Proportion Calculator
Fill in three of the four values in a/b = c/d and leave the unknown blank. The calculator cross-multiplies to find it and shows the working.
Result
- Working
- d = b × c ÷ a = 4 × 9 ÷ 3 = 12
- Proportion
- 3 ÷ 4 = 9 ÷ 12
- Value of each ratio
- 0.75
Formula
- a ÷ b = c ÷ d
- Cross-multiply: a × d = b × c
- d = b × c ÷ a
How the Proportion Calculator works
A proportion states that two ratios are equal. Multiplying across the equals sign gives a × d = b × c, and dividing by the number next to the unknown isolates it.
Proportions solve everyday scaling problems: if 3 pounds cost 4 dollars, 9 pounds cost 12 dollars, because both ratios equal 0.75.
Worked example
3 ÷ 4 = 9 ÷ d. Cross-multiplying gives 3 × d = 4 × 9 = 36, so d = 36 ÷ 3 = 12.
Frequently asked questions
How do I solve a proportion?
Cross-multiply to get rid of the fractions, then divide both sides by the number multiplying the unknown.
What is the difference between a ratio and a proportion?
A ratio compares two numbers. A proportion is an equation saying two ratios are equal.
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