Proportion Calculator
Solves a/b = c/d for a, b, c, or d using cross-multiplication with nonzero denominators where required.
Limitation: Enter the three known terms; the selected unknown is computed.
How to Solve a Proportion
Choose which letter is missing, fill in the other three values, and calculate. Details show the cross-product check a×d vs b×c.
Select the letter to solve for.
Defaults are a worked example
The defaults 3/4 = 6/8 already balance. Change solve-for and clear the unknown mentally—enter only the known three.
Cross-Multiplication
From a/b = c/d, a×d = b×c. Solving for a uses (b×c)/d when d ≠ 0; similar rearrangements cover b, c, and d.
Worked Proportion Examples
Find a in a/4 = 6/8
Known terms
Missing
a = (4 × 6) ÷ 8 = 3.
Find d in 3/4 = 6/d
Known terms
Missing
d = (4 × 6) ÷ 3 = 8.
Find b in 5/b = 10/6
Known terms
Missing
b = (5 × 6) ÷ 10 = 3.
Recipe scaling
Kitchen scaling often uses the same cross-multiply idea—pair with food tools when units are cups and grams.
Frequently Asked Questions
Still have questions about this calculation?
Try the CalculatorProportion Formulas
All four solves are rearrangements of a×d = b×c.
Formula
Cross products
a × d = b × c
Solve for a
a = (b × c) ÷ d
Solve for d
d = (b × c) ÷ a
Scientific Background
Proportions model similar figures, unit rates, and recipe scaling. For simplifying a single ratio without a second pair, use the Ratio Calculator.
Sources & review
Proportion Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.
Last reviewed
July 22, 2026
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