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CALCULATORS

Rule of Three Calculator

Enter three values in a direct proportion to calculate the fourth, and compare two values as a simplified ratio, decimal, and percent.

Tool statusRuns in your browser
Preview

Rule of three

A is to B as C is to X.

Proportion comparison

Results

Missing value (X)
20
Ratio
16:9
Decimal
1.778
Percent
177.78%

Proportion calculated.

How it works

  1. Enter your input

    Fill in the values, paste your text, or upload the file this tool works with.

  2. See results instantly

    Most tools update live as you type; a few use a single button. Either way, the result appears right on this page.

  3. Use your results

    Copy, download, or share what the tool produces — you're always in control of the output.

Privacy and processing — Runs locally in your browser. This tool does not upload your input.

What is Rule of Three Calculator?

Find the fourth value in a direct proportion, or compare two values to see their ratio, decimal, and percentage.

UNDERSTAND THE TOOL

How to use the rule of three calculator

When to use it

Use this calculator when three proportional values are known and you need the missing one, such as price versus quantity or distance versus time.

Steps

  • Enter the base pair of values.
  • Enter the known value in the second pair.
  • Read the calculated missing value and copy it if needed.

Formula

Direct proportion
X = (B x C) / A

Worked example

Missing value
x = (b * c) / a

If 4 items cost 10, what do 7 cost?

The proportional result is 17.5.

(10 * 7) / 4 = 17.5

Proportion notes

Rule-of-three assumes a direct linear relationship. If the real-world relationship is not proportional, the result is only a rough shortcut.

Frequently Asked Questions

Does it handle inverse proportion?

This first version is focused on direct proportion, the most common everyday case.

Can I compare two numbers without the rule of three?

Yes. The proportion section shows ratio, decimal value, and percentage.

Can I use decimal values?

Yes. Decimal values are supported as long as they form a valid calculation.

Can I use zero or negative values?

No. This tool only accepts positive finite values because it models direct proportion and ratio comparison.

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