Percentage Calculator

Answer five kinds of percentage question, one at a time or a whole list at once, with the working shown beside each result.

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What this tool does

Percentage Calculator covers five distinct questions: what X% of Y is, the percentage change from A to B, what percent of B the value A represents, and Y increased or decreased by X%. These are different formulas rather than relabelled versions of one, which is why the tool asks you to pick the question first and then renames the two number fields to match — Percentage and Value, From and To, or Part and Whole.

Every answer arrives with its working. The results table has three columns — the question restated, the answer, and how it was reached — so a figure like 62.5% is accompanied by the division that produced it. That is useful when the number is going into something another person will check, and it is also the quickest way to notice you asked the wrong question, because the arithmetic in the third column will not be the arithmetic you had in mind.

Two behaviours surprise people often enough to state plainly. Adding a percentage and then removing the same percentage does not return you to the original figure: 250 + 8% is 270, and 270 − 8% is 248.4, because the second percentage is taken from the larger number. And percentage change is reported as a relative change, not as percentage points — 20 to 25 comes back as +25%, which is correct for quantities and misleading if both figures were already percentages, where the honest answer is five percentage points. Where the starting figure is negative, the change is measured against its magnitude.

The calculation happens in this browser tab, with no request to any server, so the salaries, margins and medical figures people typically put into a calculator are not sent anywhere. Typed lines read both thousands conventions, so 1,234.56 and 1.234,56 are understood as the same figure, but a currency symbol is not accepted anywhere: 99.99 - 15% is answered and £99.99 - 15% is rejected as unreadable, so strip the symbol before pasting a price list. The two numeric fields are ordinary number inputs and take digits, a minus sign and a decimal point only. Answers are shown to six decimal places, or four for a percentage, which is display rounding rather than a limit on the arithmetic underneath.

How to use it

  1. Choose the question. The Question menu lists what is X% of Y, percentage change from A to B, A is what percent of B, and increase or decrease Y by X%. The two fields below it relabel themselves for whichever you pick.
  2. Enter the two numbers. Decimals and negatives are both fine; grouping commas and currency symbols are not, because these are plain number boxes. They step in hundredths, so pence and cents can be typed straight in.
  3. Or type your questions instead. Write one per line in the input box and the fields are ignored: 15% of 200, from 80 to 100, 30 is what % of 120, 250 + 8%, 99.99 - 15%, or 5 out of 8. Lines starting with # or // are skipped, so you can annotate a list.
  4. Read the answers. One row per question, with the arithmetic in the How column. A line the parser cannot understand stops the run and names its line number and text, so you correct that line rather than search the whole batch.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Taking a discount off a price

Write 99.99 - 15% and you get the price after the discount, with the amount that came off shown next to it.

Reporting movement between two periods

Percentage change from last quarter's figure to this one, stated with its direction and the absolute difference as well as the percentage.

Turning a score into a share

5 out of 8 becomes 62.5% — the same sum whether it is a test mark, a conversion rate or the proportion of a batch that failed.

Checking a list of figures in one pass

Paste a discount, a rise and a share as three lines and read one table back, instead of running three separate sums and copying each result out.

Questions

Do the salaries and prices I type go anywhere?
No. Percentage Calculator runs in this browser tab, so nothing you type is transmitted or stored, and the tool keeps working with the network disconnected.
Why doesn't taking off the same percentage undo adding it?
Because the second percentage is worked out from the new, larger figure. 250 + 8% gives 270, and 270 − 8% gives 248.4. To recover the original amount from a price that already includes tax or a markup, use VAT & Sales Tax, which divides by the rate rather than subtracting it.
Is percentage change the same as percentage points?
No, and the difference matters when your inputs are themselves percentages. A move from 20% to 25% is a rise of five percentage points, but this tool reports the relative change, +25%. Read the answer as a change in the quantity, not in the unit.
Which phrasings does the typed box understand?
Six patterns: 15% of 200, 250 + 8%, 250 - 8%, from 80 to 100 (the word from and the arrow form 80 -> 100 both work), 30 is what % of 120, and 5 out of 8. One overlap is worth knowing: the word of on its own is treated as a share, so 15 of 200 answers 7.5% rather than 30 — keep the % sign when you mean a percentage of something. Other wordings, including 15 percent of 200 spelled out, are rejected with the line number rather than guessed at.
What happens if the starting value is zero?
Percentage change from zero is undefined, so the tool says so instead of returning infinity. Asking what percent a value is of zero is refused for the same reason.
Can it chain one calculation into the next?
No. Each line is answered on its own, so a 10% rise followed by a 5% fall needs two lines, with the answer from the first typed into the second. There is no running total across the batch.
How precise are the answers?
The maths is done in double-precision floating point and the result is rounded for display — six decimal places for a value, four for a percentage. For money, round the displayed answer yourself to the convention your accounts use.

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