Rule of three (proportion) calculator
Solve simple and compound proportions, including direct and inverse proportions, with the formula.
For each other quantity, enter its value in scenario 1 and scenario 2, and whether it is directly or inversely proportional to the unknown quantity.
Processed on your device. Nothing you type is sent anywhere.
How to use: Rule of three (proportion) calculator
- For a simple proportion, enter A, B, and C: “If A corresponds to B, what does C correspond to?”
- Choose whether the quantities are directly or inversely proportional.
- For a compound proportion, enter the known value of the quantity you’re solving for and the other quantities in both scenarios.
About this tool
Quantities are directly proportional when one increases and the other increases at the same rate (more pounds, higher cost). They’re inversely proportional when one increases and the other decreases (more workers, fewer days).
Compound proportion example: 5 bricklayers build 2 houses in 10 days. How many days will 10 bricklayers take to build 6 houses? The number of bricklayers is inversely proportional (more bricklayers, fewer days), while the number of houses is directly proportional (more houses, more days): 10 × 5/10 × 6/2 = 15 days.
Frequently asked questions
How do you tell whether a proportion is direct or inverse?
Ask yourself: if one quantity doubles, does the other double or get cut in half? If it doubles, the relationship is direct; if it gets cut in half, it’s inverse.
What’s the formula for a simple proportion?
For a direct proportion, X = (B × C) ÷ A. For an inverse proportion, X = (A × B) ÷ C.
Is it really free? Is there a daily limit?
It's free with no daily limits. Processing happens on your device, so there's no per-use cost and no need to limit you.
Does it work on phones?
Yes. The site works in any modern browser on Android, iPhone, Windows, Mac, and Linux—no installation needed.
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