Scale factor (as used by AutoCAD) is always a reciprocal of the drawing scale.
AutoCad uses the unit one as the base unit.
For example, if you wish to plot a mechanical drawing at a scale of 1/2” = 1” the reciprocal of 1/2 is 2/1 which is 2. Therefore the scale factor is 2. If this drawing was to be plotted on a 11 x 17 piece of paper the limits would have to be 34,22. Text would be .25 high and other settings made for this scale to be plotted properly.
Another example would be an architectural drawing plotted at a scale of 1/4” = 1’-0”. Changing feet to inches so the units are all in inches gives us 1/4” = 12”. Dividing both sides by 12 which is the same thing as multiplying by 1/12 we obtain 1/48 = 1. The reciprocal of 1/48 is 48. The scale factor then is 48. If this drawing is plotted on a 11 x 17 piece of paper the limits would have to be 816,528 (68’,44’). Text would be 6” high (.125 times 48 = 6). Other settings would have to be made for this scale to be plotted properly.
Another example would be a metric drawing made full size. 1 = 25.4. Dividing both sides of this equation by 25.4 we get 1/25.4 = 1. The reciprocal of 1/25.4 is 25.4/1. The scale factor therefore is 25.4.
Text (originally .125) then would be made approximately 3 high (.125 X 25.4 = 3.175). Text (originally .25) then would be made approximately 6 high (.25 X 25.4 = 6.35). Other settings would have to be made for this scale to be plotted properly
The scale factor of a drawing should be determined and utilized during the time of drawing set up.
The proper scale factor is extremely important because it makes sure that text (height, etc.), dimension values (dimscale), hatch patterns, limits, and linetype scale (ltscale) are plotted at the proper size.
A drawing scale of 1” = 1” has a scale factor of 1
A drawing scale of 1/2” = 1” has a scale factor of 2
A drawing scale of 2 = 1 has a scale factor of .5
A drawing scale of 1 = 60.0’ has a scale factor of 720
A drawing scale of half size (metric) has a scale factor of 50.8