Measurement Systems and Their Equivalents

Milliequivalents are used to indicate the strength of certain drugs such as potassium chloride, sodium bicarbonate, and sodium chloride. Milliequivalents (mEq) are expressions of the number of grams of equivalent weight of a drug contained in 1 mL of a normal solution. Electrolytes are usually measured in milliequivalents.

International units are standardized units also used to indicate the strength of certain drugs. Vitamins and other substances such as heparin, insulin, and penicillin are measured in units or international units. The size of a unit varies for each drug. International units do not measure medications in terms of their physical weight or volume. Neither units nor milliequivalents can be directly converted into the metric, household, or apothecary systems.


In the 12-hour clock, the times of day are expressed as either between midnight and noon (as A.M.), or between noon and midnight (as P.M.). For example, 3:33 A.M. occurs after midnight but before noon, whereas 3:33 P.M. occurs after noon but before midnight. In this clock system, each numbered hour occurs twice per day. This clock is primarily used by the public in the United States. In the 24-hour clock, the times of day are expressed uniquely. Hours and minutes are written as follows: 3:33 A.M. is expressed as 0333; 3:33 P.M. is expressed as 1533. Therefore, a unique number (01 through 24) is used to express the 24 hours of the day. When midnight falls, the minutes during the 12:00 A.M. hour are expressed as “00,” not as “24.” Therefore, 12:33 A.M. would be expressed as “0033,” not “2433.” This clock is used in nearly every country throughout the world, and in the United States by scientists, the military, and other professionals.


For common usage, the Celsius (Centigrade) scale ranges from 0°C (freezing) to 100° (boiling), and the Fahrenheit scale ranges from 32°F (freezing) to 212°F (boiling). There is a 180° difference between the freezing and boiling points on the Fahrenheit scale and a 100° difference between the freezing and boiling points on the Celsius scale. The difference between the two scales can therefore be expressed as 180/100, which reduces to 9/5. Most of the world uses the Celsius scale, but the United States uses the Fahrenheit scale.


Most prescriptions and medication orders are written in the metric system. Medications are usually ordered in a unit of weight measurement, such as grams or grains. You must be able to convert between units of measurement within the same system or convert units of measurement from one system to another. Table 9-7 summarizes equivalent measures for volume in three different systems.