Systems of Measurement and Common Equivalents - Clinical Calculations Made Easy: Solving Problems Using Dimensional Analysis, 3rd Edition

Clinical Calculations Made Easy: Solving Problems Using Dimensional Analysis, 3rd Edition

Chapter 2.Systems of Measurement and Common Equivalents

Objectives

After completing this chapter, you will be able to:

1. Identify measurements included in the metric, apothecary, and household systems.

2. Understand abbreviations used in the metric, apothecary, and household systems.

SYSTEMS OF MEASUREMENT

Three systems of measurement are used for medication dosage administration: the metric system, the apothecary system, and the household system. To be able to accurately administer medication, you must understand all three of these systems.

THE METRIC SYSTEM

The metric system is a decimal system of weights and measures based on units of ten in which gram, meter, and liter are the basic units of measurement. However, gram and liter are the only measurements from the metric system that are used in medication administration. The meter is a unit of distance, the gram (abbreviated g or gm) is a unit of weight, and the liter (abbreviated L) is a unit of volume.

The most frequently used metric units of weight and their equivalents are summarized in Box 2.1.

BOX 2.1 Metric System Units of Weight and Equivalents

1 kilogram (kg)

1 gram (g)

1 milligram (mg)

1 microgram (mcg)

1 kg = 1000 g

1 g = 1000 mg

1 mg = 1000 mcg

Another way to understand the metric units of weight and their equivalents is to visualize the relationship between the measurements and equivalents displayed in Figure 2.1.

Figure 2.1. Metric system units of weight and equivalents.

The most frequently used metric units for volume and their equivalents are summarized in Box 2.2.

Another way to understand the metric units of volume and their equivalents is to visualize the relationship between the measurements and equivalents displayed in Figure 2.2.

BOX 2.2 Metric System Units of Volume and Equivalents

1 liter (L)

1 milliliter (mL)

1 cubic centimeter (cc)

1 L = 1000 mL

1 mL = 1 cc

Figure 2.2. Metric system units of volume and equivalents.

THE APOTHECARY SYSTEM

The apothecary system is a system of measuring and weighing drugs and solutions in which fractions are used to identify parts of the unit of measure. The basic units of measurement in the apothecary system include weights and liquid volume. Although this may be replaced by the metric system, it is still necessary to understand it because some physicians continue to order medications using this system, and they also may include Roman numerals in the medication order.

The most frequently used measurements and equivalents within the apothecary system's units of weight are summarized in Box 2.3, and the most frequently used measurements and equivalents within the apothecary system's units of volume are summarized in Box 2.4. Figure 2.3 can help you visualize the equivalents for weight and volume.

BOX 2.3 Apothecary System Units of Weight and Equivalents

1 pound (lb)

1 ounce (oz)

1 dram (dr)

1 grain (gr)

1 lb = 16 oz

1 oz = 8 dr

1 dr = 60 gr

BOX 2.4 Apothecary System Units of Volume and Equivalents

1 gallon (gal)

1 quart (qt)

1 pint (pt)

1 fluid ounce (fl oz)

1 fluid dram (fl dr)

1 minim (M)

1 gal = 4 qt

1 qt = 2 pt

1 pt = 16 fl oz

1 fl oz = 8 fl dr

1 fl dr = 60 M

1 fl oz = 1 oz

1 fl dr = 1 dr

Figure 2.3. Apothecary system of equivalents for weight and volume. Please note that the figures are not shown to scale.

THE HOUSEHOLD SYSTEM

The use of household measurements is considered inaccurate because of the varying sizes of cups, glasses, and eating utensils, and this system generally has been replaced with the metric system. However, as patient care moves away from hospitals, which use the metric system, and into the community, it is once again necessary for the nurse to have an understanding of the household measurement system to be able to use and teach it to clients and families.

The most frequently used measurements and equivalents within the household measurement system are summarized in Box 2.5. Figure 2.4 can help you visualize the equivalents.

BOX 2.5 Household Measurement System and Equivalents

1 cup

1 tablespoon (tbsp or T)

1 teaspoon (tsp or t)

1 drop (gtt)

1 cup = 8 ounces (oz)

2 Tbsp = 1 oz

3 tsp = 1 tbsp

1 tsp = 60 gtt

Figure 2.4. Household measurement system and equivalents for volume. Please note that the figures are not shown to scale.

Temperature

Clients and families are required to monitor temperature changes associated with various medical conditions. Two thermometers may be used for monitoring temperature: a Fahrenheit thermometer or a Celsius thermometer. The nurse must be able to explain both of these systems of measurement when discharging clients and families.

The most frequently used measurements for Celsius and Fahrenheit are summarized in Figure 2.5.

Figure 2.5. Conversion chart for Celsius to Fahrenheit.

Box 2.6 summarizes a method for converting between Celsius and Fahrenheit or Fahrenheit and Celsius. This easy method requires addition, subtraction, multiplication, or division.

BOX 2.6 Temperature Conversion Method

To convert from Fahrenheit to Celsius:

°C = (°F - 32) ÷ 1.8

To convert from Celsius to Fahrenheit:

°F = °C × 1.8 + 32

°C = temperature in degrees Celsius

°F = temperature in degrees Fahrenheit

COMMON EQUIVALENTS

Sometimes it is necessary to convert from one system to another to accurately administer medication. See Table 2.1 for approximate equivalents for weight and Table 2.2 for approximate equivalents for volume.

TABLE 2.1 Approximate Equivalents for Weight

METRIC	APOTHECARY
1 kg (1000 g)	2.2 lb
1 g (1000 mg)	15 gr
60 mg	1 gr

TABLE 2.2 Approximate Equivalents for Volume

METRIC	APOTHECARY	HOUSEHOLD
4000 mL	1 gal (4 qt)
1 L (1000 mL)	1 qt (2 pt)
500 mL	1 pt (16 fl oz)
240 mL	8 oz	1 cup (1 glass)
30 mL	1 oz (8 dr)	2 tbsp
15 mL	½ oz (4 dr)	1 tbsp (3 tsp)
5 mL	1 dr (60 M)	1 tsp (60 gtt)
1 mL (1 cc)	15 M	15 gtt
1 mL (1 cc)	1 M	1 gtt

PREVENTING MEDICATION ERRORS

Understanding the three systems of measurement will assist in preventing medication errors. Every nurse should have a chart that clearly identifies the conversions between the three systems of measurement to recheck answers for accuracy.

SUMMARY

This chapter has reviewed the metric, apothecary, and household systems of measurement. To assess your understanding and retention of the systems of measurement, complete the following practice problems.

Practice Problems for Chapter 2: Systems of Measurement and Common Equivalents (See page 39 for answers)

Write the correct abbreviation symbols for the following measurements from the metric system: