Proportion calculator (rule of three)
Enter the three known numbers to get the fourth. This is the rule of three used for prices, recipes and scales.
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How to use it
- Enter a, b and c.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
From A:B = C:X we get A × X = B × C, so X = (B × C) ÷ A
Example
Inputs: A = 2, B = 5, C = 6. Result: Value of X: 15; Ratio B ÷ A: 2.5.
Limits and caveats
- Assumes direct proportion. For inverse proportion, such as workers and time, X = (A × B) ÷ C instead.
Questions about this tool
If 2 kg cost 5, what do 6 kg cost?
Multiply 5 by 6 and divide by 2. The price is 15.
What is cross multiplication?
In A:B = C:X, A × X = B × C, so X = B × C ÷ A.
Does it work for map scales?
Yes. If 1 cm stands for 50 km, then 3.5 cm stands for 175 km.