Percentage Calculator
Five percentage modes: of, is what %, % change, increase, decrease.
Overview
The Percentage Calculator covers the five percentage questions people actually ask: what is X% of Y, X is what percent of Y, what is the percent change from X to Y, increase Y by X% and decrease Y by X%. Pick a mode, fill in two numbers and the answer pops out — no more eyeballing whether to multiply or divide.
It is built for shoppers calculating discounts, students doing word problems, marketers computing campaign uplift and small-business owners tracking growth. The arithmetic is trivial; getting the right arithmetic is the part that trips people up.
How it works
Each mode uses a single formula:
- X% of Y:
Y * X / 100. - X is what % of Y:
X / Y * 100. - Percent change:
(Y - X) / X * 100. - Increase by:
Y * (1 + X / 100). - Decrease by:
Y * (1 - X / 100).
Percentages are stored as ratios internally to avoid rounding glitches when chaining operations. The output keeps reasonable precision and the calculator highlights which mode is active so you don't pick the wrong one.
Examples
20% of 150 → 30
30 is what % of 150 → 20%
Change from 80 to 100 → +25%
Increase 200 by 15% → 230
FAQ
Why is a 20% increase not undone by a 20% decrease?
Because the base changes. Increasing 100 by 20% gives 120; decreasing 120 by 20% gives 96, not 100. To reverse, decrease by 20 / 1.20 ≈ 16.67%.
Is "20 percentage points" the same as "20%"?
No. A move from 30% to 50% is a 20-percentage-point increase but a 66.7% relative increase.
Can I chain percentage increases?
Yes, but they don't add. Two 10% increases compound to 21%, not 20%.
What about negative percentages?
Allowed everywhere. A -25% increase is just a 25% decrease, and a percent change can be negative when the value falls.
Why does dividing by 100 matter?
Percentages are fractions out of 100. The conversion only matters when the percent is on its own (5% means 0.05); within 5% of the formula already handles it.