---
title: "Rule of Three Calculator | Numequo"
description: "Solve a direct proportion from three known values and see every cross-multiplication step. Includes the formula and practical examples."
canonical: "https://www.numequo.com/math/proportions-ratios/rule-of-three-calculator/"
locale: "en"
---
[Numequo](https://www.numequo.com/) › [Math](https://www.numequo.com/math/) › [Proportions and ratios](https://www.numequo.com/math/proportions-ratios/)

# Rule of Three Calculator

Complete the proportion: if a first quantity corresponds to a second, discover what the new quantity corresponds to.

## Input data

- **If this quantity** - type: number; default: 3; step: 0.01
- **Corresponds to** - type: number; default: 12; step: 0.01
- **Then this quantity** - type: number; default: 5; step: 0.01

## How it works

For A : B = C : X, cross multiplication gives A × X = B × C, therefore X = (B × C) ÷ A.

## Quick examples

- **3 → 12, 5 → ?** - If this quantity: 3, Corresponds to: 12, Then this quantity: 5
- **2 kg → €7, 5 kg → ?** - If this quantity: 2, Corresponds to: 7, Then this quantity: 5
- **4 people → 300 g, 6 → ?** - If this quantity: 4, Corresponds to: 300, Then this quantity: 6

## Understanding your result

The missing value keeps the same proportional relationship as the first pair. Doubling the first quantity therefore doubles its corresponding value.

Last verified: 29 September 2026 (2026-09-29)

## Frequently asked questions

### What is the rule of three?

It is a method for finding a fourth value when two ratios are equal. It applies to directly proportional quantities.

### How do I perform the cross multiplication?

Multiply the two diagonally opposite known values, then divide that product by the known value opposite the unknown.

### Can the rule of three be used for prices?

Yes, when the unit price stays constant. For example, if 2 kg cost €7, then 5 kg cost €17.50.

### When should I not use a direct rule of three?

Do not use it when the relationship is not proportional, such as a fixed fee plus a variable price, or when one quantity decreases as the other increases.

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