How to Calculate a Percentage (Of, Is, and Change)
Published 2026-11-02
Percentage calculator is a core math search because “percent” is three different classroom problems: what is X% of Y, what percent is X of Y, and how much did a number change. Mixing them is the usual error.
Use the free tool
Run these formulas with your own numbers. Math stays in your browser.
Open Percentage Calculator →Our percentage calculator does all three. Checkout problems have their own tools: tip, discount and sales tax, so the labels match the receipt.
Open the percentage calculator
Choose the mode, enter the two numbers and read the result with the formula written out. Use the discount calculator for “20% off,” the tip calculator for gratuity and the sales-tax calculator to add or extract tax.
The three formulas
“Of” multiplies. “Is” divides. Change divides the difference by the original, then multiplies by 100. The original must not be zero.
result = Y × (X ÷ 100)
percent = (X ÷ Y) × 100
((new − old) ÷ old) × 100
Worked examples
15% of 200 is 30. That is the default “of” story on the calculator.
What percent is 40 of 50? (40 ÷ 50) × 100 = 80%.
A price from $80 to $100 is a 25% increase. From $100 to $80 is a 20% decrease — the base changed, so the percents are not mirrors.
| Story | Math | Result |
|---|---|---|
| 15% of 200 | 200 × 0.15 | 30 |
| 40 is what % of 50 | 40 ÷ 50 | 80% |
| 80 → 100 | (20 ÷ 80) × 100 | +25% |
| 100 → 80 | (−20 ÷ 100) × 100 | −20% |
Percent off vs percent of
“20% off $50” is a discount: sale price $40. “20% of $50” is $10, which happens to be the savings, not the price you pay. Use the discount calculator when you want the sale price and the dollars saved. “Percentage calculator,” “percent of” and “percent change” are three labels for the modes on one page — pick the mode that matches the English sentence. Tip, tax and markdown have their own tools so the result is labeled the way a receipt is labeled.
- Decide which of the three questions you are asking.
- Enter the numbers in that mode.
- For a tip, tax or sale price, switch to the matching money tool.
- For grades or GPA, use the grade or GPA calculators — they weight scores, not a single percent-of.
Use the free tool
Run these formulas with your own numbers. Math stays in your browser.
Open Percentage Calculator →Frequently Asked Questions
Why is a 50% decrease then 50% increase not back to the start?
Each percent uses a new base. 100 → 50 (−50%), then 50 → 75 (+50%).
Can the percent be over 100?
Yes. 150% of 10 is 15. A change from 10 to 25 is +150%.
Is percent the same as percentage points?
No. A rate from 10% to 12% rose 2 percentage points, which is a 20% relative increase.
Where is compound percent?
Repeated growth is the compound interest calculator.
Does this use eval?
No. The site never evaluates typed math as code. The percentage tool is arithmetic on the fields you fill.
Related calculators
Percentage Calculator
Find X% of Y, what percent one number is of another, and percent change.
Tip Calculator
Tip amount and per-person split for any bill.
Discount Calculator
Sale price and savings from a percent off or dollar discount.
Sales Tax Calculator
Add sales tax to a price or remove it from a total.