Percentage increases appear everywhere: a rent rise, a higher energy bill, a pay review, or a product that has gone up in price. The calculation is simple once you know which number should sit at the bottom of the fraction.

Start with the original amount. Subtract it from the new amount to find the change. Then divide that change by the original amount and multiply by 100.

For example, imagine a monthly bill rises from £80 to £92. The difference is £12. Divide 12 by the original £80 and you get 0.15. Multiply by 100 and the increase is 15%.

The important part is using the original figure as the starting point. If you divide by the new amount instead, the answer will be close but wrong. That is one of the most common mistakes people make when checking a percentage change.

A percentage increase is not the same as a percentage-point increase. If a savings rate moves from 4% to 5%, it has increased by one percentage point. Relative to the old rate, though, it has gone up by 25%. Both descriptions can be true, but they answer different questions.

For everyday checks, keep the figures in the same unit before you begin. Compare pounds with pounds, not pounds with pence, and make sure you are looking at the same period. A weekly cost compared with a monthly cost will give a misleading result.

You can use the Percentage Calculator to check the maths in a few seconds. Enter the original value and the new value, then use the result alongside the simple working above. It is useful for a quick answer, but understanding the steps makes it easier to spot a result that does not look right.