One of the key characteristics of the metric system of measuring is its arrangement using multiples of ten. For example, there are one thousand milliliters in a liter and ten decimeters in a meter. As a result, it makes sense that you should only use decimals -- which represent tenths, hundredths and other smaller increments of ten -- to represent these numbers. Converting metric fractions to decimals is simple and straightforward and will require, at most, only a basic calculator.
Divide the fraction out. For example, if you have 5/8 liter of water, divide five by eight to find that you have .625 liters of water.
Convert to smaller units to avoid a decimal altogether. For the example calculation, wherein you found you had .625 liters of water, consider that there are 1,000 milliliters in one liter of water. Keeping this in mind, you can multiple your volume in liters by 1,000 and represent it as 625 mL of water.
Represent extremely small fractions using exponential notation. For example, if you are given a mass measurement of 2/325 gram, divide it out to get .00615 grams. In exponential notation, you represent small numbers as larger numbers -- to the "ones" place, only -- multiplied by ten to a certain negative power. For the example value, consider the number 6.15 and think of how many "places" you'd need to move the decimal to the left in order to get .00615. The answer is three. As a result, you represent this value as 6.15 x 10^-3 grams.