Scale rulers are used by architects and engineers to calculate dimension in a blue print or building plan without the use of time-consuming and complicated math equations.
Determine the scale of your plans. It will be labelled on the plans and should be found in the bottom corner.
Turn the ruler to the corresponding scale factor. For instance, if the plans read ¼ inch equals one foot, you will use the ¼ inch scale on the ruler.
Measure the desired line starting from zero on the correct scale. In this case, you would need to use the ¼ inch scale.
The measurement you calculate equals how many feet the line actually is at full scale dimensions.
Try it out several times until you get the hang of it. This will increase your understanding of the scale ruler so you can use it with ease.
Be sure you are using the correct scale on the ruler. If you use the wrong scale on the ruler, all your measurements will be incorrect.