Calculated
Ratio Calculator
Sign up to favourite this calculatorSolve for a missing term in A : B = C : D, or reduce a ratio to lowest terms. Leave one term blank to solve it, or leave both right-hand terms blank to simplify.
Missing term C
Full ratio
3 : 8 = 27 : 72In lowest terms
3 : 8As a decimal (A ÷ B)
0.375In 1 : n form
1 : 2.6667In n : 1 form
0.375 : 1Save to My calculations
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Keep this calculation as a scenario, change any input above, and the Now column shows how the result moves. Keep up to three, A to C; Open puts one back in the calculator.
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Assumptions and limits
- An estimate from the numbers you entered, for guidance only. It is not financial, tax or medical advice.
- Calculator version 1, last updated 25 October 2026.
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How to use this calculator
The four fields are the four terms of A : B = C : D. What the calculator does depends on which of them you leave empty.
| To do this | Enter | You get |
|---|---|---|
| Solve for a missing term | A 3, B 8, C blank, D 72 | C = 27, so 3 : 8 = 27 : 72 |
| Simplify a ratio | A 14, B 28, C and D blank | 1 : 2 |
| Scale a ratio up | A 2, B 5, C 8, D blank | D = 20 |
| Check two ratios match | All four filled in | Equivalent, or not |
Leaving more than one term blank, other than C and D together, gives the calculator too little to work with, and it says so rather than guessing.
Prefer fraction notation? The Proportion Calculator solves the same cross-multiplication as A/B = C/D instead of A : B = C : D.
What do I need to enter?
- A and B — the two terms of the ratio you know.
- C and D — the two terms of the equivalent ratio. Leave one blank to solve for it, or leave both blank to simplify A : B. Fill all four to check whether the two ratios are equal.
Each term is a number up to 1,000,000,000,000 in size; decimals and negatives are accepted.
What do the results show?
- The main figure — the missing term, the ratio in lowest terms, or whether the two ratios are equivalent, depending on what you left blank.
- Full ratio and in lowest terms — A : B = C : D as entered, and A : B reduced by dividing both terms by their greatest common divisor.
- As a decimal (A ÷ B), 1 : n and n : 1 — the same ratio written three other ways.
- Working and share of the total — the arithmetic used, and A and B as percentages of A + B.
How ratios are solved and simplified
Two ratios are equal when their cross products are equal. That single fact is what lets any one of the four terms be recovered from the other three.
A : B = C : D when A × D = B × C
- A × D — the first term times the last, e.g. 3 × 72 = 216
- B × C — the middle two multiplied, e.g. 8 × 27 = 216
- Equal cross products — the two ratios are equivalent
Rearranging gives the missing term directly. With C unknown, C = (A × D) ÷ B, so (3 × 72) ÷ 8 = 27.
Simplifying is a different operation: divide both terms by their greatest common divisor. 14 : 28 shares a divisor of 14, so it reduces to 1 : 2. Reducing never changes the relationship — 1 : 2, 7 : 14 and 14 : 28 are all the same ratio, which is why any of them can stand for the others.
A worked example
A recipe uses flour and water in the ratio 3 : 8 and you have 72 g of water. Enter 3 and 8 as A and B, leave C blank, and put 72 in D. Cross-multiplying gives(3 × 72) ÷ 8 = 27, so you need 27 g of flour.
The share figures answer the other common question. Three parts and eight parts make eleven, so flour is 27.27% of the mixture and water 72.73%. Scaling the whole recipe to 550 g means 150 g of flour and 400 g of water.
Frequently asked questions
How do I simplify a ratio?
Divide both terms by their greatest common divisor. For 14 : 28 that divisor is 14, so the ratio is 1 : 2. Enter 14 and 28 in A and B, leave C and D blank, and the calculator does exactly that.
How do I find a missing term in a ratio?
Cross-multiply. In 3 : 8 = C : 72, multiply 3 by 72 and divide by 8, which gives 27. The rule is the same wherever the gap is: the missing term is the product of the two terms diagonally opposite it, divided by the one beside it.
What do the 1 : n and n : 1 forms mean?
They are the same ratio rescaled so one side is exactly one. 3 : 8 is 1 : 2.6667 and 0.375 : 1. The first is how map scales and mixing ratios are written; the second is useful when you want the answer per unit of the second quantity.
How do I split an amount in a ratio?
Use the share of the total shown under the inputs. For 3 : 8 the shares are 27.27% and 72.73%, because there are 11 parts in total. Splitting £550 in that ratio gives £150 and £400.
Can I use decimals in a ratio?
Yes. 1.5 : 2 is a valid ratio and simplifies to 3 : 4, because scaling both sides by the same amount never changes a ratio. The calculator scales decimals up to whole numbers before reducing.
Is a ratio the same as a fraction?
They are closely related but read differently. The ratio 3 : 8 compares two parts, while the fraction 3/8 compares one part to the whole of the other. Mixing 3 : 8 means 3 parts in 11 total, which is 3/11 of the mixture rather than 3/8 of it.
Last updated 25 October 2026.