Common weight units:

• Kilogram (kg): SI base unit of mass
• Pound (lb): US customary & imperial
• Ounce (oz): 1/16 of a pound
• Gram (g): 1/1000 of a kilogram
• Stone (st): UK body weight unit
• Metric ton (t): 1000 kilograms
• Milligram (mg): 1/1000 of a gram

Key relationships:

• 1 kg = 2.205 lb = 35.274 oz
• 1 lb = 0.4536 kg = 16 oz
• 1 oz = 28.35 g
• 1 stone = 14 lb = 6.350 kg

The Kilogram:

Originally defined in 1795 as the mass of one liter of water at 4 degrees Celsius. In 1889, it was redefined by a physical platinum-iridium cylinder kept in Paris. Since 2019, it is defined by the Planck constant.

The Pound:

Derived from the Roman libra (hence the abbreviation "lb"). The modern avoirdupois pound was standardized in England and is defined as exactly 0.45359237 kilograms since 1959.

The Stone:

An ancient unit of weight used in trade, varying by commodity and region. Standardized at 14 pounds in England in 1835 and still commonly used for body weight in the UK and Ireland.

Metric units (kg, g, mg) are used for:

• Scientific and medical measurements
• International commerce and trade
• Food labeling (most countries)
• Industrial manufacturing

Imperial units (lb, oz, st) are used for:

• Body weight (US uses lb, UK uses stones)
• Cooking and recipes (US)
• Postal and shipping (US)
• Grocery and retail (US)

Specialized units:

• Metric tons: shipping and heavy industry
• Carats: gemstones and precious metals
• Troy ounces: precious metals trading