Today, most nations of the world rely on the metric system of measurement. It's also the most widely used system for measuring amounts of drugs.
The metric system is a decimal system. That means it's based on the number 10 and multiples and subdivisions of 10. The metric system offers three advantages over other systems:
It eliminates common fractions.
It simplifies the calculation of large and small units.
It simplifies the calculation of drug doses. (See Tips for going metric.)
Beginning with the basics 
The three basic units of measurement in the metric system (along with the abbreviation for each) are the meter (m), liter (L), and gram (g):
The meter is the basic unit of length.
The liter is the basic unit of volume—it's equivalent to
of a cubic meter.The gram is the basic unit of weight—it represents the weight of 1 cubic centimeter (cm3 or cc) of water at 4°C (39.2°F).
All other units of measure are based on these three major units. When you see the root word meter, liter, or gram within a measurement, you can easily tell if you're measuring length, volume, or weight.
For example, centimeter (cm) and millimeter (mm) are units of length, centiliter (cL) and milliliter (mL) are units of volume, and kilogram (kg) and milligram (mg) are units of weight.
Three devices—the metric ruler, the metric graduate, and metric weights—are used to measure meters, liters, and grams. (See Measuring meters, liters, and grams.)
Multiples and subdivisions of meters, liters, and grams are indicated by using a prefix before the basic unit. Each prefix that's used in the metric system represents a multiple or subdivision of 10.
Consider the gram. The most common multiple of a gram is the kilogram, which is 1,000 times greater than the gram. The most common subdivision of a gram is the milligram, which represents of a gram, or 0.001 g.
Any metric measurement can be represented by a number and an abbreviation that represents the unit of measure. The abbreviation stand s for the basic unit of measure—gram (g), meter (m), liter (L)—and the prefix, such as kilo (k), centi (c), and milli (m). For example, kg stand s for kilogram, cm for centimeter, and mL for milliliter. (See What a little prefix can do.)
The metric system also includes one unusual unit of volume—the cubic centimeter (cc). Because a cubic centimeter occupies the same space as 1 mL of liquid, the two units of volume are considered equal and may be used interchangeably. However, cubic centimeters usually refer to gas volumes, and milliliters usually describe liquid volumes.
The International Bureau of Weights and Measures adopted the International System of Units in 1960 to promote the stand ard use of metric abbreviations and prevent errors in drug transcriptions. Unfortunately, some healthcare providers still use the old abbreviations.
So, you should stay alert for nonstand ard abbreviations, especially l instead of L to represent liters, and gm or GM instead of g to represent grams. As a precaution, some nurses and doctors use L in all liter-related abbreviations, such as mL and dL, but it isn't a requirement.
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