Ratio Calculator

Simplify ratios and solve proportions

A ratio compares two quantities — like 2 : 3 — and a proportion says two ratios are equal, like 2 : 3 = 10 : 15. This calculator does both. Fill only the first two boxes to simplify a ratio to its lowest terms, or leave one box blank in A : B = C : D to solve for the missing value.

How simplifying works

To reduce a ratio, divide both sides by their greatest common divisor. 8 : 12 share a divisor of 4, so they simplify to 2 : 3 — the same proportion in the smallest whole numbers. The tool also shows the ratio as a decimal and as percentages of the whole.

How solving a proportion works

Equal ratios cross-multiply: in A : B = C : D, the products A × D and B × C are equal. So if one term is unknown you can always find it. For 2 : 3 = 10 : D, cross-multiplying gives 2 × D = 3 × 10, so D = 15. This "rule of three" is one of the most useful everyday math tools.

Where ratios show up

• Cooking: scaling a recipe up or down keeps the ingredient ratios fixed.
• Maps & models: a scale like 1 : 50,000 is a ratio between map and real distance.
• Mixing: paint, concrete, fuel, and fertilizer are blended to set ratios.
• Finance & business: splitting profit, costs, or shares in a given ratio.

FAQ

Can I use decimals?

Yes. Whole-number ratios reduce with the greatest common divisor; ratios with decimals are normalised so the smaller side becomes 1, which still shows the true proportion.

What's the difference between a ratio and a fraction?

They're closely related — the ratio 2 : 3 corresponds to the fraction 2⁄3 (and the decimal 0.667). A ratio emphasises the comparison between parts; a fraction emphasises a part of a whole.

Is this ratio calculator free, and is my data private?

Yes — it's free with no sign-up, and every calculation runs in your browser. The numbers you enter are never uploaded or stored, so you can use it as often as you like.

How do I solve for a missing value in a proportion?

Fill in three of the four boxes in A : B = C : D and leave the unknown one blank. The calculator cross-multiplies to find it — for 2 : 3 = 10 : D it computes D = 15. This is the classic "rule of three" used for scaling and conversions.

Can I scale a recipe with this?

Yes — that's a common use. Set up the original amount and serving size against your target serving size as a proportion, leave the new ingredient amount blank, and the tool solves for it while keeping the ratio of ingredients exactly the same.

Does it work on mobile?

Yes. The calculator is responsive and works in any phone or tablet browser, handy for quick scaling, mixing, or conversion math wherever you are.