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Ratio Calculator

Simplify a ratio to its lowest terms, scale one up to find a missing part, or split an amount in a given ratio. Two or three parts, decimals allowed, and the arithmetic shown each time.

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The ratio

Decimals are fine — 1.5:2 simplifies to 3:4. Leave the third box empty for a two-part ratio.

Loaded with an example. Type over any field.

Simplest form

3:4

12:16 simplifies to 3:4

Divided by
4
As a decimal
0.75
Part 1 of the whole
42.9%
Part 2 of the whole
57.1%

The working

  1. Largest number that divides all of them: 4
  2. 12 ÷ 4, 16 ÷ 4 → 3:4
  3. As a single number: 3 ÷ 4 = 0.75

Check this before you rely on it. A free everyday calculator, provided as a guide — not tax, accounting, financial or legal advice. The working is shown above so you can verify it against your own figures. See our terms of use.

How it works

STEP 1

Pick what you are doing

Simplifying an awkward ratio, scaling one up to find a value you do not have, or dividing an amount between people. They are three different questions and the tabs keep them apart.

STEP 2

Type the parts

Two boxes, or three if you need them. Decimals are fine — 1.5:2 is a real ratio and it simplifies to 3:4, which is the answer most calculators of this kind get wrong.

STEP 3

Read the working

It shows what the parts were divided by, or the cross-multiplication, so the answer can be checked rather than taken on trust.

What this doesn't cover

Knowing where a calculator stops is what makes the rest of it trustworthy.

  • UnitsA ratio has none — it compares like with like. If you are mixing 2 parts to 3 parts, both have to be measured the same way. Two litres to three millilitres is not 2:3.
  • Rounding in a shareSplitting £100 three ways gives £33.33 each with a penny left over. The shares here are exact and add up to the total; if you are paying real money out, someone has to take the odd penny.
  • OrderA ratio is not a set. 2:3 and 3:2 are different answers, and the calculator keeps the order you typed rather than sorting it.
  • Aspect ratios of a specific screenThe maths is the same, but a screen described as 16:9 may not be exactly that once you account for its real pixel count. Use the actual pixel dimensions if it matters.
The guideHow to simplify a ratio, and how to share an amount in oneTo simplify a ratio, divide every part by their greatest common divisor — 12:16 divided by 4 is 3:4. To share an amount, add the parts, divide the amount by that total, then multiply by each part.Read it

Common questions

How do you simplify a ratio?

Find the largest number that divides every part exactly — the greatest common divisor — and divide them all by it. For 12:16 that number is 4, giving 3:4. This calculator shows the divisor it used, so you can see why the answer is what it is.

Can a ratio have decimals in it?

Yes, and it still simplifies. Multiply every part by ten, or a hundred, until they are whole numbers, then reduce as normal. 1.5:2 becomes 15:20, which reduces to 3:4. The calculator does this step for you and shows it in the working.

How do I divide an amount in a ratio?

Add the parts together to get the total number of shares, divide the amount by that to find what one share is worth, then multiply each part by it. £500 in 2:3 is 5 shares, £100 each, so £200 and £300.

How do I find a missing number in a ratio?

Cross-multiply. In a:b = c:d the two diagonals are equal, so a×d = b×c, and any one of the four follows from the other three. For 3:4 = 9:? that gives 3 × ? = 4 × 9, so the answer is 12.

What is a ratio as a fraction or a decimal?

For a two-part ratio, a:b is the fraction a/b, and dividing one by the other gives the decimal. 3:4 is 3/4 or 0.75. A three-part ratio has no single decimal, which is why the calculator shows a dash there instead of making one up.

What is the difference between a ratio and a proportion?

A ratio compares two or more quantities — 3:4. A proportion says two ratios are equal — 3:4 = 9:12. The middle tab solves proportions; the first simplifies ratios.

Does 2:3 mean two out of three?

No, and this is the most common misreading. 2:3 means two parts to three parts, so five parts in total — two out of five, or 40%. "Two out of three" would be written 2:1 as a ratio. The calculator shows each part as a percentage of the whole to make this unambiguous.

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