Percentage calculator

The four calculations

Almost every percentage question is one of four sums, and the hard part is usually working out which one you are being asked. Each mode above matches one of them.

• Percent of a number. A share of a known total: a tip, a commission, a tax on a known amount.
• One number as a percentage of another. A score, a market share, how much of a budget has gone.
• Change between two numbers. Growth, inflation, a pay rise, a drop in traffic.
• The figure before a change. The price before a discount, the total before tax, the salary before a rise.

Why the last one trips people up

A percentage is always taken from a base, and reversing one means dividing by that base rather than adding the same percentage back. Take 20% off 300 and you get 240; add 20% to 240 and you get 288. The two differ because the first 20% came from 300 and the second came from 240.

The same trap explains why a 50% rise followed by a 50% fall does not return you to where you started. It is the most common percentage error there is, and it is worth checking any figure that depends on reversing a change.

Working precision

Answers are shown to six decimal places where they need it, so a third of a number reads as 33.333333% rather than being rounded to something that no longer adds up. For money, round the final figure yourself at the end rather than at each step.