How to use it
- Pick the question you want to answer.
- Enter the two numbers.
- Click Calculate.
- Read the result with the full equation shown.
The three percentage questions
Almost every everyday percentage problem is one of these three:
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of $80 = $12 |
| X is what percent of Y? | X ÷ Y × 100 | 18 of 24 = 75% |
| Percent change from old to new | (new − old) ÷ old × 100 | $40 → $50 = +25% |
Choose the matching tab, type the two numbers and the calculator shows the result together with the equation, so you can check the logic. You can type numbers with thousands separators, like 1,250.50. The calculation runs in your browser.
Worked examples
Sales tax. A $249 item with 7% sales tax: 7% of 249 = 249 × 0.07 = $17.43 tax, so the total is $266.43.
Tip. A $64 restaurant bill with a 20% tip: 20% of 64 = $12.80. A quick mental check: 10% is $6.40, so 20% is twice that.
Test score. 42 correct answers out of 50: 42 ÷ 50 × 100 = 84%.
Price increase. Rent goes from $1,600 to $1,720 a month: (1,720 − 1,600) ÷ 1,600 × 100 = +7.5%.
Discount. A jacket marked down from $120 to $90: (90 − 120) ÷ 120 × 100 = −25%.
Percent change vs. percentage points
These two are often mixed up. If a savings rate rises from 4% to 5%, it went up by 1 percentage point, but by 25 percent relative to where it started ((5 − 4) ÷ 4 × 100).
Use percentage points when you compare two rates directly (interest rates, unemployment rates, poll numbers). Use percent change when you want the relative size of the move. News headlines that say a rate "jumped 25%" and others that say it "rose one point" may be describing the same change.
Common mistakes
- Ups and downs don't cancel. A 20% increase followed by a 20% decrease leaves you 4% below where you started: 100 → 120 → 96.
- Reversing a discount. If a price is 25% off and you know the sale price, divide by 0.75, don't add 25% back. $90 ÷ 0.75 = $120.
- Wrong base. Percent change is always measured against the old value. Going from 50 to 75 is +50%, but going back from 75 to 50 is −33.3%.
- Zero as the base. You can't compute a percent change from zero, and you can't ask what percent of zero something is; the calculator tells you when that happens.
Quick mental shortcuts
- 10%: move the decimal point one place left. 10% of 340 = 34.
- 5%: half of 10%. 5% of 340 = 17.
- 1%: move the decimal two places. 1% of 340 = 3.4.
- Percentages are reversible: 8% of 50 equals 50% of 8, which is 4.
- 25%, 50%, 75%: divide by 4, divide by 2, or take three quarters.
For money questions with more steps, try the tip calculator or the margin calculator.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 30% of 250 is 250 × 30 ÷ 100 = 75.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. 45 out of 60 is 45 ÷ 60 × 100 = 75%.
How do I calculate percent increase or decrease?
Subtract the old value from the new value, divide by the old value and multiply by 100. A positive result is an increase, a negative result is a decrease.
Can I enter numbers with commas?
Yes. Thousands separators like 12,500 are accepted, and you can use a decimal point for fractions.
Why can't the old value be zero?
Percent change divides by the old value, and dividing by zero is undefined. Any increase from zero is infinitely large in relative terms.
Last updated: November 12, 2026