Three are three common measurement systems in pharmacy – the metric system, the apothecary system and the avoirdupois system. The metric system includes units of weight, volume, and linear measure; the apothecary system includes units of weight and volume; and the avoirdupois system includes only units of weight.
The Metric System
The metric system is the most widely used system in pharmacy. In the metric system, the gram is the main unit of weight, the liter the main unit of volume, and the meter the main unit of length.
The Apothecary System
The apothecary system provides for the measurement of both weight and volume. This system is based on the grain (abbreviated gr) as a measurement of weight, and the fluid ounce (fl oz) as a measurement of volume.
The Avoirdupois System
The avoirdupois system is used in commerce to supply bulk chemicals and other items by weight, in ounces or pounds.