### The Metric System

Most of the units used in science are metric units. The metric system is also referred to as the International System of Units or SI, an abbreviation of its French name, Le Système International d’Unités. The metric system is based on a limited set of base units. In biology, the most commonly encountered measurements are the meter (which measures length or distance), the gram (weight), the liter (volume), and the mole (amount). Smaller and larger units are created by adding prefixes to these units to indicate the order of magnitude. Most but not all units vary from the base unit by multiples of 1000 or 1/1000.

Commonly used prefixes are listed in Table 1. For example, a kilogram is equal to 1000 grams, while a microgram is equal to 0.000001 g. Likewise, a kilometer is equal to 1000 meters, while a micrometer is equal to 0.000001 m. If we were to create our own unit called the “harvard”, a “kiloharvard” would be equal to 1000 harvards. In fact, you will find that a number of fields use the SI prefixes to modify units not traditionally considered metric units. For example, your computer’s hard drive may be able to store 100 gigabytes (GB) of information – this is equal to 100,000,000 bytes. In biology, it is often useful to talk about a length of DNA in base pairs, and large lengths of DNA may be referred to in kilobases (kb) or megabases (Mb).

The prefixes used in the metric system and their values are defined in Table 1 below. However, the values you will most frequently encounter in biology are kilo- (1000x the base unit), milli- (0.001x base), micro- (0.000001x base) and nano- (0.000000001x base).

Table 1.

 Prefix Symbol Multiple of the Base Unit Example giga- G 1,000,000,000x 1 gigawatt = 1,000,000,000 watts of power mega- M 1,000,000x 1 megabyte = 1,000,000 bytes of information kilo- k 1000x 1 kilometer = 1000 meters in length deca- da 10x 1 decaliter = 10 liters of volume centi- c 0.01x 1 meter = 100 centimeters in length milli- m 0.001x 1 liter = 1000 milliliters of volume micro- m 0.000001x 1 gram = 1,000,000 micrograms in weight nano- n 0.000000001x 1 mole = 1,000,000,000 nanomoles of substance pico- p 0.000000000001x 1 second = 1,000,000,000,000 picoseconds of time