Metric units often have prefixes. Kilometers and centimeters both have prefixes before the word “meter.” The prefixes instruct you to multiply or divide by a power of 10: kilo means multiply by 1000, so a kilometer equals 1000 meters. Centi means divide by 100, so a centimeter is one one-hundredth of a meter. In other words, there are 100 centimeters in a meter. The table in Equation 1 on the right lists the values for the most common prefixes.

Prefixes allow you to describe the unimaginably vast and small and everything in between. To illustrate, every day the City of New York produces 10 gigagrams of garbage. The distance between transistors in a microprocessor is less than a micrometer. The power of the Sun is 400 yottawatts (a yotta corresponds to the factor of 10²⁴). It takes 3.34 nanoseconds for light to travel one meter. The electric potential difference across a nerve cell is about 70 millivolts.

These prefixes can apply to any unit. You can use gigameters to conveniently quantify a vast distance, gigagrams to measure the mass of a huge object, or gigavolts to describe a large electrical potential difference.

Some of the most common prefixes − kilo, mega, and giga − are commonly used to describe the specifications of computers. The speed of a computer microprocessor is measured by how many computational cycles per second it can perform. Microprocessor speeds used to be specified in megahertz (one million cycles per second) but are now specified in gigahertz (one billion cycles per second). Modem speeds have increased from kilobits to megabits per second. (Although bits are not part of the metric system, computer scientists use the same prefixes.)

The units of measurement you use are a matter of both convenience and convention. For example, snow skis are typically measured in centimeters; a ski labeled “170” is 170 centimeters long. However, it could also be called a 1.7-meter ski or a 1700-millimeter ski. The ski industry has decided that centimeters are reasonable units and has settled on their use as a convention.

In this textbook, you are most likely to encounter kilo, mega and giga on the large side of things and centi, milli, micro and nano on the small. Some other prefixes are not as common because they just do not seem that useful. Is it easier to say “a decameter” than the more straightforward 10 meters? And, for the extremely large and small, scientists often use another technique called scientific notation rather than prefixes.