Percentage Formulas You’ll Actually Use

Percentages show up constantly — tips, discounts, raises, taxes, grades — but most people only remember one formula and try to force every problem into it. Here are the three that actually cover almost everything.

Finding a percentage of a number. To find X% of Y, convert the percentage to a decimal and multiply: (X ÷ 100) × Y. A 20% tip on a $45 bill is (20 ÷ 100) × 45 = $9.

Finding what percent one number is of another. To find what percent X is of Y, divide X by Y and multiply by 100: (X ÷ Y) × 100. If you scored 42 out of 50 on a test, that’s (42 ÷ 50) × 100 = 84%.

Finding percentage change. To find the percent increase or decrease from an old value to a new value, subtract the old from the new, divide by the old, then multiply by 100: ((New − Old) ÷ Old) × 100. If your rent went from $1,200 to $1,320, that’s ((1,320 − 1,200) ÷ 1,200) × 100 = 10%.

A common mistake is applying a percentage change formula backward — a 20% increase followed by a 20% decrease does not return you to the original number, because the second 20% is calculated from a different (higher) base.

Our Percentage Calculator handles all three of these automatically — just pick the calculation type and enter your numbers.