How do I find 15% of a number?

Multiply the number by 0.15. For example, 15% of 80 is 80 × 0.15 = 12. A quick mental trick: find 10% by moving the decimal one place left, then add half of that for the extra 5%.

What is the formula for percentage change?

Percentage change = ((New value − Old value) ÷ Old value) × 100. A positive result is an increase; negative is a decrease. For example, a price rising from $40 to $52 is a 30% increase.

How do I calculate what percentage one number is of another?

Divide the part by the whole, then multiply by 100. If 30 students out of 120 passed, that is (30 ÷ 120) × 100 = 25%.

What is the difference between percentage and percentage points?

Percentage points measure the arithmetic difference between two percentages. If interest rates rise from 3% to 5%, that is a 2 percentage point increase — but a 66.7% percentage change. The distinction matters in finance and statistics.

How do I reverse a percentage to find the original value?

Divide the final value by (1 + the percentage as a decimal) for increases, or (1 − the decimal) for decreases. If a price after a 20% increase is $120, the original was $120 ÷ 1.20 = $100.

Finance

Percentage Calculator

Three calculators in one: find what a percentage of a number is, work out what percent one value represents of another, or measure the percentage change between two figures.

Percentage (%)

Of this number

Result

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Formula used—

When to use each mode

The three tabs cover the most common situations people encounter. Use X% of Y when you know a rate and want the actual amount — like working out a 20% tip or a 15% discount. Use X is what % of Y when you have both numbers and want the proportion — like how much of a budget has been spent. Use % Change when comparing two values over time — prices, test scores, or revenue figures.

The maths behind it

Each mode uses a different arrangement of the same relationship between a part and a whole.

Find X% of Y: Result = Y × (X ÷ 100) X is what % of Y: Result = (X ÷ Y) × 100 % change from A to B: Result = ((B − A) ÷ A) × 100

Practical examples

Exam score: You answered 54 out of 70 questions correctly. What percentage did you score? (54 ÷ 70) × 100 = 77.1%.

Sale price: A jacket costs $85 and is 30% off. How much is the discount? 85 × 0.30 = $25.50 off, making it $59.50.

Sales growth: Revenue went from $42,000 to $55,000. What is the percentage increase? ((55,000 − 42,000) ÷ 42,000) × 100 = 31%.

Common questions

Multiply the number by 0.15. For example, 15% of 80 is 80 × 0.15 = 12. A quick mental trick: find 10% by moving the decimal one place left, then add half of that for the extra 5%.
Percentage change = ((New value − Old value) ÷ Old value) × 100. A positive result is an increase; negative is a decrease. For example, a price rising from $40 to $52 is a 30% increase.
Divide the part by the whole, then multiply by 100. If 30 students out of 120 passed, that is (30 ÷ 120) × 100 = 25%.
Percentage points measure the arithmetic difference between two percentages. If interest rates rise from 3% to 5%, that is a 2 percentage point increase — but a 66.7% percentage change. The distinction matters in finance and statistics.
Divide the final value by (1 + the percentage as a decimal) for increases, or (1 − the decimal) for decreases. If a price after a 20% increase is $120, the original was $120 ÷ 1.20 = $100.