A percentage question arrives in four shapes and most people cannot name which one they have. All four boxes are on screen at once, so you read the question that matches yours instead of choosing from a menu, and each one shows the arithmetic underneath its answer.

How to use it

1. 1 Find the box whose question matches yours.
2. 2 Type the numbers. A comma works as the decimal mark just like a dot.
3. 3 Read the answer, and the line underneath shows exactly how it was worked out.

The four formulas

Every answer on this page comes from one of these four lines. They are printed here so the result is checkable rather than something you have to trust.

Increase and decrease are not symmetric

A price that rises by 50% and then falls by 50% does not come back to where it started: 100 becomes 150, then 75. The reason is that the second percentage is taken from a different base. This is the single most common percentage mistake, and it is why the change box always names both the starting and the ending value.

Percent and percentage point are different

If a rate goes from 4% to 6%, that is a rise of two percentage points, but a rise of fifty percent. Both statements are true and they describe the same move. Interest rates, tax rates and poll results are reported in points; this calculator works in percent, so convert before comparing.

Nothing is sent anywhere

The arithmetic runs in the page as you type, with no request behind it. Once the page has loaded it keeps working without a connection, which is useful in a shop or a meeting room with no signal.