
Design Master HVAC, Electrical, and Plumbing use the AutoCAD system variable DIMSCALE to determine the scale of the current drawing. DIMSCALE is used to size text, leader arrowheads, and other drawing elements that are not to scale.
DIMSCALE should be set to the number of inches that should be represented by an inch on the plan when printed on paper. It answers the question, if I plot the drawing and measure a real inch on it, how many inches would that represent on the plan? To determine the DIMSCALE of a drawing, you need to convert your scale factor to inches. For a 1/8″=1′ scale factor, you first would convert 1′ to 12″, which gives you 1/8″=12″. Then, you would multiply both sides by 8 to get 1″=96″. The DIMSCALE is the number on the right side. For a 1/8″=1′ drawing, you use a DIMSCALE of 96. For a 1″=20′ site plan, you would convert the 20′ to inches and get 1″=240″, resulting in a DIMSCALE of 240. For a scale like 3/16″ = 1′, you need to get the left side to be 1″, so you would multiply both sides by (16 / 3). In this case you would get 3/16″ = 1′, which is the same as 3/16″ = 12″, which is the same as 1″ = 64″, and DIMSCALE should be set to 64. 
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