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Ratio Calculator

Ratios turn up in recipes, mixes, model scales and screen sizes. Fill A and B to simplify a ratio; leave C or D empty and the tool solves the proportion A:B = C:D for whichever value is missing.

How to use it

Enter A and B for the ratio you already have.
To scale it, fill in either C or D and leave the other blank.
Read the missing value, or the simplified form when C and D are both empty.

Examples

3 : 4 = 15 : ? → 20. A recipe for three scaled to fifteen portions.
1920 : 1080 simplifies to 16 : 9.
Mixing at 1 : 25 for 5 litres of water → 0.2 litres of concentrate.

Questions people ask

What is the difference between a ratio and a fraction?

A ratio compares parts to each other, a fraction compares a part to the whole. Two parts water to one part juice is a ratio of 2:1, but water is 2/3 of the mixture. Mixing the two up is the usual source of a bad dilution.

Can I use decimals?

Yes. Decimal inputs work throughout and the proportion is solved exactly. Simplification is most useful with whole numbers, so decimal ratios are also shown as a single decimal value you can compare directly.

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Updated: 2026-08-07