Math
Proportion Calculator
Solve A/B = C/D for any missing value. Leave one field blank and we'll find it using cross-multiplication.
A
B
C
D
Result: D
20.000000
Show steps
2 / 5 = 8 / D
D = (5 × 8) ÷ 2
D = 40 ÷ 2
D = 20.0000
Frequently asked questions
What is a proportion?
A proportion is an equation stating that two ratios are equal: A/B = C/D. If you know three of the four values, you can always find the fourth.
How does cross-multiplication work?
Cross-multiplying means multiplying across the equals sign diagonally: A × D = B × C. This gives you an equation you can solve for the missing value.
Where are proportions used?
Proportions are used everywhere: in cooking (scaling recipes), maps (scale ratios), finance (exchange rates), medicine (dosages), and statistics (percentages).
What if I get zero in a denominator?
Division by zero is undefined. If you leave a non-missing field as zero in a denominator position, the calculator will show an error.
Solve proportions of the form A/B = C/D. Leave any one of the four values blank, and we'll use cross-multiplication to find it with step-by-step working shown.