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Ratio calculator

Three ratio jobs in one place: put a ratio in its simplest form, find the missing number in a proportion (scaling recipes, maps and screen sizes), or split an amount between people in a ratio — with the working shown like a maths teacher would.

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  • Private — runs in your browser

How do you simplify a ratio?

  1. 1Choose what you want to do: simplify, solve a proportion, or share an amount.
  2. 2Type the ratio with colons (for example 12 : 18) or fill in A, B and C.
  3. 3Read the answer, the ratio as 1 : n and as percentages, and the steps underneath.

Ratio formulas

  1. 01Simplest form: divide every part by their greatest common divisor (GCD)
  2. 02Proportion A : B = C : D → D = B × C ÷ A
  3. 03Sharing an amount: one part = amount ÷ (sum of the parts); each share = its part × one part

A ratio compares quantities, so multiplying or dividing every part by the same number doesn't change it. To simplify 12 : 18, find the biggest number that divides both — 6 — and divide: 2 : 3. Decimals are turned into whole numbers first: 1.5 : 2 becomes 15 : 20, which simplifies to 3 : 4.

In a proportion two ratios are equal, so the cross products are equal: A × D = B × C. That is why D = B × C ÷ A. To share 12,000 in the ratio 2 : 3 : 5, there are 2 + 3 + 5 = 10 parts, one part is 1,200, and the shares are 2,400, 3,600 and 6,000.

Good to know

  • Use the same units on both sides before you compare: 50 cm : 2 m is 50 : 200, which is 1 : 4.
  • A ratio isn't a fraction of the whole: in 2 : 3 the first part is 2/5 (40%) of the total, not 2/3.
  • For screen and photo sizes, the proportion mode keeps the aspect ratio when you change one side.

Ratio calculator — common questions

How do I simplify a ratio?

Divide every part by the greatest number that goes into all of them. 12 : 18 has a greatest common divisor of 6, so it simplifies to 2 : 3.

How do I divide an amount in a ratio?

Add the parts of the ratio, divide the amount by that total to get one part, then multiply by each part. 12,000 in 2 : 3 : 5 is 10 parts of 1,200: 2,400, 3,600 and 6,000.

How do I solve a proportion?

Cross-multiply. If A : B = C : ?, the missing number is B × C ÷ A. For 4 : 300 = 6 : ?, it is 300 × 6 ÷ 4 = 450.

How do I turn a ratio into percentages?

Divide each part by the sum of all parts and multiply by 100. In 2 : 3 the parts are 2/5 = 40% and 3/5 = 60%.

Can a ratio have decimals?

Yes. Multiply every part by 10, 100 or 1,000 until they are whole numbers, then simplify. 0.5 : 1.25 becomes 50 : 125, which is 2 : 5.

Sources

  • Ratio and proportion — standard school mathematics

Formulas last reviewed: September 30, 2026 ·How we check our formulas

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