What is direct proportion?

Two quantities are directly proportional when one increases by a factor and the other increases by the same factor. Their ratio remains constant.

How do I find an unknown in a proportion?

Use cross multiplication for direct proportion. For inverse proportion, divide the constant product of one pair by the known value in the other pair.

Should I choose direct or inverse proportion?

Choose direct when both quantities increase or decrease together. Choose inverse when one increases while the other decreases.

What does a 1:500 scale mean?

One unit in the drawing represents 500 of the same units in reality. This is a reduction scale because the drawing is smaller than the actual object.

How is an enlargement scale written?

When the drawing is larger than the actual object, the ratio is written as n:1. A drawing eight times larger than reality is correctly shown as 8:1.

Can I use different units in a scale calculation?

Yes. Select mm, cm, m, or km independently for drawing and actual length. The calculator normalizes the units automatically.

How do I simplify a decimal ratio?

Multiply both sides by the same power of 10 to produce integers, then divide by the GCD. For example, 1.5:2.5 becomes 15:25 and then 3:5.

Tools/🧮 Calculators/Ratio & Proportion Calculator

Ratio & Proportion Calculator

Solve direct and inverse proportions, calculate scale, and simplify ratios.

Version 1.0 · Last Updated: August 8, 2026 · Maintained by ToolLarder

Ratio, proportion, and scale calculations

The Ratio & Proportion Calculator combines direct proportion, inverse proportion, unknown value, scale, and ratio simplification tools on one page and shows the calculation used for every result.

Direct proportion calculator

Two quantities are directly proportional when they increase at the same rate. In A/B = C/x, cross multiplication finds the unknown.

x = B × C ÷ A

Inverse proportion calculator

Two quantities are inversely proportional when one decreases as the other increases. The product of paired values stays constant.

A × B = C × x → x = A × B ÷ C

Find an unknown in a proportion

When three values are known, choose the direct or inverse relationship to calculate the fourth value.

A / B = C / x

Technical, map, and drawing scale calculator

Compare drawing and actual length to produce a 1:n reduction, 1:1 full-size, or n:1 enlargement scale with automatic unit conversion.

Drawing : Actual = Scale numerator : Scale denominator

Decimal ratio simplifier

Convert integers or decimals to a common integer scale, then divide by the GCD to find the equivalent ratio in lowest terms.

1.5 : 2.5 → 15 : 25 → 3 : 5

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