What exactly is the rule of three?

It is a method for solving proportions. If A relates to B the way C relates to an unknown D, then D = (B × C) / A. All you need are three numbers and you can find the fourth.

When should I use the direct version?

Use it whenever both quantities move in the same direction: more hours = more pay, more fabric = higher cost. If they move in opposite directions (more workers = fewer days), use the inverse version instead.

Can it handle decimals and large numbers?

Yes — the calculator supports decimal inputs and lets you control how many decimal places the answer displays, up to 10.

Is this the same as cross-multiplication?

Exactly. The rule of three is applied by cross-multiplying the proportion and solving for the unknown.

Rule of Three (Direct)

Solve direct-proportion problems instantly. Enter three values and find the missing fourth.

First Ratio

Value A

Unit (optional)

Value B

Unit (optional)

Second Ratio

Value C

Unit (optional)

Value X (result)

Unit (optional)

Decimal Places

Features

Instant direct-proportion solver

Enter three values and get the missing fourth in one click

Step-by-step breakdown

See the formula and working so you can double-check or learn the method

Adjustable precision

Choose how many decimal places the answer should show

Optional unit labels

Add units like kg, $, or km for clearer context

How to use this tool

1
Enter the known trio — Fill in the three values that define your proportion — for example 2 kg costs $5, and you need the price for 7 kg
2
Add units if you like — Labelling the fields with kg and $ keeps things readable, especially when you share results
3
Hit Calculate — The answer appears immediately alongside the formula used
4
Review the steps — Expand the working section to see how the cross-multiplication was performed

Common uses

Cooking & baking

Scale a recipe from 4 servings to 10 without guesswork

Shopping & budgeting

Figure out cost per unit or total cost at a bulk rate

Currency conversion

Convert amounts when you know the current exchange rate

Distance & travel

Estimate arrival time when you change cruising speed

Frequently asked questions

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