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 helps with direct and inverse rule of three for prices, recipes, work time, speed, and quantities without forcing you to move formulas into a spreadsheet. It is designed for quick everyday math, school, study, shopping, planning, and plausibility checks.
Typical searches include rule of three calculator, direct proportion, inverse proportion. The calculator shows the result, calculation path, and important intermediate steps directly in the browser. The key is whether larger source values create larger results or whether the relationship is inverse.
Typical use cases
Check everyday math quickly
Use the calculator for prices, quantities, shares, comparisons, notes, or quick decisions where a traceable result matters more than a large spreadsheet.
Explain formula and result
The core formula is: direct proportion: d = b times c / a; inverse proportion: d = a times b / c. This makes it easier to reuse the result in homework, quotes, calculations, documentation, or communication.
Compare several variants
Change inputs directly and compare the effects. This is useful for scenarios, price variants, group splits, recipe amounts, or simple business decisions.
Tips for better results
Keep input units clear
Check whether you are working with percentages, factors, money amounts, quantities, or plain numbers. Many mistakes come from mixing reference values.
Choose rounding deliberately
Rounded output is easy to read but can create small differences in intermediate steps. For binding amounts, check the required rounding rule.
How it works
How it works
Step by step
Features
Features
Local calculation
Inputs are processed directly in the browser and are not sent to a server for calculation.
Visible calculation path
Formula, substituted values, and result are shown so the calculation remains traceable.
Updates instantly
When you change inputs, the result and intermediate steps are recalculated without reloading the page.
Frequently asked questions
Frequently asked questions
Are inputs uploaded when using Rule of Three Calculator?
No. Calculation runs locally in the browser. No account, upload, or server processing is required.
Can I use a comma as decimal separator?
Yes. Localized pages accept common decimal separators; dots and commas are interpreted according to the locale.
Why can a rounded result differ slightly?
The calculator rounds displayed values for readability. Internally, decimal values and intermediate steps can contain more digits than shown.
More tools
More tools
Ratio Calculator
Enter a ratio and instantly simplify, normalize, scale, or split a total.
Percentage Calculator
Calculate percentages, parts, whole values, adjustments, and percentage changes instantly in your browser.
Weighted Average Calculator
Enter values and weights and instantly see the weighted average, weight sum, product sum, contributions, and calculation steps.