Rule of Three Calculator
A is to B as C is to X — the everyday proportion, solved live.
How this is calculated
X = B × C ÷ A (cross-multiplication)
Worked example: 3 kg costs R45 — what do 5 kg cost? X = 45 × 5 ÷ 3 = R75. The per-unit line (R15/kg here) is the same answer wearing shop clothes, and comparing per-unit prices is the fastest honest way to read supermarket shelf tags. Works identically for servings, map scales, paint coverage, and exchange amounts — anything directly proportional.
Frequently asked questions
What is the rule of three?
The classic proportion shortcut: if A relates to B, then C relates to X, where X = B × C ÷ A. Three knowns, one unknown — recipe scaling, price-per-unit, map distances, and dosage-style conversions are all this one move.
When does the rule of three apply?
Only when the relationship is directly proportional — double one side, double the other. Prices per quantity, ingredients per serving, distance per map-centimeter all qualify. Things with fixed costs or diminishing returns (taxi fares with a base charge, workforce-vs-time) do not.
What is an inverse rule of three?
When more of one means proportionally less of the other — 4 painters take 6 hours, so 8 painters take 3. That is X = A × B ÷ C instead. This page computes the direct version; flip which value you call C to handle simple inverse cases.
Why does the order of A, B, C matter?
A and B must be the matched known pair ("3 kg costs R45"), and C the new quantity in A’s units. Mixing the units up is the only way to get a wrong answer out of a right formula — the labeled fields keep the pairing straight.