About this tool
Applies the simple rule of three (cross multiplication) to find an unknown value among four proportionally related ones. You know three values and want the fourth. Useful for adjusting recipes to different portion sizes, calculating unit prices, converting scales, and any problem involving direct proportion.
How to use
- Fill in three of the four fields with the values you know.
- The missing field is calculated automatically.
- Change any of the three values and the fourth recalculates in real time.
Frequently asked questions
- How does cross multiplication work?
- In a proportion A/B = C/D, you multiply across: A × D = B × C. Knowing three of the four values, you isolate the fourth. For example, if 4 apples cost 2 euros and you want to know what 10 cost, then 4/2 = 10/X, so X = (10 × 2) / 4 = 5 euros.
- What if the relationship isn't directly proportional?
- The simple rule of three only works when two quantities vary in the same proportion (more of one means more of the other). For inverse relationships (more of one means less of the other), there's a variant called the inverse rule of three, which this tool doesn't cover.
- Can I use different units?
- Yes, as long as you're consistent. If A is in euros, C must also be in euros. Mixing units (grams with kilos, for instance) gives a wrong result. Convert beforehand or use the same unit on both sides.