Percentages come up constantly — tips, discounts, grades, growth rates. Almost every percentage question is one of three cases.
1. X% of a number
Multiply the number by the percentage divided by 100. For example, 20% of 80 is 80 × 0.20 = 16.
2. X is what % of Y
Divide X by Y and multiply by 100. For example, 30 out of 120 is 30 ÷ 120 × 100 = 25%.
3. Percent change
Subtract the old value from the new, divide by the old value, and multiply by 100. From 50 to 65 is (65 − 50) ÷ 50 × 100 = +30%. A negative result is a decrease.
The Percentage Calculator handles all three in one place. For shopping, the Discount Calculator applies percent-off pricing (including stacked discounts), and the Tip Calculator splits a gratuity across a table.
A quick trick
"X% of Y" always equals "Y% of X." So 18% of 50 is the same as 50% of 18 — which is just 9. Swapping the order can turn a hard mental calculation into an easy one.