- Choose a mode: What is X% of Y, X is what % of Y, or % change from X to Y.
- Enter your two numbers in the input fields.
- Read the live result and the worked formula shown directly beneath it.
- Switch modes any time — your numbers stay put so you can compare answers instantly.
The three percentage problems, side by side
Almost every real-world percentage question is one of these three. Here's each with its formula and a worked example.
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 80 = 12 |
| X is what % of Y? | X ÷ Y × 100 | 30 of 120 = 25% |
| % change from X to Y | (Y − X) ÷ X × 100 | 50 to 65 = +30% |
A negative result in the third row simply means a decrease — going from 65 to 50 is −23%. The tool labels increases and decreases clearly so the sign is never ambiguous.
Key features and benefits
- Three modes: percent of a number, reverse percentage, and percentage change.
- Live results as you type — no calculate button.
- Shows the formula used so you can verify the math.
- Handles decimals and negative values correctly.
- 100% private — calculations never leave your browser.
- Free, no sign-up, and works offline.
Try the Percentage Calculator now — it's free and runs entirely in your browser.
Frequently asked questions
How do I calculate the percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 15% of 80 is 80 × 15 ÷ 100 = 12. The What is X% of Y mode does this automatically.
How do I find what percent one number is of another?
Divide the first number by the second and multiply by 100. So 30 is 30 ÷ 120 × 100 = 25% of 120. The X is what % of Y mode shows the division for you.
How is percentage increase or decrease calculated?
Percentage change is (new − old) ÷ old × 100. Going from 50 to 65 is (65 − 50) ÷ 50 × 100 = +30%. A negative result means a decrease.
What's the difference between percentage change and percentage points?
Percentage points measure the simple gap between two percentages, while percentage change is relative. Moving from 10% to 15% is a rise of 5 percentage points but a 50% relative increase.
Can it handle negative numbers?
Yes. All three modes accept negative and decimal inputs. With a negative starting value, the sign convention for percentage change can be counter-intuitive, so the tool shows the raw formula to help you interpret it.
Related free tools
- Discount Calculator — sale price and amount saved.
- GST Calculator — add or remove GST at any rate.
- VAT Calculator — add or extract VAT from a price.
- EMI Calculator — monthly loan repayments.
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