Proportion Calculator

Solve proportions, find missing values, and verify ratios with direct or inverse proportion support.

Updated June 26, 2026

What is the Proportion Calculator?

Solve missing values in proportions quickly with a clean, interactive calculator built for direct and inverse relationships. Enter three values, leave one blank, and get the missing number instantly. The tool also shows verification details, so you can check whether the proportion is valid and understand how the result was found. This calculator is useful for homework, ratio problems, scale conversions, recipe adjustments, and everyday math where proportions matter.

The calculator uses proportion rules to find the missing value from four related terms. For direct proportion, it treats the relationship as: a / b = c / d If one value is missing, the tool uses cross multiplication to isolate it. For example, if d is missing, the calculator solves: d = (b \times c) / a For inverse proportion, it uses a constant-product relationship, where one quantity increases as the other decreases. This is useful when the variables move in opposite directions. The tool also includes: Step-by-step solving. Verification of the final proportion. Precision control for rounded decimal output. Copy-friendly results for quick reuse.

Enter three values in a proportion and calculate the fourth instantly.

a = 3, b = 5, c = 12, d = ?

d = 20

Verify whether two ratios are equal and see the cross-multiplication result.

a = 4, b = 6, c = 10, d = 15

Verified: true

Switch to inverse mode when one quantity increases as the other decreases.

a = 8, b = 3, c = 12, d = ?

d = 2

What is a proportion calculator?

A proportion calculator finds the missing value in a proportional relationship and checks whether two ratios are equal.

How do I solve a proportion with one missing value?

Enter three known values, leave one blank, and the calculator uses cross multiplication to compute the missing term.

What is the difference between direct and inverse proportion?

In direct proportion, both values increase or decrease together. In inverse proportion, one value rises as the other falls.

Can this calculator verify if a proportion is correct?

Yes. It calculates the cross products and checks whether both sides match.

Why does the calculator ask for only three values?

A proportion has four related parts. If three are known, the fourth can be calculated.

Can I use decimal values?

Yes. The calculator supports decimal inputs and lets you control output precision.

Is this useful for recipes and scale problems?

Yes. Proportion calculations are common in recipe scaling, map scale, construction, and unit conversions.

What if I enter an invalid value?

The calculator shows a clear error message when the input is incomplete, malformed, or mathematically invalid.

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