The Metric System The metric system is the official system of weights and measures used by Navy Pharmacy Departments for weighing and calculating pharmaceutical preparations. The metric system is becoming the accepted system throughout the world. Hospital Corpsmen need to be concerned primarily with the divisions of weight, volume, and linear measurement of the metric system. Each of these divisions has a primary 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 for multiples of, respectively, ten, one hundred, and one thousand basic units, and the prefixes micro, milli, centi, and deci for one-ten thousandth, one- thousandth, one-hundredth, and one-tenth, respectively, you have the basic structure of the metric system. By applying the appropriate basic unit to the scale of figure 6-1, you can readily determine its proper terms. For example, using the gram as the basic unit of weight, we can readily see that 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 System Although fast becoming obsolete, the apothecary system for weighing and calculating pharmaceutical preparations is still used and must be taken into consideration. It has two divisions of measurement: weight and volume. In this system, the basic unit of weight is the grain (abbreviated “gr”), and the basic unit of volume is the minim (abbreviated “m”). The Avoirdupois System The avoirdupois system is a system used in the United States for ordinary commodities. The basic units of the avoirdupois system are dram (27.344 grains), ounce (16 drams), and pound (16 ounces). Table of Weights and Measures See table 6-1, a table of weights and measures; study it thoroughly. Converting Weights and Measures Occasionally, there are times when it will be necessary to convert weights and measures from one system to another, either metric to apothecary or vice versa. Since patients can hardly be expected to be familiar with either system, always translate the dosage directions on the prescription into a household equivalent that they can understand. Household measurements are standardized, on the assumption that the utensils are common enough to be found in any home. Table 6-2 is a table of household measures, with their metric and apothecary equivalents. CAUTION: For the conversion of specific quantities in a prescription or in converting a pharmaceutical formula from one system to another, exact equivalents must be used. CONVERSION As stated earlier, in the practice of pharmacy it may be necessary to convert from one system to another to dispense in their proper amounts the substances that have been ordered. Although the denominations of the metric system are not the same as the common systems, the Bureau of International Standards has established conversion standards that will satisfy the degree of accuracy required in almost any practical situation. Ordinary pharmaceutical procedures generally require something between two- and three-figure accuracy, and the following tables of conversion (tables 6-3 and 6-4) are more than sufficient for practical use. Naturally, if potent agents are involved, you must use a more precise conversion factor for purposes of calculation. 6-11 HM3F0601 GRAM METER LITER KILO HECTO DEKA ONE LINE deci centi milli DECIMAL LINE 100 1,000 10 1 01 001 1/10 1/100 1/1000 1. Figure 6-1.—Graph comparing the metric system with the decimal equivalent.