Rule of thumb · MathematicsNº 139 / 167

Percent of means multiply by hundredths

X% of Y is (X/100) times Y — and "Y plus X%" is Y times (1 + X/100).

Why it works

Percent just means per hundred. The same algebra powers tips, discounts, VAT-style add-ons, and reverse "what percent is A of B".

When it fails

Percentages do not commute across a sequence. Up 20% then down 20% leaves you at 96%, not 100% — and a change from 3% to 5% is two percentage points but a 67% increase, which is the ambiguity most misleading statistics are built on.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

Open Percentage Calculator →

More in Mathematics

← PreviousMid-market is not what you get at the bank Next →Resistor bands read digit-digit-multiplier

How do you work out a percentage of a number?

X% of Y is (X/100) times Y — and "Y plus X%" is Y times (1 + X/100). Percent just means per hundred.

Browse all 167 rules of thumb →