Number conversions are always convenient to have around. There are many times when you may need to convert grams to pounds, miles to kilometers, or see the latest exchange rates. In OS X there are some convenient ways to convert numbers that are readily available.
The OS X calculator has a decent number converter in it. To access it, enter a number in the calculator and choose an option from the "Convert" menu (any will do, since they all display the same converter window). With the window open, you can select from a number of conversion types. Upon clicking OK, the calculator will change the entered number to the converted value.
NOTE: If you need to convert currency, be sure to click the "Update" button so the conversion is computed with the latest exchange rates.
When connected to the internet, you can easily do conversions and other small math tasks using Google, either from the Google home page or from the Safari toolbar.
Apple supplies a small Dashboard widget for converting numbers. Sometimes this can be more convenient than launching Calculator or opening a new browser window for using Google. Unfortunately, unless it is already launched Dashboard can take a few seconds to load, even with a fast internet connection.
Many times people use the Terminal to remotely connect to other computers, or perform a variety of tasks, and when doing so it may be easier to use Terminal resources for things instead of resorting to other applications. The BSD subsystem in OS X has a command called "units", which you can use to enter a variety of conversions. To use it, launch Terminal and enter "units". You will be asked for what you have, and then be asked for what you want, and it will output the conversion in two forms, both as a forward multiplier and reverse divisor.
If you do not enter a number, the converter will assume you mean one of each unit you enter, and show you the conversion factor between the two units.