Read the scale of the blueprint or drawing. You will find this in the title block at the bottom of the page you're viewing.
Rotate the architect's scale and look for the side labeled with the drawing's scale. The labels on the scale indicate the number of inches per foot for that scale, so the side labeled 1/8 is for a scale of 1/8 inch = 1 foot.
Line up the "0" line on the correct edge of the scale with the left end of the line you want to measure.
Read the scale marking that is closest to the right end of the line, but not beyond the line, to determine the length of the line in feet.
Slide the scale so that the scale marking that was just beyond the right end of the line now lines up with it.
Count the number of marks between the zero mark and the far left mark on the scale to determine how many inches each mark represents. For example, each mark represents one inch if there are twelve marks, but it represents two inches if there are only six marks.
Count the number of marks between the "0" marking and the left end of the line you are measuring. Multiply this number by the number of inches each mark represents to find the inches.
Combine the feet measurement and the inches measurement to get the total length of the line. For example, if you measured 13 feet and 5 inches, the total length is 13' 5".