What is the difference between a ratio and a proportion?

A ratio compares two quantities (3 : 4). A proportion is a statement that two ratios are equal (3 : 4 = 6 : 8). This calculator solves proportions.

How do I simplify a ratio?

Divide both sides by their greatest common divisor. 12 : 18 simplifies to 2 : 3 (both divided by 6).

How do I convert a ratio to a percentage?

For a ratio A : B, the first part as a percent of the total is A ÷ (A + B) × 100. So 3 : 7 means 30% : 70%.

Ratio Calculator

Enter values in three of the four fields below. The calculator solves for the missing value to keep the ratio equal.

By Numerly · Updated June 14, 2026

Fill in any three values. Leave the fourth blank — we'll solve it.

Solution

D = 15

How it works

A ratio A : B = C : D means A × D = B × C (cross-multiplication). Solving for any missing term: D = B × C ÷ A; C = A × D ÷ B; A = B × C ÷ D; B = A × D ÷ C.

Frequently asked

What is the difference between a ratio and a proportion?: A ratio compares two quantities (3 : 4). A proportion is a statement that two ratios are equal (3 : 4 = 6 : 8). This calculator solves proportions.
How do I simplify a ratio?: Divide both sides by their greatest common divisor. 12 : 18 simplifies to 2 : 3 (both divided by 6).
How do I convert a ratio to a percentage?: For a ratio A : B, the first part as a percent of the total is A ÷ (A + B) × 100. So 3 : 7 means 30% : 70%.

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