Percentage Calculator
Work out percentages six ways: X% of Y, what percent one number is of another, increase, decrease, difference, and the original before a change.
Result
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Enter two numbers to see the answer.
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All Calculators tools →This tool runs entirely in your browser. Nothing you enter is sent to our servers, so there is nothing for us to store or see.
About the Percentage Calculator
Six percentage calculations in one place, each with its own labelled inputs and a plain-English explanation of the answer — so you can see not only what the number is but why it is that number.
The distinction worth understanding is between percentage change and percentage difference, because they give different answers and are routinely confused. Change is directional and measured against the starting value: 50 to 100 is a 100% increase, while 100 to 50 is a 50% decrease — the same absolute movement, two different percentages. Difference is symmetric and measured against the average of the two, giving 66.7% either way. Use change when one value came first; use difference when you are comparing two independent measurements and neither is the baseline.
The reverse calculation is the one that trips people up most in practice. If a jacket is £120 after 20% off, the original was £150 — not £144. Adding 20% back on is the wrong operation, because the discount was taken from the larger number. Dividing by 0.8 is right, and that is what the "find the original" mode does.
Everything is calculated on your device as you type. Nothing is sent anywhere.
How to use the Percentage Calculator
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Choose the calculation
Pick the question you are actually asking from the dropdown. The two input labels change to match, so you always know which number goes where.
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Enter your numbers
The answer updates as you type. Decimals and negative numbers are both fine.
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Read the explanation
Beneath the result is a sentence stating the calculation in full, which is worth a glance to confirm the tool understood the question the way you meant it.
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Copy the result
The copy button puts the unrounded value on your clipboard, not the shortened display version.
Frequently asked questions
What is the difference between percentage change and percentage difference?
Percentage change is directional and measured against the starting value, so 50 to 100 is a 100% increase while 100 to 50 is a 50% decrease. Percentage difference is symmetric and measured against the average of the two values, giving 66.7% in both directions. Use change when one value came first in time; use difference when comparing two independent measurements.
How do I find the original price before a discount?
Divide, do not subtract. If an item costs £120 after 20% off, the original was £120 ÷ 0.8 = £150. Adding 20% back on gives £144, which is wrong, because the discount was calculated from the larger original figure rather than the reduced one. The "find the original" mode does this correctly.
How do I calculate a percentage increase?
Subtract the old value from the new one, divide by the old value, and multiply by 100. So from 80 to 100 is (100 − 80) ÷ 80 × 100 = 25%. The key point is that you always divide by the starting value, never by the ending one.
Why does it say percentage change from zero is undefined?
Because the calculation divides by the starting value, and dividing by zero has no answer. Conceptually, any increase from nothing is infinite in percentage terms — going from 0 to 1 is not a 100% increase, it is an increase with no meaningful percentage. Report the absolute change instead.
Can I add two percentages together?
Not directly. A 10% increase followed by another 10% increase is a 21% increase overall, not 20%, because the second increase applies to the already-larger number. Likewise a 50% rise followed by a 50% fall leaves you 25% below where you started. Use the "increase or decrease by" mode twice in succession to get it right.