Choose % of in the percentage calculator, type the percentage and the number, and it shows the answer with the working.
Calculate a percentageThe method
Percentage of a number = percentage ÷ 100 × number
- Turn the percentage into a decimal. Divide by 100, which moves the decimal point two places left: 15% is 0.15, 7.5% is 0.075, 120% is 1.2.
- Multiply by the number. 0.15 × 80 = 12, so 15% of 80 is 12.
“Percent” means “per hundred”, so 15% of something is 15 parts out of every 100. The decimal is just that fraction, 15/100, written another way.
Worked examples
| Question | Working | Answer |
|---|---|---|
| 20% of 150 | 0.20 × 150 | 30 |
| 15% of $64 | 0.15 × 64 | $9.60 |
| 7.5% of 2,400 | 0.075 × 2,400 | 180 |
| 18% GST on ₹2,500 | 0.18 × 2,500 | ₹450 |
| 0.5% of $30,000 | 0.005 × 30,000 | $150 |
| 125% of 80 | 1.25 × 80 | 100 |
A percentage over 100 gives an answer bigger than the number, and one under 1% needs care with the zeros: 0.5% is 0.005, not 0.05.
Doing it in your head
- 10%: move the decimal point one place left. 10% of 64 is 6.4.
- 5%: half of 10%. 5% of 64 is 3.2.
- 1%: move it two places. 1% of 64 is 0.64.
- 50% and 25%: halve the number, or halve it twice. 25% of 360 is 180, then 90. 75% is the two added: 180 + 90 = 270.
- Build the rest from those. 15% is 10% + 5%: 6.4 + 3.2 = 9.6. 18% is 20% − 2%: 12.8 − 1.28 = 11.52.
- Swap them. x% of y is always the same as y% of x. 8% of 50 is hard; 50% of 8 is 4, and that’s the answer.
What percent is one number of another?
Turn the question around: divide the part by the whole, then multiply by 100.
Percent = part ÷ whole × 100
42 out of 50 on a test: 42 ÷ 50 × 100 = 84%
$45 of a $180 budget spent: 45 ÷ 180 × 100 = 25%
The “whole” is whatever you are comparing against, and it goes underneath. Swapping them gives a different answer: 50 is 119% of 42. For a change between an old and a new value, the percentage increase guide covers the formula and its traps.
A percentage of a percentage
Multiply the two decimals together, then turn the result back into a percent.
40% of 25% = 0.40 × 0.25 = 0.10, which is 10%
That answers questions like “25% of the class chose science, and 40% of those chose physics: what share of the whole class chose physics?” The answer is 10% of the class.
It’s also why two discounts in a row aren’t added together:
| $200 item | Working | Price |
|---|---|---|
| 20% off | 200 × 0.80 | $160 |
| Then 10% off that | 160 × 0.90 | $144 |
| Total | 0.80 × 0.90 = 0.72 of the price | 28% off, not 30% |
The discount calculator shows the sale price and the saving for a single discount.
Reverse percentages: finding the original
When you know the number after a percentage was added or taken off, divide by the multiplier. Never take the same percentage off the new number.
| You know | Working | Original |
|---|---|---|
| A jacket is $60 after 25% off | 60 ÷ 0.75 | $80 |
| A bill is ₹1,180 including 18% GST | 1,180 ÷ 1.18 | ₹1,000 (GST ₹180) |
| Pay is $49,920 after a 4% rise | 49,920 ÷ 1.04 | $48,000 |
| 30 people is 12% of a group | 30 ÷ 0.12 | 250 people |
The usual mistake with the jacket is to add 25% back to $60, which gives $75. That’s wrong because the 25% was taken from $80, a bigger number.
Everyday examples
- A tip. 18% of a $46.50 restaurant bill is 0.18 × 46.50 = $8.37. Rounding the bill to $46 in your head first gives $8.28, close enough to decide.
- Sales tax added at the till. A $35 item with 8.25% tax: 0.0825 × 35 = $2.89 in tax, so $37.89 in total. Multiplying by 1.0825 gives the total in one step.
- Marks needed to pass. If a 75-mark paper needs 40% to pass, that’s 0.40 × 75 = 30 marks. A score of 54 out of 75 is 54 ÷ 75 × 100 = 72%.
- A deposit. A 10% deposit on a ₹6,50,000 car is 0.10 × 6,50,000 = ₹65,000, leaving ₹5,85,000 to pay or borrow.
- Interest for one year. 4.5% a year on $12,000 is 0.045 × 12,000 = $540 for the first year. Over several years interest is usually earned on interest too, so the total isn’t just $540 × years.
In every case the steps are the same: turn the percentage into a decimal, multiply, and then add or subtract the result if the question asks for a new total.
In Excel or Google Sheets
Spreadsheets understand the % sign, so =15%*80 gives 12. With the number in A1 and the percentage in B1:
- Percent of a number: =A1*B1, with B1 typed as 15% (or =A1*B1/100 if B1 holds plain 15).
- What percent: =A1/B1, then format the cell as a percentage.
- Original before a discount: =A1/(1-B1).
If an answer comes out 100 times too big or small, check whether the cell holds 15% or 15; they are different numbers to the spreadsheet.
Common mistakes
- Forgetting to divide by 100. 15 × 80 = 1,200 is not 15% of 80.
- Misplacing small percentages. 0.5% is half of one percent, 0.005 as a decimal.
- Adding percentages of different things. 10% of your income plus 10% of your partner’s isn’t 20% of either income.
- Percent versus percentage points. A rate going from 4% to 5% rose by 1 percentage point, which is a 25% increase.
- Rounding too early. Keep the full decimal until the last step, especially with money.