Rule of Three Calculator
Enter three values and instantly solve for the fourth using direct or inverse proportion, with a clear calculation path.
Calculate a proportion
Both quantities change by the same ratio.
Labels
Values in the same column must already use the same unit. These fields do not convert units.
corresponds to
corresponds to
Enter three valid values. The field marked as calculated is solved automatically.
Relation
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Scaling
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Good to know
Good to know
The rule of three calculator solves direct and inverse proportion problems directly in your browser. With direct proportion (“the more, the more”), both quantities change in the same ratio, for example quantity and price or portions and ingredients. With inverse proportion (“the more, the less”), one quantity increases while the other decreases correspondingly, for example workers and duration or speed and travel time.
Typical searches include rule of three calculator, direct proportion, and inverse proportion. Calculation runs locally in the browser. Important: values within a column must already be in the same unit — the tool does not convert units, and labels are plain text.
Typical use cases
Calculate directly proportional prices and quantities
Use the direct rule of three for recipe quantities, purchase prices, or other cases where both quantities grow in the same ratio.
Plan work time and staffing with inverse proportion
Use the inverse rule of three to calculate how the duration changes when more or fewer workers work on a task.
Give custom labels for your specific case
Freely name the two quantities, for example “pieces” and “$”, to make the calculation path directly traceable for your use case.
Tips for better results
Correctly determine direct or inverse
Ask yourself whether a larger value in the first column also produces a larger target value (direct) or a smaller one (inverse) before starting the calculation.
Align units before entering values
Don't mix values like 1 kg and 500 g in one column without converting first — the labels are plain text and don't convert units.
How it works
How it works
Step by step
Features
Features
Direct or inverse proportion
Choose the type of relationship that matches your problem, with examples for both cases.
Solve for any value
Mark which of the four values you're solving for; the other three are entered.
Calculation path and formula shown
The formula, proportionality factor, and calculation path are shown transparently and can be copied.
Frequently asked questions
Frequently asked questions
What's the difference between direct and inverse rule of three?
With direct proportion, both quantities change in the same ratio (“the more, the more”); with inverse proportion, one increases while the other decreases (“the more, the less”).
Does the tool convert units?
No. Labels are plain text; values within a column must already be in the same unit.
Are my inputs uploaded?
No. Calculation runs entirely locally in the browser.
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