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Units of Measure

Pharmaceutical calculations involve four different systems of measure: the International System of Units (SI), formerly known as the metric system, apothecaries’, avoirdupois’, and household. SI is an international decimalized system of measurement that recognizes the following units: gram, liter, and meter. SI is a decimal system, in the sense that all multiples and submultiples of the base units are factors of powers of ten of the unit. Examples of the SI system include:

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Name Prefix Factor
Mega M 106
Kilo k 103
Deci d 10−1
Centi c 10−2
Milli m 10−3
Micro μ 10−6
Nano n 10−9
Pico p 10−12

The apothecaries’ system is a traditional system of measurement using drams (liquids) and grains (solids) and is occasionally found in prescriptions. For example, a prescribed medicine being sold in four ounce (image iv) bottles or five grains (V gr.) of aspirin. The system consists of two basic units, grains for solids and minims for liquids. Examples of the apothecaries’ system are listed in Table 72-1.

TABLE 72-1Apothecaries’ System of Measurea

The avoirdupois system is a system of weights or mass commonly used in the United States for measuring body weight and in selling products. The avoirdupois ounce equals 437.5 grains. Sixteen ounces (7000 grains) corresponds to 1 pound (lb) (Table 72-2).

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