The Metric SystemThe metric system is the official system of weightsand measures used by Navy Pharmacy Departmentsfor weighing and calculating pharmaceuticalpreparations. The metric system is becoming theaccepted system throughout the world. HospitalCorpsmen need to be concerned primarily with thedivisions of weight, volume, and linear measurementof the metric system. Each of these divisions has aprimary or basic unit and is listed below:Basic unit of weight is the gram, abbreviated “g”Basic unit of volume is the liter, abbreviated “l”Basic linear unit is the meter, abbreviated “m”By using the prefixes deka, hecto, and kilo formultiples of, respectively, ten, one hundred, and onethousand basic units, and the prefixes micro, milli,centi, and deci for one-ten thousandth, one- thousandth,one-hundredth, and one-tenth, respectively, you havethe basic structure of the metric system. By applying theappropriate basic unit to the scale of figure 6-1, you canreadily determine its proper terms. For example, usingthe gram as the basic unit of weight, we can readily seethat 10 g equals 1 dekagram, 100 g equals 1 hectogram,and 1000 g is referred to as a kilogram. Conversely,going down the scale, 0.1 g is referred to as a decigram,0.01 g is called a centigram, and 0.001 g is a milligram.The Apothecary SystemAlthough fast becoming obsolete, the apothecarysystem for weighing and calculating pharmaceuticalpreparations is still used and must be taken intoconsideration. It has two divisions of measurement:weight and volume. In this system, the basic unit ofweight is the grain (abbreviated “gr”), and the basicunit of volume is the minim (abbreviated “m”).The Avoirdupois SystemThe avoirdupois system is a system used in theUnited States for ordinary commodities. The basic unitsof the avoirdupois system are dram (27.344 grains),ounce (16 drams), and pound (16 ounces).Table of Weights and MeasuresSee table 6-1, a table of weights and measures;study it thoroughly.Converting Weights and MeasuresOccasionally, there are times when it will benecessary to convert weights and measures from onesystem to another, either metric to apothecary or viceversa. Since patients can hardly be expected to befamiliar with either system, always translate thedosage directions on the prescription into a householdequivalent that they can understand. Householdmeasurements are standardized, on the assumptionthat the utensils are common enough to be found in anyhome. Table 6-2 is a table of household measures, withtheir metric and apothecary equivalents.CAUTION:For the conversion of specificquantities in a prescription or in converting apharmaceutical formula from one system toanother, exact equivalents must be used.CONVERSIONAs stated earlier, in the practice of pharmacy it maybe necessary to convert from one system to another todispense in their proper amounts the substances thathave been ordered. Although the denominations of themetric system are not the same as the commonsystems, the Bureau of International Standards hasestablished conversion standards that will satisfy thedegree of accuracy required in almost any practicalsituation. Ordinary pharmaceutical proceduresgenerally require something between two- andthree-figure accuracy, and the following tables ofconversion (tables 6-3 and 6-4) are more thansufficient for practical use. Naturally, if potent agentsare involved, you must use a more precise conversionfactor for purposes of calculation.6-11HM3F0601GRAMMETERLITERKILOHECTODEKAONELINEdecicentimilliDECIMAL LINE1001,000101010011/101/1001/10001.Figure 6-1.—Graph comparing the metric system with thedecimal equivalent.