Determine what measurement system you are using. There are two measurement systems, either metric or the English measurement system. The metric measurement system is based on 10 and uses terminology such as millimeter, centimeter and decimeter. The English system is not based on a particular number, but uses terminology such as inches, yards and miles.
Determine the unit of measurement you are using. For example, if you are measuring the length of a sandwich, it is probably done in centimeters for the metric system and inches for the English system.
Take the measurement. Using the sandwich example, say the length of the sandwich is 6 inches according to your English standard ruler.
Determine the unit you are trying to find the fraction of for the equation. This is most likely the next highest unit class in the system. Otherwise, the measurement would not be a fraction, but a whole number. Staying with the sandwich example, say you are measuring the fraction of a foot. You would need to figure out what fraction of a foot is 6 inches.
Place the measurement in the numerator of the equation. In the sandwich example, this is 6.
Determine the number of smaller units -- in this case, inches -- that would make up the larger unit in the comparison (a foot). It takes 12 inches to make 1 foot. You would put this number in the denominator to get the following fraction: 6/12.
Reduce the fraction, if necessary, by dividing both the numerator and denominator by the greatest common factor. The greatest common factor is the number that you can divide both the numerator and denominator by and still get a whole number. In the fraction 6/12, 6 is the largest number that will divide into both the numerator and denominator equally. So 6/6 = 1, and 12/6 = 2.
You would rewrite the equation as 1/2. Finish the problem by reapplying your larger unit measurement. In the example, it was feet. So the sandwich is 1/2 a foot long.