CAD Scale Factors 101

CAD Scale Factors 101

In Computer-Aided Design (CAD) practices, it is a standard norm for users to draw architectural structures such as buildings at their actual, full-scale dimensions for the sake of simplicity and accuracy. This means, for example, when a CAD user is designing a door, they would draw it in its actual size which could be 3 feet wide and 7 feet tall, rather than reducing it to a smaller size.

However, these full-scale drawings eventually have to be reproduced on physical sheets of paper that are much smaller in size than the real-world objects they represent. This disparity in size makes it necessary to introduce CAD scale factors.

The scale factor serves to systematically reduce the size of the drawing so that it fits the paper, while also providing a conversion factor that allows anyone viewing the drawing to understand the actual size of the object or building it represents.

An interesting aspect of working with scale in CAD, particularly in AutoCAD software, is the use of a special suffix, ‘XP’, which is part of the Viewport Scale as indicated in certain charts. This suffix is specific to AutoCAD and is used to alter the scale within a particular viewport.

The viewport is a viewing area in the CAD interface where users can see their work. Typically, when you want to change the scale of your drawing, you would be in the ‘paper space’ on a sheet, then you would enter the ‘model space’ within the viewport.

In AutoCAD, the procedure to change the scale is to type ‘Z’ or ‘Zoom’ and then input a scale factor with the ‘XP’ suffix. For instance, entering ’96XP’ scales the drawing to 1/8″ = 1′-0″ in paper space. This means, in the final drawing, an inch on paper represents eight feet in real life.

This procedure is succinctly encapsulated in Autodesk’s statement, “You can change the view scale of the viewport by using the XP option of the ZOOM command when model space is accessed from within a layout viewport.” Simply put, this means that you can alter the scale of the drawing in the viewport by using the XP option of the ZOOM command after you’ve entered the model space from within the viewport.

Calculating CAD Scale Factors

For the transformation of an architectural drawing scale into a scale factor, follow these steps:

  1. First, pick the scale you wish to work with. For instance, we’ll use 1/8″ = 1′-0″.
  2. Now, flip the fraction (i.e., make it the reciprocal) and multiply the result by 12. In this case, changing 1/8 to 8/1 and multiplying by 12 gives us a scale factor of 96.

When you want to translate an engineering drawing scale to a scale factor, the process is a bit different:

  1. Initially, select the scale you prefer. As an example, we’ll use 1″ = 20′.
  2. Next, convert the length in feet to inches by multiplying by 12. With our example, multiplying 20 (feet) by 12 results in a scale factor of 240.

Architectural Scales

DRAWING SCALESCALE FACTORVIEWPORT SCALEDECIMAL SCALE
1/16″ = 1′-0″1921/192xp.0625″ = 1′-0″
3/32″ = 1′-0″1281/128xp.09375″ = 1′-0″
1/8″ = 1′-0″961/96xp.125″ = 1′-0″
3/16″ = 1′-0″641/64xp.1875″ = 1′-0″
1/4″ = 1′-0″481/48xp.25″ = 1′-0″
3/8″ = 1′-0″321/32xp.375″ = 1′-0″
1/2″ = 1′-0″241/24xp.50″ = 1′-0″
3/4″ = 1′-0″161/16xp.75″ = 1′-0″
1″ = 1′-0″121/12xp1″ = 1′-0″
1 1/2″ = 1′-0″81/8xp1.5″ = 1′-0″
3″ = 1′-0″41/4xp3″ = 1′-0″

This table provides the correlation between the drawing scale, scale factor, viewport scale, and decimal scale for various architectural measurements.

Engineering Scales

DRAWING SCALESCALE FACTORVIEWPORT SCALE
1″ = 10′-0″1201/120xp
1″ = 20′-0″2401/240xp
1″ = 30′-0″3601/360xp
1″ = 40′-0″4801/480xp
1″ = 50′-0″6001/600xp
1″= 60′-0″7201/720xp
1″ = 70′-0″8401/840xp
1″ = 80′-0″9601/960xp
1″ = 90′-0″10801/1080xp
1″ = 100′-0″12001/1200xp

This table gives a detailed breakdown of how the drawing scale correlates with the scale factor and viewport scale for various engineering measurements.