Percentage Calculator (What % of, % of a number, reverse)
Three basic percentage calculations in one: what percent A is of B, a given percent of a number, and the whole from a part and its percentage. Enter two numbers to instantly get the percent or amount, with your choice of decimal places and rounding.
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Input
Calculation type
Result
A as a percent of B
15%
30 is 15% of 200.
How it works
- 'What percent is A of B?' divides the compared value A by the base value B and multiplies by 100 (percent = A ÷ B × 100). Example: 30 is 15% of 200.
- 'What is X% of a number?' multiplies the base by the percentage and divides by 100 (amount = base × percent ÷ 100). Example: 15% of 800 is 120.
- 'A is X% of what?' divides the part by the percentage and multiplies by 100 to recover the whole (whole = part ÷ percent × 100). Example: if 120 is 15%, the whole is 800.
- A base value or percentage of 0 cannot be computed (it would divide by zero); check your inputs in that case.
- Results are shown rounded to the number of decimal places you choose, using the selected rounding mode (round half up, round down, or round up).
Worked examples from everyday shopping
Say an $80 jacket is 25% off. Choose the "% of a number" mode, enter 80 as the base value and 25 as the percentage, and you get the discount: $20. The price you pay is 80 − 20 = $60. A shortcut is to enter 75 (100 − 25) as the percentage, which gives the sale price directly.
Tipping works the same way. An 18% tip on a $64.50 restaurant bill is 64.50 × 18 ÷ 100 = $11.61.
The "A is X% of what" mode answers reverse questions, such as removing tax from a tax-inclusive price. In countries with a 20% VAT, a receipt total of 144 includes the tax, so the net price is the amount that 144 is 120% of. Enter 144 as the part value and 120 as the percentage, and the answer is 120. A common mistake is to take 20% off the gross price instead: 144 × 0.8 = 115.20, which is wrong because the tax was charged on the net price, not on the total.
Why percentages do not simply add up
Every time a percentage is applied, the base changes. That is why successive percentages multiply rather than add.
- 20% off, then an extra 10% off at the register: 0.8 × 0.9 = 0.72, so the total discount is 28%, not 30%.
- A 20% price increase followed by a 20% decrease: 1.2 × 0.8 = 0.96, leaving the price 4% below where it started.
- If store A charges 1,200 and store B charges 1,000, A is 120% of B (20% more expensive), but B is about 83.33% of A (about 16.67% cheaper). Before you calculate, decide which number is the 100% baseline and put that in the base value field.
Choosing decimal places and rounding
If you score 47 out of 60 on a quiz, the exact result is 78.333...%. With two decimal places and standard rounding the tool shows 78.33%; with zero decimal places it shows 78%.
For things that only come in whole units, pick the rounding direction based on the goal. Suppose a motion needs at least 60% approval from 37 members. The "% of a number" mode gives 22.2 votes. Since 22 votes would fall short of 60%, the correct threshold is 23, which you get with zero decimal places and Round up. When you must stay under a limit, such as spending no more than a certain percent of a budget, Round down is the safe choice.
Quick checks before you calculate
- Enter percentages as whole percent values: type 15 for 15%, not 0.15.
- Make sure both amounts use the same unit, for example grams with grams and minutes with minutes. Mixing units throws the answer off by a factor of 60 or 1,000.
- A result above 100% is perfectly normal when the compared value is larger than the base value.
Frequently asked questions
- How do I find what percent A is of B?
- Divide the compared amount A by the base amount B and multiply by 100 (percentage = A ÷ B × 100). For example, 30 is 15% of 200.
- What is the difference between finding a percent of a total and working the total back out?
- To get a percent of a total, use total × percent ÷ 100 (15% of 800 is 120). To reverse it, divide the part by the percent and multiply by 100 (if 120 is 15%, the total is 800).
- Can it round results and handle edge cases?
- Yes. You can choose the number of decimal places and the rounding mode (round, floor, or ceiling). Note that it cannot compute when the base amount or the percentage is zero, since that would divide by zero.
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