Proportion Calculator Formula

Understand the math behind the proportion calculator. Each variable explained with a worked example.

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Formulas Used

D (missing value)

d = (b * c) / a

A/B

ratio1 = a / b

C/D

ratio2 = c / ((b * c) / a)

Cross Product (A*D)

cross_product = a * ((b * c) / a)

Variables

VariableDescriptionDefault
aA (numerator 1)3
bB (denominator 1)4
cC (numerator 2)12

How It Works

How to Solve a Proportion

Formula

If A/B = C/D, then D = (B * C) / A

A proportion is an equation stating that two ratios are equal. Cross-multiplication gives A*D = B*C. Solve for any missing value by rearranging. This is fundamental to scaling, unit conversion, and similar triangles.

Worked Example

If 3/4 = 12/x, find x.

a = 3b = 4c = 12
  1. 01Cross-multiply: 3 * x = 4 * 12
  2. 023x = 48
  3. 03x = 48 / 3 = 16
  4. 04Check: 3/4 = 0.75, 12/16 = 0.75. Confirmed.

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