Everyday maths
Percentage calculator: three questions to set up correctly
A practical way to identify the percentage, the part, and the whole before you calculate.
Updated July 20, 2026 · 4 min read
Start by naming the numbers
Most percentage mistakes happen before the calculation begins. Decide which number is the whole amount, which is the part of that amount, and whether you are being asked for a percentage or a resulting value.
For example, in the question 'What is 18% of 750?', 750 is the whole and 18 is the rate. In '135 is what percentage of 750?', 135 is the part and 750 is the whole.
Use the right formula for the question
To find a percentage of a number, divide the rate by 100 and multiply it by the whole. To find what percentage a part represents, divide the part by the whole and multiply by 100.
Percentage change has one more step: subtract the original value from the new value, divide by the original value, then multiply by 100. The original value is the key reference point.
Check whether the answer makes sense
A 10% result should be about one tenth of the whole, while 50% should be about half. A quick estimate can flag a misplaced decimal or an accidentally reversed division.
When precision matters, keep the full result while working and round only once at the end. Follow the rounding instruction from the relevant invoice, report, exam, or form.