U.S. Customary Units and the Apothecary System

Key terms

Objective 1

Apothecary System

One of the oldest measurement systems used to calculate medications based on grains and minims.

Avoirdupois System

The English system for measuring weight, in which 1 pound is 16 ounces; also used in the United States.

Units

Basic measurement used to determine the strength of medications.

U.S. Customary System (Household Measurements)

System of measurement based on common kitchen measuring utensils.

Objective 1

Working with medications has always been a challenge in the United States. Physicians, based on their educational background, may use the metric system, the apothecary system, or the U.S. Customary System (household measurements) and the avoirdupois system when prescribing medications.

As metric measurements become the predominant format for medication packaging, the use of the apothecary system will become minimal. The apothecary system is an approximate system and is less accurate than the metric system. Furthermore, the use of abbreviations and symbols in the apothecary system has a greater potential for error.

In this chapter, we continue to use ratio and proportion strategies (see Chapter 5) to set up our conversion problems. In addition, we refer to the metric conversions from Chapter 7. Conversion between the metric system and the U.S. Customary System (household measurements), or the avoirdupois system, is needed by caregivers who are dispensing liquid medication in a home setting. Figure 8-1 demonstrates measuring tools commonly used in health care and home settings for dispensing liquid medication.

Figure 8-1 Common measuring tools for dispensing liquid medication. (From Fulcher RM, Fulcher E: *Math calculations for pharmacy technicians: A worktext*, St. Louis, 2007, Saunders.)

Objective 2

Apothecary system

The apothecary system is an old English system of measurement that became popular in the United States in the 18th century. The base units in the apothecary system are the grain (weight), minim (volume), and inch (length) (Box 8-1). A grain is the dry weight measurement equal to a large grain of wheat. Minims are based on the amount of liquid that equals the weight of a grain, and the inch is the approximate length of two wheat grains.

BOX 8-1 APOTHECARY MEASUREMENTS

Weight*	Volume	Length
Grain	Minim	Inch
Scruple^†	Fluid dram	Yard
Dram	Fluid ounce	Mile
Ounce	Pint
Pound	Quart
	Gallon

^*From smallest measurement to greatest measurement.

^†Scruples were never used as a pharmacology measurement.

The apothecary system is a familiar measurement system in the United States because household equivalents were developed for conversion from apothecary measures (Table 8-1). Because of the lack of consistent accuracy, however, the apothecary system is being phased out in favor of the metric system in the field of pharmacology.

TABLE 8-1

Equivalents in the Apothecary System

Type of Substance	Abbreviation	Unit of Measure	Common Equivalents
Volume/liquid		minim	—
	fl3	fluid dram	lx (60)
	fl	fluid ounce	3 viii (8)
Mass	gr	grain	—
	3	dram	gr lx (60)
		ounce	3 viii (8)
	#	pound	xii (12)*