Formulas
- X% of Y = Y × X ÷ 100. Example: 15% of 200 = 30.
- X is what % of Y = X ÷ Y × 100. Example: 30 is 25% of 120.
- Percentage change = (new − old) ÷ old × 100. Example: from 80 to 100 is a 25% increase.
Everyday uses
- Discounts: a 30% discount on $59.90 is $17.97, so you pay $41.93.
- Tips: 18% of a $64 bill is $11.52.
- Price increases and salary raises: use percentage change to compare before and after.
A common mistake
A 50% drop followed by a 50% rise does not bring you back to the start: 100 → 50 → 75. Percentages are always calculated on the current value.
Frequently asked questions
How do I calculate a percentage in my head?
Find 10% by moving the decimal point one place left, then multiply. For 15% of 80: 10% is 8, 5% is 4, total 12.
How do I add a percentage to a number?
Multiply by (1 + percentage ÷ 100). Adding 8% tax to $50: 50 × 1.08 = $54.
What is the difference between percent and percentage points?
If a rate goes from 5% to 7%, it rose 2 percentage points, which is a 40% increase.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.