Weight conversion is one of those tasks that feels trivial until a recipe, a shipping form and a bathroom scale all use different units in the same afternoon. The metric side is straightforward decimal arithmetic. The imperial side is not: sixteen ounces to a pound, fourteen pounds to a stone, and two different definitions of "ton" depending on which side of the Atlantic you are on.
Strictly, these are units of mass rather than weight — weight is a force and changes with gravity, mass does not. Outside physics the distinction is universally ignored, and a kitchen scale in Denver reads the same as one in Amsterdam because it is calibrated for mass.
Key conversion factors
| From | To | Multiply by |
|---|---|---|
| Kilograms | Pounds | 2.20462 |
| Pounds | Kilograms | 0.453592 |
| Grams | Ounces | 0.035274 |
| Ounces | Grams | 28.3495 |
| Stone | Pounds | 14 |
| Stone | Kilograms | 6.35029 |
| US ton (short) | Kilograms | 907.185 |
| Metric tonne | Kilograms | 1,000 |
| UK ton (long) | Kilograms | 1,016.05 |
Mental shortcuts that are close enough
- Kilograms to pounds: double it and add ten percent. 70 kg → 140 + 14 = 154 lb, against a true 154.3.
- Pounds to kilograms: halve it and subtract ten percent. 150 lb → 75 − 7.5 = 67.5 kg, against a true 68.0.
- Ounces to grams: multiply by 28, then nudge up slightly.
- A litre of water weighs one kilogram, which makes water the easiest thing in the kitchen to convert.
Three tons, one word
The short ton used in the United States is 2,000 lb. The long ton still used in some UK contexts is 2,240 lb. The metric tonne is 1,000 kg, or about 2,205 lb. They differ by up to 12%, which is enough to matter on a freight invoice. When a document says "ton" without qualification, the country of origin is usually the best clue — but on anything contractual it is worth asking.
Where precision actually matters
- Baking: flour and sugar by weight rather than volume is the single biggest improvement most home bakers can make; a "cup" of flour varies by up to 20% depending on how it is packed.
- Medication dosing: paediatric doses are calculated per kilogram, so a pounds-to-kilograms slip is a clinical error, not a rounding issue.
- Shipping: carriers bill on the greater of actual and volumetric weight, and thresholds are hard cutoffs.
- Freight and structural work: an error at tonne scale compounds quickly.
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