SI prefixes are letters or words that are placed before a unit in order to modify its meaning. This gives us the ability to scale our units up or down so that the quantities we use in a particular setting are simpler. This is similar to other unit decisions, where we pick the unit to be convenient for the problem that we are attempting to solve or the the situation that we find ourselves in. For example, it’s more sensible to talk about 2 kilometers of distance instead of 2,000,000 millimeters. Another way to think about this is that these prefixes allow us to "hide" some of the scientific notation in the units in order to simplify the communication of values.

Here is a chart of some of the SI prefixes:

The prefixes allow you to easily relate values back to the base unit. For example, \(1 \text{ megavolt}\) is equivalent to \(10^6 \text{ volts}\text{.}\) Similarly, \(1 \text{millivolt}\) is equivalent to \(10^{-3} \text{ volts}\text{.}\) However, if you’re going from the base unit to the prefix, you will need to change the sign of the exponent. So \(1 \text{ volt}\) is the same as \(10^{-6} \text{ megavolts}\) and is equivalent to \(10^3 \text{ millivolts}\text{.}\)

You can also create conversion factors by using two prefixes and the base unit. For example, to convert megavolts to millivolts, you would start with \(1 \text{ volt} = 1 \text{ volt}\) and simply replace both sides with the appropriate equivalent: \(10^{-6} \text{ megavolts} = 10^3 \text{ millivolts}\text{.}\) From here, it’s just a matter of building the correct fraction for the desired outcome.

In these examples, we’ve used the full names of the units. But in practice, the units have abbreviations. For example, instead of writing "volts" we would just use "V". So a 5V batter is a "5 volt" battery. When using the abbreviated units, you also use the abbreviated symbol for the prefix. So "MV" is "megavolt" and "mV" is millivolt. Notice that the capitalization matters.