How to use the percentage calculator
- Type the first value into Number A.
- Type the second value into Number B.
- Read the three results. They update as you type:
- A% of B: treats A as a percentage and applies it to B.
- A is this percent of B: shows how big A is compared with B.
- Percent change from A to B: treats A as the old value and B as the new value.
Decimals and negative numbers work too. Use Copy result to paste all three answers somewhere else.
Formula
A% of B = A × B ÷ 100
A as % of B = A ÷ B × 100
% change A → B = (B − A) ÷ |A| × 100- A, B: the two numbers you enter.
- |A|: the absolute value of A. Using it means a move from −50 to −25 counts as an increase, which matches everyday meaning.
Worked examples
Example 1: a discount
A jacket costs $80 and is 15% off. Enter A = 15 and B = 80.
- 15% of 80 = 80 × 15 ÷ 100 = $12 off.
- You pay 80 − 12 = $68.
Example 2: a test score
You scored 42 out of 48. Enter A = 42 and B = 48.
- 42 ÷ 48 × 100 = 87.5%.
Example 3: a price change
Your electricity bill went from $120 to $138. Enter A = 120 and B = 138.
- (138 − 120) ÷ 120 × 100 = 15% increase.
If it later falls from $138 back to $120, enter A = 138 and B = 120. The change is (120 − 138) ÷ 138 × 100 = −13.04%. The fall is a smaller percentage than the rise because it starts from a larger number.
How to interpret the results
- A negative percent change is a decrease. −25% means the new value is a quarter smaller than the old one.
- A percent of more than 100% means A is bigger than B. 150% means A is one and a half times B.
- Rounding: results show up to four decimal places. For money, round to two decimals. For reports, one decimal place is usually enough.
Tips and common mistakes
- Know your base. “20% of what?” is the key question. A percentage change is always relative to the starting value.
- Percentage points aren’t percent. A rate going from 4% to 5% rose by 1 percentage point, but by 25%.
- Changes don’t cancel out. A 50% fall followed by a 50% rise leaves you at 75% of where you started, not 100%.
- Stack discounts by multiplying. 20% off and then another 10% off is 28% off in total (0.8 × 0.9 = 0.72), not 30%.
- Round at the end. Rounding intermediate steps can shift the final answer, especially with several calculations in a row.
Assumptions and limitations
- Percent change uses A as the starting value. If you want the change measured from B instead, swap the numbers.
- Percent change from zero, and “what percent of zero”, are undefined, so the calculator shows a dash.
- For averages of percentages, such as average growth over several years, a simple percent change is not enough. Compound growth needs a separate calculation.