How to use the percentage calculator
- Pick the row that matches your question.
- Type your two numbers into that row. The answer appears on the right as you type.
- Read the small line under each result to see the exact calculation used.
Formula
Four calculators in one
Most percentage problems are one of four questions. Each row of the calculator answers one of them:
- What is X% of Y? Tips, tax, commission, discounts.
- X is what % of Y? Test scores, budget shares, conversion rates.
- Percentage change from A to B. Price rises, salary increases, growth.
- X is Y% of what? Working back to an original price or total.
The formulas
Percent of a number
X% of Y = (X ÷ 100) × Y. 15% of 200 = 30.
What percent one number is of another
(Part ÷ Whole) × 100. 30 of 120 = 25%.
Percentage change
((New − Old) ÷ |Old|) × 100. 80 → 100 = +25%, 100 → 80 = −20%.
Reverse percentage
Part ÷ (Percent ÷ 100). If 45 is 30% of a total, the total is 45 ÷ 0.3 = 150.
Common mistakes
- Using the wrong base. Percentage change always divides by the original value.
- Adding percentages that apply in sequence. A 10% rise followed by another 10% rise is +21%, not +20%.
- Mixing up percent and percentage points. An interest rate moving from 4% to 5% is a 1-point rise but a 25% increase.
Where percentages show up
Shopping discounts (try the discount calculator for stacked offers), exam marks, loan rates, nutrition labels and statistics all use the same arithmetic. For heavier maths, such as powers, roots and statistics, open MathKit.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 200 is 0.15 × 200 = 30.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. 30 out of 120 is 30 ÷ 120 × 100 = 25%.
How is percentage change calculated?
Subtract the old value from the new value, divide by the old value and multiply by 100. Going from 80 to 100 is a 25% increase; from 100 to 80 is a 20% decrease.
Why isn't a 50% increase cancelled by a 50% decrease?
Because each percentage applies to a different base. 100 + 50% = 150, but 150 − 50% = 75. The decrease is calculated on the larger number.
What is the difference between percent and percentage points?
Percentage points measure the plain difference between two percentages (5% to 7% is +2 points), while percent change measures relative change (5% to 7% is a 40% increase).