Percentage Calculator — Percent Of, Change & More
Three of the most common percentage questions in one place: what is X% of a number, what percent one value is of another, and the percent increase or decrease between two numbers. Type your figures and the answers update instantly — handy for discounts, tips, test scores, markups, and tracking change over time.
What is X% of Y
Multiply the number by the percentage over 100. 15% of 80 is 0.15 × 80 = 12. This is the everyday case behind discounts (20% off), tips, sales tax, and commission. The result is in the same units as the base number.
X is what percent of Y
Divide the part by the whole and multiply by 100. 30 out of 200 is 30 ÷ 200 × 100 = 15%. Use it for test scores, completion rates, market share, or what fraction of a budget a line item takes.
Percent change
Subtract the old value from the new, divide by the old, and multiply by 100. From 120 to 150 is a +25% increase; from 150 to 120 is a −20% decrease. The two aren’t symmetric — the base you divide by changes.
Common mistakes
Confusing percent change with percentage points (a rise from 5% to 7% is +2 points but +40%); dividing by the new value instead of the old; and forgetting that a 50% fall needs a 100% rise to recover.
Frequently asked questions
- What’s the difference between percent and percentage points?
- A move from 5% to 7% is a rise of 2 percentage points, but a 40% increase in relative terms. Use points when comparing two percentages directly, and percent change when describing how much one number grew.
- Why isn’t a 20% rise cancelled by a 20% fall?
- Because the base changes. 100 up 20% is 120; 120 down 20% is 96, not 100. To undo a 20% fall you need a 25% rise — the larger base on the way down makes the percentages asymmetric.
- How do I work out the original price before a discount?
- Divide the sale price by one minus the discount. A $80 item at 20% off was 80 ÷ 0.8 = $100. Subtracting 20% of $80 gives the wrong answer because the discount applied to the higher original price.
How we calculate it
Percent of: number × percent ÷ 100. Percent one value is of another: part ÷ whole × 100. Percent change: (new − old) ÷ old × 100, so it is signed — positive for an increase, negative for a decrease — and always relative to the original value.
What it doesn't do
- Compound percentage growth over many periods (use Compound Interest)
- Tax brackets and effective rates (use the Tax Estimator)
- Percentage points vs percent — read the labels carefully
Last reviewed: 2026-05