How to use
- What is B% of A? Enter a value and a percent, for example 15% of 200.
- A is what percent of B? Enter the part and the whole, for example 45 out of 50.
- Percentage change from A to B: enter the original value and the new value to see the increase or decrease.
- Increase or decrease A by B%: enter a value and a percent to see both the increased and the decreased value at once.
- Results appear as soon as both boxes are filled. You can enter negative numbers and use a period or a comma as the decimal point.
Formulas
- B% of A = A × B ÷ 100
- A as a percent of B = A ÷ B × 100
- Percentage change from A to B = (B − A) ÷ |A| × 100. A positive result is an increase and a negative one is a decrease.
- A increased by B% = A × (1 + B ÷ 100), and A decreased by B% = A × (1 − B ÷ 100)
Worked examples
- 15% of 200: 200 × 15 ÷ 100 = 30.
- Test score: 45 out of 50 is 45 ÷ 50 × 100 = 90%.
- Sales growth: from $8,000 to $10,000 is (10,000 − 8,000) ÷ 8,000 × 100 = a 25% increase.
- Sale price: $80 with 20% off is 80 × (1 − 0.2) = $64. The same tool shows that $80 increased by 20% is $96.
Common mistakes
- Percentage change depends on the starting value. A rise from 200 to 250 is a 25% increase, but a fall from 250 back to 200 is a 20% decrease.
- Increasing and then decreasing by the same percent does not return the original value: 250 decreased by 20% is 200, but 200 increased by 20% is 240.
- A percent is not a percentage point. If a rate goes from 2% to 3%, it rose by 1 percentage point, which is a 50% increase.
- To undo a percent increase, divide instead of subtracting: a price of 120 after a 20% increase was 120 ÷ 1.2 = 100, not 120 − 20% = 96.
- The change is divided by the absolute value of the starting number, so going from −50 to −25 counts as a 50% increase.
Frequently asked questions
How do I calculate percentage increase?
Subtract the old value from the new value, divide by the old value and multiply by 100. For example, from 200 to 250 is (250 − 200) ÷ 200 × 100 = 25% increase.
How do I calculate percentage decrease?
Use the same formula; a negative result is a decrease. For example, from 250 to 200 is (200 − 250) ÷ 250 × 100 = −20%, a 20% decrease.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. For example, 45 out of 50 is 45 ÷ 50 × 100 = 90%.
How do I calculate a percentage of a number?
Multiply the number by the percent and divide by 100. For example, 15% of 200 is 200 × 15 ÷ 100 = 30.
How do I calculate a discounted price?
In “Increase or decrease A by B%”, enter the original price as A and the discount as B; the decreased value is the sale price. For example, $80 with 20% off is $64.
Is a percent the same as a percentage point?
No. If an interest rate rises from 2% to 3%, it went up by 1 percentage point, but by 50% as a percentage change.
Why can’t I calculate the change from 0?
Percentage change divides by the original value, and division by zero is undefined. The same applies to “A is what percent of B?” when B is 0.