Percentage Calculator
Find what X% of a number is, what percent one number is of another, percent increase or decrease, or the whole from a known part.
How to use the percentage calculator
- 1
Pick a tab: X% of Y, X is what % of Y, % change, or Find the whole.
- 2
Enter the two numbers requested for that mode.
- 3
Your answer updates instantly as you type — no button to click, and it works with decimals and negative numbers too.
The four percentage formulas, with examples
1. X% of Y (find the part)
Formula: (percent ÷ 100) × number
Example 1: What is 20% of 150? (20 ÷ 100) × 150 = 0.2 × 150 = 30.
Example 2: A restaurant bill is $64.00 and you want to leave a 18% tip. (18 ÷ 100) × 64 = 0.18 × 64 = $11.52 tip, for a total of $75.52.
Example 3: A $250 jacket is 30% off. The discount amount is (30 ÷ 100) × 250 = $75, so the sale price is $250 − $75 = $175.
2. X is what percent of Y (find the percent)
Formula: (part ÷ whole) × 100
Example 1: 30 is what percent of 150? (30 ÷ 150) × 100 = 0.2 × 100 = 20%.
Example 2: You scored 42 out of 50 on a test. (42 ÷ 50) × 100 = 0.84 × 100 = 84%.
Example 3: Out of 220 total orders, 11 were returned. (11 ÷ 220) × 100 = 5% return rate.
3. Percent change (increase or decrease)
Formula: ((new − original) ÷ original) × 100
Example 1 (increase): Rent goes from $1,200 to $1,350. ((1,350 − 1,200) ÷ 1,200) × 100 = (150 ÷ 1,200) × 100 = +12.5%.
Example 2 (decrease): A stock drops from $80 to $68. ((68 − 80) ÷ 80) × 100 = (−12 ÷ 80) × 100 = −15%, a 15% decrease.
Example 3 (why order matters): A price rises 20%, then falls 20%. Starting at $100: up 20% gives $120, then down 20% of $120 (not of $100) gives $120 − $24 = $96 — not back to $100, because each percent change is calculated against the value at that step, not the original.
4. Find the whole (reverse percentage)
Formula: part ÷ (percent ÷ 100)
Example 1: 30 is 20% of what number? 30 ÷ (20 ÷ 100) = 30 ÷ 0.2 = 150.
Example 2: A $12 tip was 15% of the bill. The bill total is 12 ÷ 0.15 = $80.
Example 3 (reversing an increase): After a 20% price increase, an item costs $72. Because $72 represents 120% of the original price (100% + 20%), the original price is 72 ÷ 1.20 = $60 — not 72 minus 20% of 72, which would incorrectly give $57.60.
Percentage quick reference
| Percent | Fraction | Decimal | Of $100 |
|---|---|---|---|
| 10% | 1/10 | 0.10 | $10 |
| 12.5% | 1/8 | 0.125 | $12.50 |
| 20% | 1/5 | 0.20 | $20 |
| 25% | 1/4 | 0.25 | $25 |
| 33.3% | 1/3 | 0.333 | $33.33 |
| 50% | 1/2 | 0.50 | $50 |
| 75% | 3/4 | 0.75 | $75 |
A fast mental-math trick: 10% of any number just moves the decimal point one place left (10% of 150 = 15). From there, 5% is half of that (7.50), and 20% is double it (30) — useful for estimating tips and discounts without a calculator.
Everyday uses for percentage math
Percentages show up constantly because they let you compare values of very different sizes on the same scale. A $5 discount means something different on a $10 item (50% off) than on a $500 item (1% off) — the percentage tells you the real significance of the number in a way the raw dollar amount can't.
Shopping and discounts: sale tags, coupon codes, and cashback offers are almost always expressed as a percentage of the original price — use the "X% of Y" mode to find the discount amount, then subtract it from the price.
Tipping and bills: restaurant tips are conventionally 15-20% of the pre-tax bill, and splitting a bill with tax and tip across a group is easiest once you know the exact percentage amount rather than estimating.
Grades and test scores: a score reported as "42/50" is easier to compare across different tests once converted to a percentage (84%) — use the "X is what percent of Y" mode.
Finance and investing: interest rates, loan APRs, investment returns, and inflation are all percentages, and percent change over time (this quarter vs. last quarter, this year vs. last year) is the standard way financial performance is reported.
Statistics and surveys: survey results, poll numbers, and error rates are reported as percentages of a total sample so that results from different sample sizes remain directly comparable.
Curious how your weight relates to a healthy range instead of raw pounds or kilograms? Our BMI Calculator applies the same idea of comparing a value against a standard scale.
Splitting a restaurant bill and figuring out the tip? Try the Tip Calculator →
Frequently asked questions
Related guides
The four percentage formulas you'll actually use — finding a part, finding a percent, percent change, and reversing a percentage — each with worked examples.
Read the guide →The exact formula for percent change, why order and direction matter, and worked examples covering raises, price hikes, discounts, and investment losses.
Read the guide →Why 'a 3% rise' and 'a 3 percentage point rise' mean very different things, with examples from interest rates, tax brackets, and poll numbers.
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