Proportion (rule of three) calculator

How it works

Enter three known values of a proportion to find the fourth (the rule of three).

The formula

If a : b = c : x, then x = b × c ÷ a.

Worked example

If 3 kg cost 4.5, then 7 kg cost 4.5 × 7 ÷ 3 = 10.5.

Good to know

• Use it only when the quantities change in the same ratio (direct proportion).