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.