The Metric System in Medication Administration

In nursing, the metric system is a decimal system used for all measurements of volume, length, and weight. These measurements are interrelated, making it important to understand their relationships, especially for dosage calculations. Mastery of the system allows seamless conversion between units by shifting the decimal point, thereby simplifying equations and minimizing the number of steps that one must do to perform dosage calculations.

Review of the Metric System

When considering dosages, values range from whole numbers up into the millions to fractions in the thousandths.

Important Points for Writing Numbers

Basic Clinical Use Metric Measurements

When working with units in dosage administration, the three commonly used units are:

• Length: meter
  • Abbreviated as: m
• Volume: liter
  • Abbreviated as: L
    • Notice that it is capitalized
• Weight: gram
  • Abbreviated as: g
    • Don't confuse this and write it as gm, which is incorrect.

Knowing that these common units of measurement are your starting point, you will be able to easily differentiate smaller and larger units. You will see the units combined with prefixes that will help you understand and know that the unit of measure is either smaller or larger than the given basic unit. The other thing to know is that you will only be dealing with three smaller measures and one larger measure that the basic units.

Metric Measures and Their Abbreviations

Prefixes

kilo- is the only prefix that you will encounter in the clinical setting that is a larger measure compared to the basic unit. This is commonly used for body weight and is the recommended unit of measure for infants and children.

The only three smaller measures compared to the basic units that you will encounter are milli-, micro-, and centi-.

These prefixes will be seen in combination with the basic units, and the advantage of using the basic metric units of measure is that you know exactly what you are measuring. For example, (prefix)kil0- gram(suffix) – kilogram is measuring weight since we know that gram is a basic unit of measure of weight. The prefix kilo- is telling us that the weight measurement is bigger than the standard unit of gram. So, when you look and compare 1 kilogram to grams, we know that this is equivalent to 1000 grams.

The table below further provides the conversions between the prefixes to the base units. Notice that each prefix converts the measurement by +/- 1000. In other words, it moves the decimal place to the right or left three times.

One important thing to remember is that if you are converting from one unit to a smaller unit, you will move the decimal point three places to the right and the number must get larger. Example of this would be going from gram to milligram. 1 gram is equal to 1000 mg. The opposite is also true. If you are converting from a smaller unit to a larger unit, your number will get smaller. Example of this would be going from milliliters to liters. 500 mL is equal to 0.5 L.

Medical Unit Conversions

Commonly Used Units for Weights

From Greater to Lesser

• kg = kilogram
• g = gram
• mg = milligram
• mcg = microgram

Commonly Used Units for Volume

From Greater to Lesser

• L = liter
• mL = milliliter

Omitting Errors During Practice

• Fractional dosages should include a zero in from of the decimal point.
  • ex. 0.2 mg
• Get rid of unnecessary zeros when writing metric dosages.
  • ex. 0.20 mg is wrong and should be written as 0.2 mg.
• If the dosage that you receive from the physician is unreasonably large or small, questions the order and double check.
• Conversions between g, mg, mcg, mL and L units of measure require moving the decimal point three places.