The SI metric system is the standard weight measurement system across the majority of the globe, with only the US and a few small countries not using it as their national standard. The base unit of the SI metric system is the kilogram which is based on the International Prototype Kilogram (IPK) held in Sèvres, France.
The kilogram is the only base metric unit to use a prefix and other metric weight units use the standard metric prefixes (centi, deci, milli etc).
Table of Metric Weight Units
| Unit | Abbreviation | As Grams | Useful Information |
| microgram | μg | 0.000001 | Used in food and pharmaceutical labelling |
| milligram | mg | 0.001 | Used in food and pharmaceutical labelling |
| centigram | cg | 0.01 | Rarely used |
| decigram | dg | 0.1 | Rarely used |
| gram | g | 1 | Grams are commonly used in retail sales |
| decagram | dag | 10 | Rarely used |
| hectogram | hg | 100 | Rarely used |
| kilogram | kg | 1,000 | Metric system base weight unit |
| megagram | Mg | 1,000,000 | The tonne (metric ton) is 1 megagram |
The avoirdupois weight system is the basis of both the British Imperial and US Customary weight systems. It was first recorded in Britain in the Late 13th or early 14th century and is thought to derive from an old French or old Florentine weight system used by wool merchants.
During the reign of Henry VIII an English statute made the avoirdupois system mandatory and Elizabeth I added the grain of 1/7000th of a pound to the system giving a system that is recognisable as the British Imperial weight units. Although the stone had been part of the avoirdupois system since the 14th Century, it was not formerly incorporated into the British Imperial system until the Weights and Measures Act of 1835 and thus did not make it into the US Customary units.
Table of British Imperial Weight Units
| Unit | Abbreviation | As Kilograms | Useful Information |
| grain | gr | 0.0000648 | Used for precious metals and gunpowder |
| dram | dr | 0.0017718 | 1/16th of an ounce, previously known as the part |
| ounce | oz | 0.02835 | Often used in cooking measures in both the US and UK |
| pound | lb | 0.453592 | Avoirdupois weight system base unit |
| stone | st | 6.35029 | Not part of US customary system |
| quarter | qtr | 12.70058 | Quarter of a hundredweight, also 2 stone |
| hundredweight | cwt | 50.80235 | Defined as 8 stone in the British Imperial system |
| ton | t | 1016.04691 | Also known as the long ton to differentiate it from the US short ton |
The US Customary weight system was effectively the British system as used in 1776 at the time of American independence with minor modifications made thereafter. The major differences between the US Customary system and the British Imperial system are in weights above the pound, with the US keeping the 100lb hundredweight that was abolished by the British Weights and Measures Act of 1824. The quarter and ton also differ from the British system as they are a quarter of a hundredweight and twenty hundredweight respectively.
Table of US Customary Weight Units
| Unit | Abbreviation | As Kilograms | Useful Information |
| grain | gr | 0.0000648 | Used for precious metals and gunpowder |
| dram | dr | 0.0017718 | 1/16th of an ounce, previously known as the part |
| ounce | oz | 0.02835 | Often used in cooking measures in both the US and UK |
| pound | lb | 0.453592 | Avoirdupois weight system base unit |
| quarter | qtr | 11.33981 | Quarter of a hundredweight, 25lb in the US Customary system |
| hundredweight | cwt | 45.35924 | Defined as 100lb in the US Customary system |
| ton | t | 907.18474 | Also known as the short ton to differentiate it from the British long ton |