Math
Proportion Calculator
Use the proportion calculator to solve a/b = c/x, the common classroom setup for similar figures, recipes, rates, and maps.
Try an example
Result
Missing denominator x
—
- Scale factor
- —
Student quick launch
Grade planning, algebra checks, and formulas students reach for most.
Study path
Use this calculator with
Follow these when you want the formula behind the answer, a short lesson, or nearby tools in the same topic.
What the proportion calculator solves
Use the proportion calculator to solve a/b = c/x, the common classroom setup for similar figures, recipes, rates, and maps.
Formula
How to use it
- Enter the known The top number of a fraction; it counts how many parts you have. and The bottom number of a fraction; it says how many equal parts make one whole. for the first ratio.
- Enter the matching numerator in the second ratio.
- Read the missing denominator and scale factor.
How to read the answer
The answer is the value that keeps both ratios equal. The scale factor tells how much the first numerator was multiplied to become the second numerator.
Common mistakes and edge cases
- Keep matching quantities in the same numerator or denominator position.
- Do not divide by zero.
- Cross multiplication works only when the two ratios are set up consistently.
Worked examples
Similar triangle side
Missing denominator x
12
Recipe scaling
Missing denominator x
15
Frequently asked questions
What proportion form does this solve?+
It solves a/b = c/x for the missing denominator x.
Can I solve for a different missing spot?+
Yes, rearrange the labels so the missing value is in the x position, or use the same cross multiplication idea.
About this calculator
- Written by
- mathcheck editorial team
- Last reviewed
- May 8, 2026
Method
- Uses the values entered by the user and stable formulas documented on the page.
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Last updated: May 8, 2026