What is a proportion?

A proportion is an equation stating that two ratios are equal: A/B = C/D. This means A multiplied by D equals B multiplied by C (cross-multiplication). Proportions are used whenever two quantities scale together at the same rate.

How does cross-multiplication work?

Cross-multiplication converts A/B = C/D into A × D = B × C. If one value is unknown, you isolate it by dividing both sides by its coefficient. For example, if A is unknown: A = (B × C) / D.

Which value should I leave blank?

Leave blank exactly the value you want to find. The tool detects which field is empty and solves for that variable. If more than one field is blank, the tool will prompt you to fill in more values.

Can the values be decimals or fractions?

Yes. You can enter any real number including decimals such as 1.5 or 3.14. The solver handles decimal arithmetic and rounds the result to up to 6 significant decimal places.

What if B or D is zero?

Division by zero is undefined. The solver will display an error if B or D is zero, since those are the denominators of the proportion. A and C may be zero without issue.

Can I use this to solve percentages?

Yes. Percentages are proportions out of 100. For example, "what is 30% of 80?" becomes 30/100 = x/80, so leave the C field blank (x) and enter A=30, B=100, D=80 to get x=24.

Proportion Solver

Name: Proportion Solver
Availability: InStock
Author: Nham Vu

Enter three values of A/B = C/D and instantly solve for the missing fourth value using cross-multiplication.

Enter the Proportion

Proportion

A (numerator)

C (numerator)

B (denominator)

D (denominator)

Leave exactly one field blank — that is the unknown to solve for.

Enter three values and leave one blank, then click Solve.

The blank field will be calculated for you.

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Summary

Enter three values of A/B = C/D and instantly solve for the missing fourth value using cross-multiplication.

How it works

Enter three of the four values A, B, C, or D in the proportion A/B = C/D.
Leave exactly one field blank — that is the unknown to solve for.
Click "Solve" or type in any field to compute the result automatically.
Read the answer and follow the step-by-step cross-multiplication working.
Click "Copy Result" to copy the solved value to your clipboard.
Use "Example" to load a sample proportion and see how it works.

Use cases

Scale a recipe up or down while maintaining ingredient ratios.
Convert between units using a known conversion ratio.
Find a missing measurement when two ratios must be equal.
Check whether two ratios are proportional to each other.
Solve rate and speed problems in math or science homework.
Calculate map distances using a known scale factor.
Determine unknown side lengths in similar triangles.
Work out percentage problems by setting up a ratio to 100.

Frequently Asked Questions

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