Proportion Calculator
Use this free online proportion calculator to solve for the missing value in A/B = C/D. Enter three known values and get the unknown instantly using cross multiplication.
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Result
Enter values above and click Calculate to see your result.
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Formula
How it works: Cross-multiply and divide to find the missing value. A × D = B × C.
Review and Methodology
This calculator runs locally in your browser. Inputs are converted into the units required by the formula, and the result is paired with supporting references so you can verify the method before using it for planning or estimates.
Worked Example
What Is a Proportion?
A proportion is a mathematical statement that two ratios are equal. If you know three of the four values in a proportion, you can find the missing one using cross multiplication. Proportions are one of the most practical tools in everyday mathematics.
- If 2 apples cost $3, then 6 apples cost $9 because 2/3 = 6/9
- Cross multiplication works by setting the products of the diagonals equal: A x D = B x C
- Proportions are the basis for unit conversions, map scales, and recipe scaling
- Any equivalent fractions form a proportion: 1/2 = 3/6 = 50/100
Proportions are used daily in cooking, construction, medicine (dosage calculations), currency conversion, and anywhere you need to scale a known relationship to a new size.
You can also calculate changes using our Ratio Calculator, Fraction Calculator or Percentage Calculator.
Frequently Asked Questions
What is a proportion?
A proportion is an equation stating that two ratios are equal. For example, 1/2 = 3/6 is a proportion because both ratios simplify to the same value.
How do I solve a proportion?
Use cross multiplication: multiply the means and extremes, then solve for the unknown. In A/B = C/D, multiply A x D and B x C, set them equal, and solve.
When are proportions used?
In recipes (scaling ingredients), maps (converting scale distances to real distances), unit conversions, medicine (dosage by weight), and many other real-world applications.
What is cross multiplication?
Cross multiplication is a shortcut for solving proportions. In A/B = C/D, you multiply diagonally: A x D = B x C. Then solve for the unknown variable.
Can proportions have more than two ratios?
Yes. Extended proportions like A:B:C = D:E:F compare three or more quantities simultaneously. Each pair of corresponding values must maintain the same ratio.
Can I use this Proportion Calculator on my own web page?
You can. Look for the "Embed" button near the top of this calculator. It lets you pick a size, border style, and color palette, then gives you an iframe tag to paste into any webpage. The widget is responsive, loads fast, and costs nothing. More details at calculory.com/services/embed-calculators.
AI Assistant
Ask about this calculator
I can help you understand the proportion calculator formula, interpret your results, and answer follow-up questions.
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