1
Enter three values
Leave only the value you want to find blank.
Free Proportion Calculator
A proportion calculator solves a missing value in two equal ratios. Enter three values in a/b = c/d and get the answer with cross-multiplication steps you can check.
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Equivalent ratios
Enter three values
Leave the value you want to solve blank. Decimals and negative numbers are supported.
Proportion calculator result
c = 15
c = 15
Completed proportion
3 / 4 = 15 / 20
Step-by-step work
- 1. Cross multiply using a × d = b × c.
- 2. 3 × 20 = 60.
- 3. Divide by 4: c = 60 ÷ 4.
- 4. c = 15.
- 5. Check: 3 × 20 = 4 × 15 = 60.
Educational tool: verify important calculations independently. Fractions with a denominator of zero are undefined.
How to Solve a Proportion
2
Cross multiply
Multiply the values across each diagonal.
3
Divide and check
Isolate the unknown, then compare both cross-products.
Proportion Calculator FAQ
What is a proportion calculator?
A proportion calculator finds a missing value in two equal ratios. It uses the rule a × d = b × c, then isolates the unknown value.
How do you solve a proportion by cross multiplication?
Multiply the numerator of the first ratio by the denominator of the second, and multiply the other diagonal. Set those products equal and divide to isolate the missing value.
Can I solve for any part of the proportion?
Yes. Leave any one of a, b, c, or d blank and enter numbers in the other three boxes. The calculator solves the blank term.
Can a proportion include decimals or negative numbers?
Yes. The calculator accepts whole numbers, decimals, scientific notation, and negative values, as long as neither denominator is zero.
Why can a denominator not equal zero?
Division by zero is undefined, so a fraction with zero on the bottom cannot be part of a valid proportion.