How to use the percentage calculator
- Pick the question you want answered from the Calculation menu. The two input labels change to match it.
- Type your two numbers. Decimals and negative numbers are fine; commas are ignored.
- Press Calculate. The answer appears on the right with the formula used, so you can check it on paper. Reset clears everything.
Use “percent of” for things like a 15% deposit or a 7% commission, “what percent” for test scores or budget shares, and “percent change” for price moves, pay raises, or traffic growth.
Quick mental checks: 10% of any number just moves the decimal point one place left (10% of 80 is 8), 5% is half of that, and 1% moves it two places. Combine them to sanity-check the tool: 15% is 10% plus 5%, so 15% of 80 is 8 + 4 = 12. Percentages also work in reverse, since X% of Y always equals Y% of X. That means 8% of 50 is the same as 50% of 8, which is 4.
How it's calculated
All three modes come from the same idea: a percent is a fraction of 100.
X as a percent of Y = (X ÷ Y) × 100
Percent change = (new − original) ÷ |original| × 100
Percent change is measured against the original value, which is why a rise from 50 to 100 is +100% but a fall from 100 back to 50 is only −50%. The tool divides by the absolute value of the original so the sign of the answer always tells you the direction: positive is an increase, negative is a decrease. Percent change from zero is undefined, so the tool asks for a non-zero starting value.
Worked example
Percent of: a $80 jacket needs a 15% deposit. 15 ÷ 100 × 80 = 12, so the deposit is $12.
What percent: you answered 25 questions out of 200 in a practice set. 25 ÷ 200 × 100 = 12.5%.
Percent change: a monthly bill went from $50 to $65. (65 − 50) ÷ 50 × 100 = +30%. If it later drops from $80 to $60, that is (60 − 80) ÷ 80 × 100 = −25%.
FAQ
How do I work out a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 20% of 150 is 0.20 × 150 = 30.
How do I calculate percent increase?
Subtract the original value from the new value, divide by the original value, and multiply by 100. Going from 40 to 50 is (50 − 40) ÷ 40 × 100 = 25%.
Why is a 50% drop not undone by a 50% rise?
Because each percentage is measured from a different starting point. 100 falls 50% to 50, but a 50% rise from 50 only gets you to 75. You would need a 100% rise to get back to 100.
What is the difference between percent and percentage points?
Percentage points are the plain difference between two percentages. A rate moving from 4% to 5% rose by 1 percentage point, which is a 25% relative increase.