Turn the ruler until you find the its architecture side. Engineering rulers have more than two sides, so the proper side of the ruler must be placed on the blueprint. The architecture side of the ruler, generally used for construction projects, has the number "1/8" printed on the upper corner of the ruler.
Place the ruler in the plans, on the object that you wish to measure. Align the ruler with the object to get a proper reading.
Record the blueprint's scale for use in the next step. The scale of the blueprint will usually appear as a small measurement being equal to a large one. An example of a scale is "1 inch = 5 feet."
Record your measurement, and multiply that measurement by the scale number from your construction plans to get the accurate size needed for your construction. For example, if you have a scale of "1 inch = 5 feet" and your measurement is 5 inches, you multiply the measurement by five to come up with the accurate length of 25 feet.