Everyday Calculators
Percentages, discounts, ratios and basic statistics in one page, with the formula shown.
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What is a percentage of a number
One number as a percentage of another
Percentage change
The change is divided by the starting value, so 80 to 100 is a 25 per cent increase.
About this tool
Four small calculators that come up constantly in ordinary work: percentages in all three directions, a discount and tax calculation, proportion scaling, and descriptive statistics for a pasted set of numbers. Each one shows the formula it is using, because the most common percentage mistake is using the wrong denominator and the answer looks reasonable either way.
How to use it
- 1Pick the calculator you need from the four tabs.
- 2Enter your numbers — the result updates as you type.
- 3For the statistics tab, paste the numbers separated by anything you like: spaces, commas or new lines.
Frequently asked questions
Why is 80 to 100 a 25 per cent increase, not 20?
Because percentage change is measured against the starting value. The change is 20, and 20 divided by 80 is 0.25. Dividing by the ending value instead is the classic error, and it produces an answer that looks plausible, which is why this page prints the formula alongside the result.
Is the standard deviation the sample or the population one?
Sample, which divides by n minus one. If your numbers are a sample drawn from something larger, that is the right choice, because using n underestimates how spread out the underlying data is. If your numbers are the entire population and not a sample, say so in whatever report you are writing, because the difference shrinks as the set grows.
In what order does the discount calculator apply tax?
Discount first, then tax on the discounted price, which is how retail pricing normally works. Doing it the other way round gives a higher figure. Where discounts and tax interact differently — some jurisdictions tax the pre-discount price — you will need the local rule, so the page states the order it used.