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The rule of three is probably the most used school calculation: if a quantity A corresponds to a value B, how much belongs to a quantity C? This calculator solves exactly that via the unitary method. First the value of a single unit of A is found, then it is multiplied by C. That lets you quickly convert prices, quantities, recipes or scales.
The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. The value per unit is B divided by A; the result is that value times C, that is B times C divided by A. Example: 3 kilograms cost 12, you want 5 kilograms - one unit costs 4, five cost 20. The results are plain numbers without a fixed unit; you decide what they mean. Change an input and everything updates instantly.
An honest note: this is the direct (proportional) rule of three - more of A means more of the result. The inverse rule of three, where more of A means less (for example more workers, less time), is not modelled by this calculator. For plain percentage questions use the percentage calculator. The results are unitless numbers.
In daily life the rule of three is everywhere: scaling a recipe for four up to six, converting a map scale (1 to 25000 means 1 centimetre equals 250 metres), carrying a price per kilo over to the amount bought, or working out paint and material for a larger area. Wherever two quantities stand in a fixed ratio, it is the fastest tool.
Specifications
Specifications
Input formats
Form inputs (no file)
Processing
Locally in your browser (JavaScript)
File upload
None
In 3 steps
Enter the known quantity A.
Enter its value B.
Enter the sought quantity C and read off the result.
Limitations: The direct (proportional) rule of three via the unitary method; the inverse rule of three (more A, less result) is not modelled. The results are unitless numbers whose meaning you set yourself. For plain percentage questions use the percentage calculator.
FAQ
Are my inputs uploaded?
No. The calculation runs entirely locally in the browser (pure JavaScript); nothing is sent or stored.
How does the rule of three work?
Via the unitary method: value per unit = B divided by A, result = value per unit times C. In short: result = B times C divided by A.
Can it do the inverse rule of three?
No, this calculator solves the direct (proportional) rule of three. With the inverse rule more of A means less result - that is not modelled here.
What unit is the result in?
In none fixed - the results are plain numbers. You decide whether they mean euros, kilograms, items or something else.