The basic unit of weight in the metric system is the gram. NOTE: The abbreviation for gram is “g.”

The basic unit of volume in the metric system is the liter, abbreviated “1.”

The basic linear unit of the metric system is the meter, abbreviated “m.”

By using the prefixes deka, hecto, and kilo for multiples often, one hundred, and one thousand basic units, and the prefixes micro, mini, centi, and deci for one ten thousandth, one thousandth, one hundredth, and one tenth, you have the basic structure of the metric system. By applying the appropriate basic unit to the scale of figure 8-1, you can readily determine its proper terms. For instance, using the gram as the basic unit of weight, we can readily see that 10 g would be 1 dekagram, 100 g would equal 1 hectogram, and 1000 g are called a kilogram. Conversely, going down the scale, 0.1 g is then called a decigram, 0.01 g a centigram, and 0.001 g is called a milligram. NOTE: In the metric system, no units or their abbreviations are capitalized.

Although fast becoming obsolete, the apothecary system is still used and must be taken into consideration. It has two divisions of measurement: weight and volume. The basic unit of weight is the grain, abbreviated gr, and never capitalized; and the basic unit of volume is the minim.

This system is the one used in the United States for weight only and is used in commercial buying and selling. The pound as we know it when going to the market is the 16-ounce pound of the avoirdupois system. The basic unit of the avoirdupois system is also the grain.

Table 8-1 is a table of weights and measures; it should be thoroughly studied and memorized.

Figure 8-1.—Metric system.

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 understand. Therefore, the household measurements are standardized, assuming that the utensils are common enough to be found in any home.

Table 8-2 is a table of household measures, with their metric and apothecary equivalents.

Table 8-2.—Table of metric doses with approximate equivalents