Percentage Calculator
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Work out a percentage of a number, or what changed between two numbers.
- Pick which of the four questions you are asking
- Fill in the two numbers — the labels tell you which is which
- Copy the working, not just the number, if you are sending it on
About this tool
"Percentage calculator" is four different sums wearing one name, and which one you meant is not guessable from two numbers. So they are named apart here rather than hidden behind a single box: What is 15% of 80? What percent of 80 is 12? What changed between 80 and 92? What is 80 plus 15%? Pick the sentence you are actually asking, and the labels on the two fields change with it — because which number goes where is the entire difficulty with percentages, and a tool that will not say is one you have to check by trying it. The answer comes with its working written out — "15% of 80 = 12" — which is the part you paste into an email, and also how you notice you picked the wrong one of the four. The sum worth care is percentage change. A rise from 80 to 92 is up 15%, but the fall back from 92 to 80 is down 13%, not 15%, because the base changed. That asymmetry is the most common mistake made with percentages anywhere, so the result always says which number it was a percentage of. It answers as you type, in your browser, and nothing is sent anywhere.
Frequently asked questions
Why is going up 15% and back down again not 15%?
Because the base changes. From 80 to 92 is a rise of 12, and 12 is 15% of 80. Coming back down, that same 12 is measured against 92 instead, which is about 13%. Every percentage is a percentage of something, and that something is what moved.
How do I work out a discount?
Use "plus or minus a percent" with a negative number: 80 with −15% gives 68. It is the same sum as adding a percentage, which is why there is no separate mode — a discount is just a negative increase, and two buttons for one sum is two places to make a mistake.
Can I type 12,5 instead of 12.5?
Yes. Both separators work, since half the world uses a decimal comma. A comma grouping three digits — 1,500 — is read as a thousands separator rather than a decimal point, which is the only ambiguous case and the one worth getting right.
Why does it not just give one answer box?
Because the classic percentage error is not a miscalculation, it is answering a different question from the one that was asked, confidently. Four fields labelled "number" and "percent" would compute something for any pair you typed. Naming the question is most of what this tool does.
Do I get 0.30000000000000004 like a spreadsheet sometimes shows?
No. Results are rounded to twelve digits, which is well past anything you typed and short of where binary representation error lives, so the answer is clean without being wrong.
Is there an API?
Yes, free with an account — the same four modes and the same code that runs in this tab. Useful when the percentages are in a spreadsheet or a script rather than in front of you.
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