Systems of Measurement
Safe medication administration is based on the safe calculation of drug dosages. In this chapter, you will be introduced to foundational knowledge for working with both the metric system and the household system in the calculation of drug dosages. Understanding and working with the systems of measurement commonly used in health care is an essential component of nursing.
Learning Objectives
By the end of this section, you will be able to:
- Make unit conversions in the U.S. system
(pounds, teaspoons, tablespoons, cups, ounces) - Make unit conversions in the metric system
(micrograms, milligrams, milliliters, liters, cubic centimeters, kilograms, grams) - Convert between the U.S. and the metric systems of measurement
- Convert between Fahrenheit and Celsius temperatures
- Use military time
- Convert between military and traditional time
In this section, we will see how to convert different types of units, such as feet to miles or kilograms to pounds. The basic idea in all of the unit conversions will be to use a form of , the multiplicative identity, to change the units but not the value of a quantity.
Make Unit Conversions in the U.S. System
There are two systems of measurement commonly used around the world. Most countries use the metric system. The United States uses a different system of measurement, usually called the U.S. system. We will look at the U.S. system first.
The U.S. system of measurement uses units of inch, foot, yard, and mile to measure length, and pound and ton to measure weight. For capacity, the units used are teaspoon, tablespoon, ounce, cup, pint, quart, and gallon.
Both the U.S. system and the metric system measure time in seconds, minutes, or hours.
The equivalencies among the basic units of the U.S. system of measurement are listed in Table 7.2. The table also shows, in parentheses, the common abbreviations for each measurement.
In many real-life applications, we need to convert between units of measurement. We will use the identity property of multiplication to make these conversions. We’ll restate the Identity Property of Multiplication here for easy reference.
Identity Property of Multiplication:
For any real number , and
To use the identity property of multiplication, we write in a form that will help us convert the units. For example, suppose we want to convert inches to feet. We know that foot is equal to inches, so we can write as the fraction . When we multiply by this fraction, we do not change the value but just change the units.
But also equals . How do we decide whether to multiply by or . We choose the fraction that will make the units we want to convert from divide out. For example, suppose we wanted to convert inches to feet. If we choose the fraction that has inches in the denominator, we can eliminate the inches.
On the other hand, if we wanted to convert feet to inches, we would choose the fraction that has feet in the denominator.
We treat the unit words like factors and ‘divide o