Percentage calculator
Pick the question, enter X and Y, and read the answer with the formula used.
15% of 80 is
12.0000
80 × 15 ÷ 100
How to use it
- Pick the question you're asking. Each one puts X and Y in the order the sentence reads.
- Enter X and Y. Percentages go in as plain numbers: 15 for 15%.
- Read the answer. The formula row shows exactly what was calculated so you can check it.
Worked example
15% of 80: 80 × 15 ÷ 100 = 12.
12 is what percent of 80: 12 ÷ 80 × 100 = 15%.
From 80 to 92: (92 − 80) ÷ 80 × 100 = a 15% increase. Going back from 92 to 80 is only a 13.04% decrease, because the base changed.
How this is calculated
X% of Y is Y × X ÷ 100. X as a percent of Y is X ÷ Y × 100. The percent change from X to Y is (Y − X) ÷ X × 100. Y increased by X% is Y × (1 + X ÷ 100), and decreased is Y × (1 − X ÷ 100). Results are rounded to four decimal places and percentages to three, so the number you read is the one you'd get on paper.
What is X% of Y: Y × X ÷ 100
X is what percent of Y: X ÷ Y × 100
Percent change from X to Y: (Y − X) ÷ X × 100
Y increased by X%: Y × (1 + X ÷ 100); decreased: Y × (1 − X ÷ 100)
Results are rounded to four decimal places; percentages to three.
Rates, sources and limits
Last reviewed 13 Sep 2026 by the CalcRabbit team. Estimates only, not financial, tax or medical advice.
Frequently asked questions
Why isn't a 15% increase undone by a 15% decrease?
Because the percentage is taken of a different base each time. 80 up 15% is 92; 92 down 15% is 78.20. To get back to 80 from 92 you need a 13.04% decrease. This is why a stock that falls 50% has to rise 100% to recover.
How do I work out a percentage in my head?
Find 10% by moving the decimal point one place left, then scale. 15% of 80: 10% is 8, 5% is 4, so 15% is 12. For 15% of 78, use 15% of 80 minus 15% of 2.
What's the difference between percent and percentage points?
If a rate goes from 4% to 6%, that's a rise of 2 percentage points but a 50% increase in the rate. Use the percent change mode for the second figure.
Does this handle negative numbers?
Yes. Percent change uses the absolute value of the starting number as the base, so going from −20 to −10 shows as a 50% increase, which is the usual convention.