Restaurant bill
An 18% tip on a $240 dinner
Six people, one card. The tip comes to $43.20, so the card gets charged $283.20, or $47.20 per person.
240 × 0.18 = 43.20
Type two numbers into a plain question, read the answer, check the working. Six question types sit side by side: a tip on a bill, a quiz score, a rent rise, the gap between two quotes, a sale price, the pre-tax figure on a receipt.
Pick a question
Your question
What is % of ?
For tips, commissions, deposits, or any slice of a total.
is what percent of ?
Part first, whole second. A score of 42 out of 50 goes in as written.
Going from to is what percent change?
The first number is the base. Press Swap to see the reverse: 1,595 down to 1,450 is a 9.09% drop, not 10%.
What is the percent difference between and ?
No direction here. Both numbers get measured against their average, 400.
What is %?
Pick minus for a sale price, plus for a markup or a tax.
Which number % gives ?
Built for receipts. A $115 total with 15% tax added started at $100. Taking 15% off 115 gives 97.75, a common slip.
Answer
43.2
18% of 240 is 43.2.
The working
Side by side
Each card loads its numbers into the calculator above, so you see the same working on screen. Change one value to match your own case.
Restaurant bill
Six people, one card. The tip comes to $43.20, so the card gets charged $283.20, or $47.20 per person.
240 × 0.18 = 43.20
Rent renewal
The landlord calls this a small bump. The increase is $145 a month, a 10% rise, or $1,740 more over a year.
(1595 − 1450) ÷ 1450 × 100 = 10%
Receipt check
The total includes 15% tax. Divide by 1.15 to get $100. Taking 15% off 115 lands on $97.75, which undercounts by $2.25.
115 ÷ 1.15 = 100
Quiz score
84%, a B on the usual 10-point US scale. Each question is worth 2%, so two more right answers would reach an 88.
42 ÷ 50 × 100 = 84%
Two quotes
A 10% difference against the average. From the cheaper quote, $420 sits 10.5% higher. From the dearer one, $380 sits 9.5% lower. Pick the base before you quote a figure in an email.
40 ÷ 400 × 100 = 10%
Sale price
You pay $44.80, saving $19.20. A second 20% coupon on top brings the price to $35.84, which is 44% off the tag, not 50%.
64 × 0.70 = 44.80
Six formulas, all built from one idea: a percent is a number divided by 100. The third column uses the sample numbers loaded in each mode.
| Question | Formula | With sample numbers |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 240 × 18 ÷ 100 = 43.2 |
| X is what % of Y? | X ÷ Y × 100 | 42 ÷ 50 × 100 = 84% |
| Percent change | (New − Old) ÷ |Old| × 100 | 145 ÷ 1450 × 100 = 10% |
| Percent difference | |A − B| ÷ ((A + B) ÷ 2) × 100 | 40 ÷ 400 × 100 = 10% |
| Add or remove X% | Y × (1 ± X ÷ 100) | 64 × 0.70 = 44.8 |
| Reverse a percentage | Result ÷ (1 ± X ÷ 100) | 115 ÷ 1.15 = 100 |
A savings rate moving from 4% to 5% gets described two ways. Both are correct. Headlines mix the two up often, so check which one a report means.
The gap between two percentages. Use points when both numbers are already rates: interest, tax, unemployment, poll results.
The relative change in the rate itself, since 1 is a quarter of 4. Run the Percent change mode with 4 as the start, 5 as the new value, to see the same result.
The arithmetic is short. The mistakes come from picking the wrong base, or from edge cases the calculator refuses to fake.
A 20% drop followed by a 20% rise leaves 100 at 96, not 100. Multiply the factors instead: 0.8 × 1.2 = 0.96.
A share price falling from 200 to 100 lost 50%. Climbing back to 200 needs a 100% gain, because the base is now half the size.
Tax was added to the pre-tax price, not the total. Divide the total by 1 + rate. Use the Reverse mode for receipts.
Sales going from 0 to 12 units have no percent change, since the math divides by the start. The calculator shows a message instead of an infinite figure. Report the raw count.
Profit moving from −200 to −100 reads +50% here, because the calculator divides by the absolute value of the start. Some finance reports show −50% for the same move. Match the convention your report already uses.
Answers round to the decimals you pick, up to 6. The math runs on standard browser numbers, accurate to about 15 digits. Fine for prices, grades, growth rates. Invoices with strict cent rounding rules belong in accounting software.
The fractions below cover most shop signs, recipe scaling, exam marks. Learn a handful by heart to sanity-check any answer before you copy the result.
| Fraction | Decimal | Percent |
|---|---|---|
| 1/2 | 0.5 | 50% |
| 1/3 | 0.333… | 33.33% |
| 2/3 | 0.667… | 66.67% |
| 1/4 | 0.25 | 25% |
| 3/4 | 0.75 | 75% |
| 1/5 | 0.2 | 20% |
| 1/8 | 0.125 | 12.5% |
| 1/10 | 0.1 | 10% |
| 1/20 | 0.05 | 5% |
| 1/40 | 0.025 | 2.5% |
Short answers with the numbers worked through.
Multiply the number by the percent, then divide by 100. For 18% of 240: 240 × 18 = 4,320, then 4,320 ÷ 100 = 43.2.
Percent change has a direction. The old value is the base, so 380 to 420 is a 10.5% rise. Percent difference has no direction. Both values get compared with their average, so 380 vs 420 is a 10% difference either way round. Use change for before-and-after figures, difference for two readings of equal standing.
Divide the total by 1 plus the tax rate as a decimal. A $115 total at 15% tax: 115 ÷ 1.15 = $100. Subtracting 15% from $115 gives $97.75, which is wrong because the tax was added to $100, not to $115.
The base shrinks after the loss. 200 down to 100 is a 50% drop measured from 200. Getting from 100 back to 200 is a 100% rise measured from 100.
No. An interest rate moving from 4% to 5% rose 1 percentage point, which is a 25% increase in the rate itself. Points measure the gap between two percentages. Percent measures the relative change.
Percent change divides by the starting value, so a start of zero has no answer. The page shows a message instead of a misleading infinite figure. Quote the raw change instead, for example up 12 units from zero.
No. Every calculation runs in your browser. Your numbers leave the page only when you press Copy link, which puts them in the URL so whoever opens the link sees the same question.