Percentage Calculator
The three percentage questions that come up constantly, with the arithmetic shown rather than just the answer: what one percentage of a number is, what share one number is of another, and what changed between two values. Results keep more decimals than you need, because rounding early is how percentage arguments go wrong.
The percentage in the first question, the amount in the second, the new value in the third.
The amount the percentage applies to.
How percentage calculator works
- Ask the questionA percentage of a number, a share of a number, or a change between two numbers.
- Enter the two valuesNegative numbers are fine, and needed for a fall.
- Read the workingThe result, the remainder, and the fraction behind it.
What you get
Work out a percentage of a number, a share, or a change. Everything happens inside this page: the file is read by your browser, transformed in memory and handed straight back to you as a download. There is no upload queue, no waiting for a server, and nothing left behind when you close the tab.
Supported formats
This tool works on text you paste or type, so there is no file format to worry about. Nothing you type is sent anywhere.
Limitations, stated up front
- No compound interest, tax, VAT or discount chains. Those are a different calculation, and pretending otherwise produces quiet errors.
- The result is exact for the decimal values you enter, not for a rounded version of them.
Frequently asked questions
Why is a 50% rise not undone by a 50% fall?
Because the base changes. A hundred that rises by 50% is 150; a 50% fall on 150 is 75. To return to a hundred you need a fall of a third. This is the commonest mistake with percentages, and Furtu prints the reversing figure whenever a change is an increase.
Why is there no answer when the base is zero?
Because the share of zero is undefined, not infinite, and a change from zero has no percentage. Both cases return an explanation instead of a number, since infinity in a spreadsheet cell is a value that quietly poisons every formula around it.
Do percentages of percentages add up?
No. A 20% rise followed by a 20% fall is not a round trip — it is 1.2 × 0.8 = 0.96, so you have lost 4%. The base has moved each time, and Furtu shows the multiplication rather than leaving it implicit.
Why so many decimal places?
Because the exact value is usually not a tidy number, and rounding first compounds the error. Fifteen per cent of 233 is 34.95 exactly, while sixteen per cent of 233 is 37.28, and neither is 37. Furtu shows what the arithmetic produced, and the significant figures are yours to decide.
Are my numbers uploaded?
No. The calculation runs in your browser, so a margin figure or a salary number stays on your device.