How do you solve a proportion?

Cross-multiply: for A/B = C/D, multiply A × D and B × C, set them equal, then divide to isolate the unknown term.

What happens if I fill in all four values?

The calculator needs exactly one blank field to know which term to solve for - leave the value you want to find empty.

What is a real-world example of a proportion?

If a recipe for 4 people needs 2 cups of flour, a proportion (2/4 = x/10) finds that 5 cups are needed to serve 10 people.

Math CalculatorsClient-Side Calculation Engine

Proportion Calculator

Solve missing term in proportion equation A/B = C/D.

Math Calculators

Proportion Calculator

Solve for the missing term X in proportions of the form A/B = C/D with cross-multiplication steps.

Proportion Equation: (A / B) = (C / D). Leave one field blank to solve.

D (Solve)

Solved Value

D = (20 × 15) ÷ 10 = 30.0000

What is the Proportion Calculator and How Does It Work?

The Proportion Calculator solves for a missing value in a proportion of the form A/B = C/D. Leave exactly one of the four values blank and the calculator cross-multiplies to find it instantly.

Calculation Formula

A / B = C / D → Missing term solved via cross-multiplication

In a true proportion, the cross products are equal (A × D = B × C). Rearranging that equation isolates whichever single term is unknown.

Where:

A, B, C, D= The four terms of the proportion, with exactly one left blank to solve for

Worked Calculation Examples

Solving for D

Input:A=10, B=20, C=15, D=?

Output:D = 30

D = (B × C) / A = (20 × 15) / 10 = 30.

Solving for C

Input:A=4, B=8, C=?, D=16

Output:C = 8

C = (A × D) / B = (4 × 16) / 8 = 8.

Common Real-World Use Cases

Recipe & Ingredient Scaling

Scale a recipe designed for one serving size up or down while keeping ingredient ratios constant.

Map Reading & Scale Drawings

Convert a distance on a scaled map or blueprint into a real-world distance.

Unit Price Comparisons

Find the equivalent price at a different quantity to compare deals fairly.

Classroom Algebra

Solve standard "solve for x" proportion problems with visible cross-multiplication steps.

How to Use the Proportion Calculator

1Enter three of the four values in the proportion A/B = C/D.
2Leave the value you want to solve for blank.
3The calculator cross-multiplies and shows the solved value along with the formula used.

Pro Tips & Suggestions

•Exactly one field must be left blank - filling in all four or leaving more than one blank will not produce a result.
•The cross-multiplication formula shown can be copied to check your work by hand.

Frequently Asked Questions

Explore Category

Math Calculators

Essential mathematical calculators for fractions, algebra, formulas, and general math.

View All Category Tools

Related Calculators & Tools

mathLIVE

Ratio Calculator

Simplify ratios to lowest terms, convert ratios to fractions and percentages, and solve equivalent ratios.

Instant calculationReady →

percentageLIVE

Percentage Calculator

Calculate percentages, percentage changes, increases, decreases, and reverse percentage values quickly.

Instant calculationReady →

mathLIVE

Fraction Calculator

Add, subtract, multiply, and divide fractions, mixed numbers, and decimals with step-by-step simplification and visual fraction inputs.

Instant calculationReady →

mathLIVE

Average (Mean) Calculator

Calculate the arithmetic mean, sum, count, minimum, maximum, and range of any set of numbers.

Instant calculationReady →

mathLIVE

Median Calculator

Find the middle number (median), mode, range, and sorted numerical sequence of any set of data.

Instant calculationReady →

mathLIVE

Prime Number Checker

Check whether an integer is prime or composite and view its complete factor list and prime factorization.

Instant calculationReady →