What is a proportion?

A proportion is an equation stating that two ratios are equal: a/b = c/d. It means the relationship between a and b is the same as the relationship between c and d.

How does cross-multiplication work?

Cross-multiplication rearranges a/b = c/d to a × d = b × c. If d is unknown, then d = (b × c) / a. The calculator applies whichever variant matches the blank field.

Can I leave more than one field blank?

No. A proportion has four terms; you must supply exactly three to solve for the one unknown. Leaving two or more blank makes the equation underdetermined.

What if my answer is a repeating decimal?

The result is rounded to 6 significant figures. For exact fractions, try entering whole numbers — the tool will also display the GCD-reduced fractional form.

Can any value be zero?

The denominators b and d cannot be zero (division by zero is undefined). The numerators a and c can be zero, but only if the corresponding numerator on the other side is also zero.

Proportion Calculator

Name: Proportion Calculator
Availability: InStock
Author: Nham Vu

Enter any three values in a/b = c/d and instantly solve for the missing fourth using cross-multiplication.

Enter Three Values

Leave exactly one field blank — that is the unknown.

Fill in three values and click Solve

The missing fourth value will appear here

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Summary

Enter any three values in a/b = c/d and instantly solve for the missing fourth using cross-multiplication.

How it works

Enter values in any three of the four fields (a, b, c, or d).
Leave exactly one field blank — that is the unknown you want to solve for.
Click "Solve" or press Enter; the tool applies cross-multiplication to find the missing value.
The answer is displayed as a decimal rounded to 6 significant figures.
A step-by-step explanation shows the cross-multiplication work.
Use "Clear All" to reset all fields and start a new proportion.

Use cases

Scale a recipe: if 3 cups feed 4 people, how many cups feed 7?
Convert units: if 1 inch = 2.54 cm, how many cm is 8.5 inches?
Find a missing dimension when scaling images or blueprints.
Calculate medication dosages proportionally.
Solve percent problems: what is 35% of 240?
Determine map distances using a stated scale ratio.
Check whether two ratios are equivalent.
Homework help for middle- and high-school ratio problems.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu