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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 uses 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:

  • Mega = 106

  • Kilo = 103

  • Deci = 10−1

  • Centi = 10−2

  • Milli = 10−3

  • Micro = 10−6

  • Nano = 10−9

  • Pico = 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 70-1.

TABLE 70-1Apothecaries’ 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 70-2).

TABLE 70-2Avoirdupois System of Measurea

A common household unit of measure includes the teaspoon and tablespoon. A teaspoon is equivalent to 5 mL and a tablespoon is equivalent to 3 teaspoons. Other household liquid measurements are listed in Table 70-3.

TABLE 70-3Household System of Measure

Number Systems


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