Proportion Calculator
Solve any missing term in the proportion a/b = c/d.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Enter three values
Leave exactly one box empty.
Calculate
The missing term is solved via cross multiplication.
Verify
With all four filled, the calculator checks the proportion.
What Is a Proportion Calculator?
A proportion states that two ratios are equal: a/b = c/d. This calculator solves for any one missing term when the other three are known, and also tells you whether a given set of four numbers actually forms a proportion by checking the cross products.
Proportions are everywhere: scaling recipes, working out prices per unit, map distances, and mixing solutions. The rule is simple — cross multiplication: a × d must equal b × c.
How It Works
Leave exactly one of the four boxes empty and the calculator solves for it using the cross-product rule. Fill all four and it verifies whether the proportion holds.
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
How do I solve a proportion?
Use cross multiplication: if a/b = c/d and d is missing, d = b × c ÷ a.
What is cross multiplication?
Multiplying the numerator of one ratio by the denominator of the other: a/b = c/d means a × d = b × c.
How do I know if two ratios are proportional?
Compare their cross products. If a × d equals b × c, they form a proportion.
What if I leave two boxes empty?
Exactly one unknown can be solved. Leave one blank; the calculator will prompt you otherwise.
Where are proportions used in real life?
Scaling recipes, calculating unit prices, map reading, photography cropping and dosage calculations.