Solve proportional ratio equations with our free Rule of Three Calculator. Calculate unknown value X in direct and inverse proportions with step-by-step cross-multiplication and 100% offline privacy.
Choose Direct Proportion (if values scale together) or Inverse Proportion (if one increases as the other decreases).
Type your starting numbers into fields Value A, Value B, and Value C in the ratio grid.
Instantly view calculated value X alongside the step-by-step mathematical cross-multiplication solver.
Your numerical inputs are processed entirely inside your browser using client-side JavaScript. Nothing ever travels over a network to any server — making BunchTool 100% private.
Calculations run locally in real time as you edit values A, B, or C or toggle between Direct and Inverse modes. Solutions update instantly without delays.
Supports both linear scaling (Direct: X = B×C/A) and complementary scaling (Inverse: X = A×B/C) with automatic equation walkthroughs.
The Rule of Three is a fundamental mathematical method for finding an unknown fourth term (\(X\)) in a proportion when three terms (\(A\), \(B\), and \(C\)) are known.
The mathematical formulas are:
\(\text{Direct Proportion (A is to B, as C is to X): } X = \frac{B \times C}{A}\)
\(\text{Inverse Proportion (A requires B, as C requires X): } X = \frac{A \times B}{C}\)
Practical Applications:
• Recipe Scaling (Direct): If 4 servings require 200g flour, 6 servings require \(X = \frac{200 \times 6}{4} = 300\text{g}\).
• Worker Hours (Inverse): If 3 workers complete a job in 12 days, 6 workers take \(X = \frac{3 \times 12}{6} = 6\text{ days}\).