Weight Conversion Reference

The exact definitions (since 1959)

1 pound — everything follows from this

0.45359237 kg exactly

1 ounce (avoirdupois)

0.028349523125 kg = 1/16 lb

1 stone

6.35029318 kg = 14 lb exactly

1 US short ton

907.18474 kg = 2000 lb

1 UK long ton

1016.0469088 kg = 2240 lb

1 metric tonne

1000 kg exactly

1 kilogram

2.20462 lb / 35.274 oz

1 grain

0.06479891 g = 1/7000 lb

⚠ The two units that share a name

1 troy ounce

31.1034768 g - gold, silver, platinum

1 avoirdupois ounce

28.349523125 g - everything else

Troy is heavier by

about 9.7%

Short vs long ton

2000 lb vs 2240 lb - 12% apart

Short ton vs tonne

907.18 kg vs 1000 kg

⚠ If a source says 'ton'

find out which one before using it

Within each system

1 pound

16 ounces = 7000 grains

1 stone

14 pounds = 224 ounces

1 US short ton

2000 pounds

1 kilogram

1000 g = 1,000,000 mg

1 metric tonne

1000 kg

Rough conversions for doing it in your head

Kg to pounds

x 2.2 (double and add a tenth)

Pounds to kg

halve, then take off a tenth

Grams to ounces

divide by 28, or by 30 for a rough idea

Ounces to grams

x 28 (the true figure is 28.35)

Stone to kg

x 6.35

⚠ For anything that matters

use the exact figures above

Where weight mistakes actually bite

• A precious-metal price read in ordinary ounces instead of troy
• 'Ton' used without saying whether it is short, long or metric
• A recipe in ounces converted with the troy factor
• Body weight in stone read as a decimal - 10 st 7 lb is not 10.7 st
• Shipping limits quoted in kg against a scale reading pounds
• Fluid ounces treated as ounces of weight - a volume, not a mass
• Net versus gross weight on a package label

10 st 7 lb is not 10.7 stone

The same trap as feet and inches, and it hides just as well because the number looks plausible. Seven pounds is half a stone, so 10 st 7 lb is 10.5 stone. Convert the pounds to a fraction of a stone first, or work entirely in pounds and convert once at the end.

A fluid ounce is not an ounce

A fluid ounce measures VOLUME and an ounce measures WEIGHT. They coincide only for something with the density of water, and only in the US system - the UK fluid ounce is a different size again. A recipe asking for 8 fl oz of honey does not want 8 oz on a scale.