How Does It Work?
- Choose direct or inverseDirect means both quantities move the same way — more items, more cost. Inverse means they move in opposite directions — more workers, less time.
- Enter the three values you knowRead it as a sentence: if A gives B, then C gives what?
- Read the fourth valueThe answer appears with the multiplication and division written out so you can check it.
What Is It Used For?
The rule of three is the quickest way to scale one known relationship up or down. It is the maths behind recipe conversions, unit prices, map scales, fuel estimates and dosage checks — anywhere two quantities keep a constant ratio.
- Scaling a recipe from 4 servings to 7.
- Working out the price of 5 items when 3 items cost a known amount.
- Converting a map scale or a plan measurement to real distance.
- Estimating how long a job takes with a different number of people (inverse).
- Checking a per-unit price against a bulk price before you buy.
Examples
| Problem | Type | Answer |
|---|---|---|
| 3 shirts cost 45 → 5 shirts cost? | Direct | 75 |
| 200 g flour serves 4 → serves 6? | Direct | 300 g |
| 3 workers take 45 min → 5 workers? | Inverse | 27 min |
| A car at 90 km/h takes 4 h → at 120 km/h? | Inverse | 3 h |