Systems of Measurement

Three systems of measurement are used in medication administration: 

Metric System is based on the decimal system and uses the gram as the basic unit of weight.  The liter is the basic unit of volume, and the meter is the basic unit of length. For correct metric notation, when an amount is less than one, a zero should be placed in front of the decimal point. For example: 0.5mg.

Accepted abbreviations for metric system measures frequently used in dosage calculations are as follows:

Metric Abbreviations for Weight
kilogram kg
gram g
milligram mg
microgram mcg

Metric Equivalencies for Weight
1 kilogram = 1000 g
1gram = 1000 mg
1 milligram = 1000 mcg

When converting within the metric system, you can simply move the decimal point three places to the left or right, depending whether you are changing to a larger or smaller unit of measure.  The method I have found useful is to consider the alphabet:


The letter L represents larger unit, the letter S represent smaller unit. To convert from large to small, the decimal moves three places in the direction you move from L to S, or to the right. Example 1 illustrates that move. To convert from a smaller unit to a larger one, the decimal moves three places in the direction you move from S to L, or to the left. Example 2 illustrates that move.

Example 1: change 200mg to mcg

200 mg goes to 200,000mcg

Example 2: change 200 mg to g

200. 0 mg goes to 0.2 g

Metric Abbreviations for Volume
liter L
milliliter mL
cubic centimeter cc
Metric Equivalencies for Volume
1000 mL = 1 L
1cc = 1 mL (and can be used interchangeably)

The Apothecary System uses grains, drams and ounces as units of weight.  Lowercase Roman numerals are used when amounts are greater than one; fractions are used when amounts are less than one.  The abbreviation precedes the amount, for example, grains x or grains 1/150.  One-half may be expressed as ss, thus: iiiss = 3 1/2.

Apothecary Abbreviations
grain gr
dram dr
ounce oz

Equivalencies that should be memorized when converting from apothecary to metric are:

Apothecary & Metric Equivalencies

1 ounce

30 cc = 30 mL
1 dram 4 mL
1 grain 60 mg

Household measures are commonly used to measure medications at home. Abbreviations and equivalencies follow.

Household measure Abbreviation Metric measure Apothecary measure
1 cup C 1 C = 240 cc = 8 oz
1 tablespoon T (tbs) 1 T = 15 cc or 15 mL  
1 teaspoon t (tsp) 1 t = 5 cc  
drop gtt depends on size of drop  
pound lb 2.2 lb = 1 kg 1 lb. = 16 oz

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