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SnapRatios

Ratio & proportion tools

Proportion Calculator

Leave one value empty in a : b = c : d and get the exact answer with full cross-multiplication working.

Write x (or leave empty) in the position you want to solve.

4 : 7 = x : 21

Exact result

x = 12

Solved by cross-multiplication

Working

  1. Cross-multiply: c = (a × d) ÷ b
  2. 4 × 21 = 84
  3. 84 ÷ 7 = 12

Cross multiplication

A proportion states that two ratios are equal. If a : b = c : d, then a × d = b × c. Rearranging that equation isolates whichever value is unknown. The calculator refuses to guess when more than one value is missing, and reports when a solution would require dividing by zero.