Unit conversions are done by using a conversion factor. This is an expression of how many sub units are in a single unit. The math is done by multiplying the number you have by its conversion factor. For example, if you were trying to convert 3 troy ounces to grams you would use its conversion factor. 1 troy ounce weighs 31.1 grams. Since troy ounce is the unit you are trying to get rid of, you put it on the bottom, this is written:
3troy ounce=31.1grams
1Troy ounce

Next you cancel the units

3 troy ounce=31.1grams
1 troy ounce

Finally multiply the numbers that are left and don’t forget to carry the label that is left.
3x3.31grams=9.93 grams
So there you have it. 3 troy ounces weighs 9.93 grams.
Here is a video explaining how it is done. A faster way to do this is to use a conversion calculator, like this onehttp://education.yahoo.com/reference/weights_and_measures/ ApplicationThe ability to convert units is very helpful because sometimes you dont have access to a conversion calculator so you can simply do them with a normal calculator.