Proportion Calculator
The rule of three: three values of a proportion, direct or inverse, give the fourth, or check whether four are in proportion
Leave one box empty, it's worked out from the other three; fill all four to check them. You can also type or paste 3/250 = 6/x.
Missing value
Direct: the two grow together. 3 eggs for 250 g of flour, so 6 eggs for 500 g: 250 × 6 ÷ 3.
Inverse: one grows as the other shrinks. 3 painters take 12 days, so 6 painters take 6 days: 3 × 12 ÷ 6.
This is the rule of three, or cross-multiplication: in a direct proportion a / b = c / d, so a × d = b × c; in an inverse one a × b = c × d.