The units of mass
The kilogram is the SI base unit of mass, and since 2019 it has been defined by the Planck constant rather than by the platinum cylinder kept near Paris for 130 years. The pound is defined as exactly 0.45359237 kilograms, so imperial and metric masses convert exactly. Note that these are units of mass, not weight: weight is a force and changes with gravity, which is why a kilogram of anything weighs a sixth as much on the Moon while still being a kilogram.
| Unit | Symbol | Equals |
|---|---|---|
| Milligrams | mg | 0.000001 kg |
| Grams | g | 0.001 kg |
| Kilograms | kg | 1 kg |
| Tonnes | t | 1,000 kg |
| Ounces | oz | 0.02834952 kg |
| Pounds | lb | 0.45359237 kg |
| Stone | st | 6.35029318 kg |
How to convert by hand
Every mass conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Multiply by the source unit's factor to reach kilograms, then divide by the target's. The stone, still common in the UK and Ireland for body weight, is 14 pounds or 6.35 kg.
The equivalents people look up most
| Conversion | Result |
|---|---|
| A bag of sugar | 1 kg · 2.2 lb |
| A newborn baby | 3.4 kg · 7 lb 8 oz |
| An adult | 70 kg · 154 lb · 11 st |
| A small car | 1,200 kg · 1.2 t |
| A letter | 20 g · 0.7 oz |
Common mistakes
- Rounding a kilogram to 2.2 pounds. It is 2.2046, and over a tonne that shortfall is over 4 pounds.
- Mixing up ounces of mass with fluid ounces of volume. They measure different things and share only a name.
- Treating mass and weight as the same. On Earth the numbers coincide, which is why nobody notices until they leave.
- Forgetting that a stone is 14 pounds, not 10. Body weights in the UK are quoted as 11 st 4 lb, not as a decimal.