Three ways to work with percentages: find a percentage of a number, find what percentage one number is of another, or calculate percentage change.
15% of 200
30
15% × 200 = 30
Tip: The calculator updates instantly as you type.
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What is the Percentage Calculator?
A percentage calculator is a tool that computes the relationship between two numbers as a percentage. It answers questions like 'What percent is 25 of 100?' (25%), 'What is 15% of 200?' (30), or 'If 80 is 40% of a number, what is that number?' (200). Percentages express parts of a whole as fractions of 100.
How it works
The calculator uses three core formulas depending on what you're solving for. To find what percent A is of B, divide A by B and multiply by 100. To find a percentage of a number, multiply the number by the percentage and divide by 100. To find the original number when you know a percentage of it, divide the known value by the percentage and multiply by 100. The tool lets you pick which calculation you need and fills in the answer automatically.
Part is the smaller amount, Whole is the total, and Percentage is the value between 0 and 100. Choose the formula based on which value you're solving for.
Examples
Input
Result
Notes
What percent is 18 of 45?
40%
(18 ÷ 45) × 100 = 40%. Useful for grades, surveys, or comparing partial quantities.
What is 25% of 80?
20
(25 ÷ 100) × 80 = 20. Common for discounts, commission calculations, and tax estimates.
If 30 is 20% of a number, what is that number?
150
(30 ÷ 20) × 100 = 150. Used when you know a part and its percentage but need the whole.
How to use the Percentage Calculator
Select the type of percentage calculation you need (percent of total, percentage of amount, or find the whole).
Enter the known numbers into the input fields.
Click Calculate and the result appears instantly.
Copy the answer or use it to continue another calculation.
Verify the result makes sense—percentages over 100% mean the part is larger than the whole.
For repeated calculations, adjust inputs and recalculate without reloading the page.
Benefits
Eliminates mental math errors—no need to remember percentage formulas or do long division by hand.
Saves time on everyday money tasks like splitting bills, calculating tips, applying discounts, and working out tax.
Perfect for students checking homework, tracking grade percentages, or understanding test score distributions.
Handles all three percentage problem types, so you don't need three different tools.
Free, instant, and works offline in your browser—no sign-up or ads interrupting your work.
Clear step-by-step logic shows *how* the answer was reached, not just the final number.
Tips & common mistakes
Common mistakes
Forgetting to divide by 100 when converting a percentage to a decimal—15% is 0.15, not 15, when used in multiplication.
Reversing the numerator and denominator: 'What percent is 10 of 40?' should be (10 ÷ 40) × 100, not the other way around.
Confusing percentage *increase* with the new total—a 20% increase on £100 is £120, not £20.
Assuming percentages must be under 100%—a quantity can absolutely be 150% or 250% of another if it's genuinely larger.
Tips
Always sanity-check your answer: if you're calculating 5% of 200, expect a small number (10), not something close to 200.
For quick mental math, remember that 50% = half, 25% = quarter, 10% = one-tenth—use these anchors to estimate before calculating.
When comparing discounts, calculate the final price (not just the discount amount) to see which deal saves you more money.
In finance and retail, percentages are often compounded—a 10% increase followed by a 10% decrease does not return you to the original number.
Frequently asked questions
What's the difference between percentage and percentile?
Percentage is a number out of 100 (e.g., 75%). Percentile is a ranking—the 75th percentile means you scored as well as or better than 75% of others. This tool calculates percentages, not percentiles.
Can a percentage be over 100%?
Yes. If you earn £120 when the goal was £100, you've reached 120% of your target. Percentages over 100% simply mean the part is larger than the original whole.
How do I calculate percentage change or percentage increase?
Use the formula: ((New Value − Old Value) ÷ Old Value) × 100. This tool's 'Percent of Total' mode works for this—enter the change as the part and the original value as the whole.
Why does 10% off twice not equal 20% off?
Because the second discount applies to the already-reduced price, not the original. 10% off £100 = £90; 10% off £90 = £81. A straight 20% off £100 = £80. Compounding matters.
Is this calculator accurate for very large or very small numbers?
Yes. The calculator uses standard floating-point arithmetic and works reliably from fractions up to billions, though display limits may round very long decimals.
Can I use this for calculating grades or test scores?
Absolutely. If you scored 87 out of 100, enter 87 as the part and 100 as the whole to see you achieved 87%. It also scales for different total points—85 out of 120 = 70.83%.