How to use a ruler
Every line is a fraction of an inch. Count the number of lines between 2 inches and 3 inches (or between any other inch mark for that matter, they are all the same). That number is going to be your denominator. Most rulers it is going to be 16 marks between each inch.
Now count the number of lines after the last inch mark when doing your measurement. That is your numerator.
N/D
numerator/denominator
So if there is 16 lines in between each inch and you measured at four lines, the fraction of an inch would be 4/16 th of an inch. you can either plug that into a calculator to get 0.25 inches, or reduce the fraction to give you 1/4 th of an inch. If you were at 5/16 th of an inch, put that into a calculator and you get 0.3125.