math

Proportion Calculator

Solve the proportion a : b = c : x for the unknown x using cross-multiplication.

2 : 3 = 8 : x — Cross-multiply: 2 · x = 3 · 8, so x = (3 × 8) ÷ 2 = 12.

How it works

1Enter the three known values a, b, and c in the proportion a : b = c : x.
2The calculator cross-multiplies: a · x = b · c, then isolates x = (b × c) ÷ a.
3The result panel shows x rounded to 4 decimal places along with the full working step.

Scale a recipe: if 2 cups make 3 servings, how many cups are needed for 8 servings?
Map distances: convert a measured length on a scale drawing to real-world distance.
Unit conversion: determine an equivalent quantity when the conversion ratio is known.

What is a proportion?

A proportion states that two ratios are equal — a : b = c : x means the ratio of a to b equals the ratio of c to x.

Why does the calculator return — when a is 0?

With a = 0 the equation becomes 0 · x = b · c, which is undefined (or impossible unless b · c = 0), so the result is meaningless.

Can I use decimal or negative values?

Yes. The calculator handles any real numbers for a, b, and c, including decimals and negatives, as long as a ≠ 0.

Calculate the area and perimeter of a rectangle from its width and height, with a labeled diagram.

Calculate the area and circumference of a circle from its radius, with a labeled diagram.

Calculate the area (Heron’s formula) and perimeter of a triangle from its three sides.

Calculate the area and perimeter of a trapezoid from its two parallel sides, height, and legs.