Percentage Calculator
Four percentage questions, each with its own form and formula.
The arithmetic happens in this page. Nothing you type goes anywhere.
Percentages are the arithmetic everybody learned and nobody trusts themselves on at the moment it matters: working out a deposit, checking a payslip, deciding whether a price rise is worth arguing about.
More about Percentage Calculator
Four questions are laid out as four separate forms rather than one calculator with a mode dropdown. They look similar and they are not the same question. "What is 20% of 80" and "8 is what percent of 40" use different inputs in different roles, and a single form that quietly relabels its fields makes you stop and work out which one you are looking at.
Each answer shows the formula that produced it. That is not decoration: the commonest percentage mistake is not arithmetic but setup, and seeing (X ÷ Y) × 100 next to your numbers is what catches a part and a total entered the wrong way round.
Two questions have no answer, and the tool says so rather than inventing one. Nothing is a percentage of zero, because every number would be. And there is no percentage increase from zero, because no multiple of nothing reaches something. Tools that return infinity or a blank there are hiding a real answer: the question was malformed.
- Four separate forms, one per question, so you never work out which mode you are in
- The formula shown beside every answer, which catches a reversed part and total
- Percentage increase and decrease measured against the magnitude of the base
- Undefined questions refused with a reason, not answered with infinity
- Works from the keyboard, with the answer updating as you type
How to use it
- 1Find the form that matches your question.
- 2Type the two numbers.
- 3Read the answer, and check the formula underneath if it looks surprising.
- 4Swap the two numbers if the formula shows you entered them the wrong way round.
Questions
What is 20% of 80?
It is 16, from (20 ÷ 100) × 80. The first form does this one.
Why can I not get a percentage of zero?
Because every number would be an infinite percentage of nothing. The question has no answer, so the tool says so instead of showing infinity.
How is a percentage increase from a negative number measured?
Against the magnitude of the starting figure, so −50 to 50 is a 200% change. Measuring against the signed value flips the direction and reads as a fall.
Is a 20% rise cancelled by a 20% fall?
No. A 20% rise then a 20% fall leaves you 4% down, because the fall applies to a larger number than the rise did.
Why show the formula?
Because the usual percentage mistake is entering a part where a total belongs. Seeing the formula next to your numbers catches that immediately.