How to find a percentage on a calculator
Three different questions get called finding a percentage, and each is one or two presses on the calculator above. Here are all three, plus why the % key behaves differently on different calculators.
12 × 12 = 144
Adding 12 to itself 12 times gives the same answer.
Key sequences differ from one calculator to the next, so what follows describes the arithmetic in principle rather than any particular machine. The decimal method below uses nothing but the four basic keys and behaves identically everywhere. The % key is the part that varies, and no single description of it holds for every model — test yours rather than trusting a rule someone quotes.
A percentage of a number
Move the decimal point two places left, then multiply. 15% becomes 0.15, so 15% of 240 is 240 × 0.15 = 36. This works on any calculator on earth, because it uses nothing but multiplication.
What percentage one number is of another
Divide the part by the whole, then multiply by 100. 36 out of 240 is 36 ÷ 240 = 0.15, and 0.15 × 100 = 15%. The part always goes in first. If the answer comes back above 100, you divided the wrong way round.
Percentage increase or decrease
- Subtract the old value from the new one.
- Divide that difference by the old value.
- Multiply by 100.
From 80 to 92: 92 − 80 = 12, then 12 ÷ 80 = 0.15, then × 100 gives a 15% rise. Dividing by the new value instead is the usual slip, and it always makes a rise look smaller than it was.
Why the % key is unreliable
That key does not do the same job on every machine. On many calculators it simply divides by 100 the moment you press it. On others it reads the operation in progress and finishes it for you, so 200 × 15 % returns 30, while 200 + 15 % returns 230 on some models and 200.15 on others.
Test yours on something you can check in your head — 50 × 10 % should be 5 — or skip the key entirely and use decimals, which never surprise you.
The check that takes two seconds
10% is one decimal place: 10% of 240 is 24. 1% is two places: 2.40. Build a rough version of your answer from those and compare. 15% is 10% plus half of 10%, so 24 + 12 = 36 — which matches. If your calculator says 360 or 3.6, you have a decimal point in the wrong place.
Two common jobs use exactly these steps: the discount percentage calculator and the win percentage calculator. More on the calculators page.