# An introduction to metric prefixes

This won’t give all the metric prefixes to you, but then, you won’t generally need them all. Most of the time, you’re working with just a few basic prefixes: kilo-, centi-, milli-, and micro-. In the internet age, we’re also a little more familiar with mega-, giga-, and even tera- (some of you may even have heard of peta-). In microbiology, being familiar with nano- , pico-, and even the Angstrom are useful, but if you can remember how to do the basic format, then it’ll be easier to put all the rest of these into place.

I know this is a little much to take in at once, but I’ll walk you through it. Each vertical line on this longer horizontal one represents a power of 10, or 10 multiplied by itself a given number of times, which is the same this as 10 raised to a given power or 10 to a given exponent.. At the central point, marked by the U, 10 is multiplied by itself 0 times. 10 to the 0 power equals 1, so this represents the base unit. That’s what the U represents: Unit. That’s also what the b below it means: base. Move one space to the left, and you get 10 to the first power, or 10. Move one space to the right, and you get 10 raised to -1, or 0.1 (one tenth). This pattern continues: Two spaces to the left is 10 to second, or 100, two to the right is 0.01 (one one-hundredth). Three spaces take you to 1000 on the left and 0.001 on the right.

That’s what makes the metric system so simple - everything is a factor of 10. There are no quarters or eighths or other fractional measurements to figure out. It’s all about moving a decimal point back and forth, and if you can draw a number line and keep the prefixes straight, then the rest becomes gravy.

The base unit of length in the metric system is the meter. If you’re used to working in the US system, then visualize a yard-stick. A meter is just longer than a yard - just over 39 inches (there’s also around 2 kilometers for every 3 miles, but I’m getting ahead of myself). We’ll start with the meter as our base unit and add our prefixes onto that. (The other units shown are L for liter, the measure for volume, and g for gram, which is the metric unit of mass. Note that in SI, which is the scientific standard, the basic unit of mass is actually the kilogram. However, for simplicity, we’ll just work with the meter).

1 meter (1 m) is just that, 1 meter. Since the metric system operates on a base 10 system, then moving the decimal right or left by one unit adds a prefix. Move the decimal one place to the left, and you have 10 meters (10 m) or 1 dekameter (1 dam). Move the decimal one place to the right, and now you have 0.1 meters (0.1 m) or 1 decimeter (1 dm). If you start 1 meter and move the decimal 2 units to the left, you have 100 meters (100 m) or 1 hectometer (1 hm). Go to the right instead, and you have 0.01 meters (0.01 m) or 1 centimeter (1 cm).

Continue following this pattern: 1000 meters are 1 kilometer (1 km). 1/1000 of 1 meter, or 0.001 meter is 1 millimeter (1 mm).

There are names for the prefixes for 10,000 and 100,000, as well as .0001 and .00001, but these are so rarely used that I must admit I don’t know them. Instead, we skip ahead to 1,000,000 meters or 1 megameter (1 Mm), 1,000,000,000 meters or 1 gigameter (1 Gm), 1,000,000,000,000 meters or 1 terameter (1 Tm), and 1,000,000,000,000,000 meters or 1 petameter (1 Pm). You’ll know that we really don’t use any of these prefixes with meters - once we get to lengths so vast that they are measured as millions, billions, trillions, and so on, of meters, we actually start moving over to different units, a derived unit, the light year (the distance light travels in one year).

On the other side of the decimal, however, in discussing fractions of a meter, 0.000 001 meters is 1 micrometer (1μm). This is sometimes also referred to as 1 micron.The next unit, 0.000 000 001 meters is 1 nanometer (1 nm) and 0.000 000 000 001 meters is 1 picometer (1 pm). Finally, there’s a very special unit of length, 0.1 nanometers or 0.000 000 000 1 meters that is equal to 1 Ångström (1 Å). Note: this is not Am, Angstrommeters, and there are no Angstromgrams or Angstromliters. This is the one little exception - it is a special unit of length named for a Swedish physicist. Officially, it should contain the dots (diacritics) over the o and the ring over the A, even though these are often omitted when being written in English.

So, to summarise:

I remember this with the following mnemonic:

King Henry died by drinking chocolate milk, mother. That sentence gives me the most frequently used prefixes, (kilo, hecto, deka, base units, deci, centi, milli, micro) which is enough to build the basic metric line. I use the comma to remind me to leave spaces between milli- and micro-, and for most things, that’s enough. Even working in microbiology, I really only need to learn the bottom half of the table, pictured below, and that’s a lot easier to learn once you have the basics mastered.

Do you have to use this? Not at all. If you have your own mnemonic, use that. If you can learn it another way, go with it. I’m just offering you the tools that have served me.

If nothing else, remember: once you start with length, mass, and time (meters, grams, and seconds) every other unit is derived (made) from that, and everything is in powers of 10. You may like ounces, pounds, tablespoons, inches, feet, yards, and so on, but keep in mind that there are more units and harder conversions between them than metric. At the very least, metric offers ease of use that the US system doesn’t. The more you use it, the more intuitive it gets. Also, if you’re in science or medicine, you MUST learn it.