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Proportion Calculator

Solve a/b = c/x for whichever value is missing.

By CalcHub Editorial · Last reviewed

Quick answer

  • Formula: a/b = c/d, solved by cross-multiplication: the missing value makes a × d = b × c
  • Worked example — Missing value: 6.00 (full example below).

Proportion Calculator

Worked example result
Missing value6.00
Completed proportion2/4 = 3/6

2/4 = 3/x gives x = (4 × 3) ÷ 2 = 6. Check it: both sides equal 0.5, so the proportion 2/4 = 3/6 is true.

Diagram: how the Proportion Calculator works — you enter Which value is missing?, a, b, c, d; it applies the formula a/b = c/d, solved by cross-multiplication: the missing value makes …; and the worked example gives Missing value: 6.00.
How the Proportion Calculator works — the inputs, the formula, and a worked example (Missing value: 6.00).

A proportion states that two ratios are equal: a/b = c/d. If you know any three of the four values, the fourth is fixed — this calculator finds it by cross-multiplication.

Choose which value is missing, fill in the other three, and the missing value is solved for you. Proportions underpin scale drawings, unit conversions, similar triangles and recipe scaling.

The formula

a/b = c/d, solved by cross-multiplication: the missing value makes a × d = b × c

Worked example

2/4 = 3/x gives x = (4 × 3) ÷ 2 = 6. Check it: both sides equal 0.5, so the proportion 2/4 = 3/6 is true.

Worked example for the Proportion Calculator: with Which value is missing? d / x (bottom right), a 2, b 4, the result is Missing value: 6.00; Completed proportion: 2/4 = 3/6.
The worked example, with the example values entered and the results they give.

The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.

Frequently asked questions

What is cross-multiplication?

For a/b = c/d, cross-multiplication says a × d = b × c. Solving for the unknown is then one division: for example d = (b × c) ÷ a. It works because multiplying both sides by b × d clears both denominators.

Where are proportions used in real life?

Scaling recipes, reading maps (1 inch = 50 miles), converting currencies and units, enlarging photos without distortion, and dosing medicine by body weight all rely on proportions.

Category: Math calculators

Sources & methodology

This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.

Content last reviewed by CalcHub Editorial.

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