Rule of Three Calculator guide
How to Convert Units and Currency with the Rule of Three
Any conversion is a rule of three built on one known equivalence. If 1 US dollar is 0.92 euros, then 250 dollars is x, giving x = (0.92 × 250) ÷ 1 = 230 euros. The same layout converts kilometres to miles, kilograms to pounds, or Celsius intervals — anything where a fixed rate applies.
Every conversion has the same shape
To convert anything, you need one known equivalence — one dollar is 0.92 euros, one mile is 1.609 kilometres, one kilogram is 2.205 pounds. That single fact is the entire input.
From there, the rule of three handles any amount. There is no separate method for currency and distance and weight; it is one method with different numbers in it.
Converting currency
You have 250 US dollars and the rate is 0.92 euros to the dollar.
- $1 → €0.92
- $250 → x
Working it out
x = (0.92 × 250) ÷ 1 = €230.
When one side of the proportion is 1, the division does nothing and the whole thing collapses to a single multiplication. That is why simple conversions feel easier than other proportions — they are the same operation with the hard part already removed.
Converting back the other way
Going from euros to dollars uses exactly the same equivalence, just with the columns swapped.
€0.92 → $1, and €230 → x, giving x = (1 × 230) ÷ 0.92 = $250. Back where we started, which is a useful check that you set it up correctly.
The rule of thumb: if you are converting to the currency whose rate is listed, multiply. If you are converting from it, divide. Writing the two rows out means you never have to remember which.
Useful equivalences
| To convert | Use | Example |
|---|---|---|
| Miles to kilometres | 1 mile = 1.609 km | 5 miles = 8.05 km |
| Kilograms to pounds | 1 kg = 2.205 lb | 70 kg = 154.3 lb |
| Inches to centimetres | 1 in = 2.54 cm | 12 in = 30.48 cm |
| Litres to US gallons | 1 L = 0.264 gal | 40 L = 10.57 gal |
| Ounces to grams | 1 oz = 28.35 g | 8 oz = 226.8 g |
The one conversion this does not work for
Temperature between Celsius and Fahrenheit is the exception, and it is worth knowing why.
Converting 20°C is not a proportion, because the two scales do not share a zero point — 0°C is 32°F, not 0°F. So there is a fixed offset in the way, and a proportion cannot handle offsets. You need the full formula: F = C × 9/5 + 32.
The general principle is the one from the direct-proportion guide: a rule of three needs both quantities to be zero at the same moment. Zero dollars is zero euros, and zero miles is zero kilometres, so those work. Zero Celsius is not zero Fahrenheit, so that one does not.
Temperature *differences* are fine, though — a rise of 10°C really is a rise of 18°F, because the offset cancels out when you subtract.
