Everyday Tools

Proportion Calculator

Solve a basic proportion by leaving one value blank in a / b = c / d.

Updated April 16, 2026
Free online tool
Planning and research use

Why this page exists

Ratio problems get easier when a missing proportion value can be solved directly instead of being rearranged by hand each time. This calculator helps visitors solve a simple proportion in the form a / b = c / d by filling in three values and leaving one blank.

Interactive tool

Run the estimate

Enter your numbers and read the result first, then use the sections below to understand what affects the outcome.

Calculator

Proportion calculator

Solve a basic proportion by leaving one value blank in a / b = c / d.

Result

c = 10

Estimated missing proportion value using standard cross-multiplication on a / b = c / d.

Solved missing valuec = 10

Completed proportion6 / 9 = 10 / 15

Values useda=6, b=9, c=10, d=15

Cross-product check90 = 90

Solving 6 / 9 = 10 / 15 by cross multiplication gives c = 10.
The completed proportion is valid because the cross products match in this solved form.
Use this for direct ratio-style problems only, because the calculator expects one simple missing value in a proportion with exactly four terms.

This is standard cross-multiplication math. Leave exactly one value blank so the calculator can solve the missing part of the proportion.

Planning note

Last updated April 16, 2026. Use this tool to compare scenarios and plan ahead, then confirm important details with the lender, employer, insurer, contractor, or other qualified provider involved in the final decision.

How it works

What the calculator is doing

Enter three values in the proportion and leave one field blank.

The calculator uses standard cross multiplication to solve the missing value.

It shows the solved value and the completed proportion for quick checking.

Understanding your result

This is standard cross-multiplication math. The calculator expects exactly one missing value and can reject cases where the needed divisor is zero.

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Examples

Ways people use this tool

Example scenarios help turn a quick estimate into a more useful comparison or planning step.

Solve a missing value in a simple ratio equation

This can help when a worksheet, recipe, or scale problem is naturally written as one proportion.

Check homework or classroom steps

A solved proportion and cross-product check can make practice problems quicker to verify.

Use it beside ratio and percentage tools

Proportion work often fits naturally beside ratio, percentage, and fraction calculations.

FAQ

Common questions

How does this calculator solve the missing value?

It uses standard cross multiplication on the proportion a / b = c / d after you leave exactly one value blank.

Why do I need to leave only one field blank?

Because the calculator needs three known values to solve the fourth value cleanly in a simple proportion.

What happens if one of the needed divisors is zero?

The calculator flags that case because the missing value cannot be solved cleanly with simple cross multiplication when the needed divisor is zero.

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Keep comparing

Use these related tools to compare nearby scenarios, check a second estimate, or keep narrowing down the right decision.

Everyday ToolsUpdated April 11, 2026