# Rule of three calculator

> Solve a direct proportion from three values.

[Open tool](https://www.toolcabana.com/gu/rule-of-three) · [ગણતરી અને નાણાં](https://www.toolcabana.com/gu/category/calculators-and-finance)

Tool ID: rule-of-three. Requested language: gu. Description language: en. Complete guide translation: no; untranslated sections use English.

## Overview (en)

The Rule of three calculator takes three known values A, B and C and returns the missing fourth value D. In direct proportion it solves A / B = C / D, giving D = B × C / A. In inverse proportion it solves A × B = C × D, giving D = A × B / C. The formula with your numbers is shown with the result.

## Supported tasks (en)

Solve a direct proportion from three values.

## Steps (en)

1. Adjust the values; an initial result is calculated from the defaults.
2. Change the basic settings or open Advanced mode to fine-tune the result.
3. Review and copy or download your result. Pause Live results to run manually. Sources or individual settings above 20,000 characters require an explicit run.

## Settings

- A (en; key: a; type: number)
- B (en; key: b; type: number)
- C (en; key: c; type: number)
- Proportion (en; key: mode; type: select): Direct [en], Inverse [en]

## Limitations (en)

Use finite numbers, valid dates and the ranges shown by each control. Invalid values produce an error without a downloadable result.

## Privacy and connections (en)

This operation processes its source in your browser. Copy and download are explicit actions; source input is not saved in an account or history.

## Questions (en)

### How do I solve a rule of three problem?

Enter the known pair as A and B and the new value as C, then pick Direct. If 5 items cost 10, then 15 items cost D = 10 × 15 / 5 = 30. The result also displays the formula with your numbers filled in.

### When should I use inverse proportion?

Use Inverse when one quantity goes down as the other goes up, such as workers and days to finish a job. The calculator keeps A × B equal to C × D, so 5 workers taking 10 days means 15 workers take 5 × 10 / 15 days.

### Why does the calculator say A must be greater than zero?

A is a divisor in the direct formula, so it must be greater than zero in both modes. In inverse mode, C is the divisor and must also be greater than zero. B can be any finite number.

### Where is my input processed?

This operation processes its source in your browser. Copy and download are explicit actions; source input is not saved in an account or history.

### What are the input limits?

Use finite numbers, valid dates and the ranges shown by each control. Invalid values produce an error without a downloadable result.

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