Drawing Scale in CAD

by Mark Middlebrook, www.markcad.com.

This article summarizes the relationship between drawing scale and drawing scale factor and describes how to work with them in new and existing drawings. The article is aimed at people who create construction documentation for buildings (e.g., architectural and structural drafters) using AutoCAD or AutoCAD LT.
If you enjoy reading this kind of stuff (or don't enjoy it, but find it helpful anyway), then you'll probably also be interested in one of my AutoCAD for Dummies books .

Drawing scales and drawing scale factors

CAD users employ two different ways of talking about a drawing’s intended plot scale: drawing scale and drawing scale factor:

Drawing scale is the traditional way of describing a scale; for example 1/8"=1’-0" or 1=20. The measurement to the left of the equal sign or colon indicates a paper measurement and the number to the right indicates a CAD drawing and real world measurement.

Drawing scale factor is a single number that represents a multiplier, such as 96 or 20. The drawing scale factor for a drawing is the conversion factor between a measurement on the plot and a measurement in a CAD drawing and the real world.

Obviously, the drawing scale and drawing scale factor are just two different ways of describing the same relationship. The drawing scale factor is the multiplier that changes the first number in the drawing scale into the second number.

Drawing to scale

Scaling is handled in CAD exactly the opposite from the way it is in manual drafting. In manual drafting, you squeeze real-world objects (the building perimeter, footings, 2x4s, and so on) down by a specific scale factor, like 96 or 16, so that they fit nicely on a sheet of paper. Naturally, you always draw text and other annotations the size you want them to appear on the paper (e.g., 1/8" high), regardless of the scale of the drawing.

In CAD, on the other hand, you draw real-world objects at their actual size and stretch annotations up by the scale factor. That means that 1/8" text in a 1/8"=1'-0" plan will be 12" high in the AutoCAD drawing, while the same text in a 3/4"=1'-0" detail drawing will be 2" high. When you plot, everything gets scaled back down to fit on the paper. This approach seems peculiar at first to someone who was schooled in manual drafting, but it's actually more sensible and offers many advantages in CAD drafting. As a result, even so-called "not to scale" drawings should be set up and drawn to a specific scale.

Thus, most CAD drawings are created "full size" (i.e., using true building dimensions). Drafters then squeeze them down to paper size in one of three ways:

By plotting using a scale factor that’s the inverse of the drawing scale factor (e.g., 1/96 for a 1/8" = 1’-0" drawing)

By creating a 1=1 plot sheet and xrefing the plan or details into the plot sheet at inverse of the drawing scale factor (e.g., 1/96).

By creating a paper space setup and zooming the plan or details in the paper space viewport by inverse of the drawing scale factor (e.g., 1/96xp).

Choosing a drawing scale factor for a new drawing

In CAD, as in manual drafting, you choose a drawing scale for a new drawing by considering the size of the sheet you’ll eventually plot on and the size of what you’re representing (e.g., a building’s plan dimensions) and.

If you know the sheet size that you’re going to use and real-world size of the building plan you’re going to draw, and you want to find out the largest drawing scale factor you can use:

Divide the X dimension of the building by the X dimension of the sheet.

Divide the Y dimension of the building by the Y dimension of the sheet.

Take the larger number from the calculations in steps 1 and 2.

Round up to the nearest "real" drawing scale factor that’s used in architectural drafting.

For example, suppose you want to draw a 60x40-foot (or, 720x480-inch) framing plan and print it on 11-x-17-inch paper. You divide 720 by 17 and 480 by 11 to get 42.35 and 43.64, respectively. The larger number, 43.64, corresponds in this example to the short dimension of the house and the paper. The nearest larger common architectural drafting scale factor is 48 (corresponding to 1/4" = 1’-0"), which leaves a little bit of room for the plotting margin and title block.

If you know the desired sheet size and drawing scale factor, you can calculate the available drawing area easily. Simply multiply each of the sheet’s dimensions (X and Y) by the drawing scale factor. For example, if you choose an 11-x-17-inch sheet and a drawing scale factor of 96 (corresponding to a plot scale of 1/8" = 1’-0"), you multiply 17 times 96 and 11 times 96 to get an available drawing area of 1,632 inches x 1,056 inches (or 136 feet x 88 feet). If your sheet size is in inches but your drawing scale is in millimeters, you need to multiply by an additional 25.4 to convert from inches to millimeters. For example, with an 11-x-17-inch sheet and a scale of 1 = 200 mm (drawing scale factor = 200), you multiply 17 times 200 times 25.4 and 11 times 200 times 25.4 to get 86,360 x 55,880 mm or 86.36 x 55.88 m.

The tables later in this article list the available drawing areas for a range of sheet sizes and drawing scales. You can use those tables to help you decide on an appropriate paper size and drawing scale, and revert to the calculation method for situations that the tables don’t cover.

Determining the drawing scale factor of an existing drawing

When you plot or add text or other annotations to an existing drawing, you usually need to know the drawing’s scale factor (i.e., the multiplier corresponding to the drawing scale.) Here are some methods that you can use.

Ask the creator of the drawing. This usually is the most direct way to determine the plot scale, unless of course you don’t have access to the person who created the drawing.

Look for text or a scale bar on the drawing that indicates the scale. There is some possibility that the scale shown on the drawing is wrong, but it’s at least a good starting guess.

Use a scale to measure distances on the plotted drawing. If you were lucky enough to receive a hardcopy version of the drawing, and you suspect that it was plotted at the proper scale, then lay your scale down on an object whose size you know. By trying different sides of the scale, you’ll often come up with a match.

Check the DIMSCALE system variable setting. If the person who created the drawing set it up correctly, then the DIMSCALE system variable usually will be set to the drawing scale factor. You can inspect the DIMSCALE setting by typing DIMSCALE and then pressing ENTER at the AutoCAD command prompt. You’ll see the current setting displayed in angled brackets like so:
```
Enter new value for DIMSCALE <96>:
```
Don’t change the value; just make a note of it and press ESC to leave the setting as it is.

Measure the title block. Use the DIST command to measure the X and Y dimensions of the title block. Divide these dimensions by known sheet sizes (e.g., 36" x 24" or 42" x 30") and see whether a recognizable drawing scale factor, or something close to it, pops out.

None of these methods is infallible, but by checking with several of the methods, you usually can zero in on the right scale factor.

Scale charts

You can use the following charts to help you set up new drawings or investigate the drawing scales of existing drawings.

The first two charts list drawing scale factors and AutoCAD text heights for common architectural drafting scales.

Drawing scale and text height (imperial units)

Drawing Scale	Drawing Scale Factor	1/8" Plotted Text Height	3/32" Plotted Text Height
1/16" = 1’-0"	192	24"	18"
1/8" = 1’-0"	96	12"	9"
1/4" = 1’-0"	48	6"	4-1/2"
1/2" = 1’-0"	24	3"	2-1/4"
3/4" = 1’-0"	16	2"	1-1/2"
1" = 1’-0"	12	1-1/2"	1-1/8"
1-1/2" = 1’-0"	8	1"	3/4"
3" = 1’-0"	4	1/2"	3/8"

Drawing scale and text height chart (millimeters)

Drawing Scale	Drawing Scale Factor	3 mm Plotted Text Height	2.5 mm Plotted Text Height
1=200 mm	200	600 mm	500 mm
1=100 mm	100	300 mm	250 mm
1=50 mm	50	150 mm	125 mm
1=20 mm	20	60 mm	50 mm
1=10 mm	10	30 mm	25 mm
1=5 mm	5	15 mm	12.5 mm

The next two charts list AutoCAD model space limits sheet for common architectural drafting scales and architectural sheet sizes.

Drawing scale and sheet size (imperial units)

Drawing Scale

8-1/2" x 11"
(Letter size)

11" x 17"
(Ledger size)

24" x 36"
(Arch. D)

30" x 42"
(Arch. E)

36" x 48"
(Large E)

1/16" = 1’-0"

136’ x 176’

176’ x 272’

384’ x 576’

480’ x 672’

576’ x 768’

1/8" = 1’-0"

68’ x 88’

88’ x 136’

192’ x 288’

240’ x 336’

288’ x 384’

1/4" = 1’-0"

34’ x 44’

44’ x 68’

96’ x 144’

120’ x 168’

144’ x 192’

1/2" = 1’-0"

17’ x 22’

22’ x 34’

48’ x 72’

60’ x 84’

72’ x 96’

3/4" = 1’-0"

11’-4" x 14’-8"

14’-8" x 22’-8"

32’ x 48’

40’ x 56’

48’ x 64’

1" = 1’-0"

8’-6" x 11’

11’ x 17’

24’ x 36’

30’ x 42’

36’ x 48’

1-1/2" = 1’-0"

5’-8" x 7’-4"

7’-4" x 11’-4"

16’ x 24’

20’ x 28’

24’ x 32’

3" = 1’-0"

2’-10" x 3’-8"

3’-8" x 5’-8"

8’ x 12’

10’ x 14’

12’ x 16’

Drawing scale and sheet size (millimeters)

Drawing Scale

210 x 297 mm (A4)

297 x 420 mm (A3)

420 x 594 mm (A2)

594 x 841 mm (A1)

841x 1189 mm (A0)

1 = 200

42,000 x 59,400 mm

59,400 x 84,000 mm

84,000 x 118,800 mm

118,800 x 168,200 mm

168,200 x 237,800 mm

1 = 100

21,000 x 29,700 mm

29,700 x 42,000 mm

42,000 x 59,400 mm

59,400 x 84,100 mm

84,100 x 118,900 mm

1 = 50

10,500 x 14,850 mm

14,850 x 21,000 mm

21,000 x 29,700 mm

29,700 x 42,050 mm

42,050 x 59,450 mm

1 = 20

4200 x 5940 mm

5940 x 8400 mm

8400 x 11,880

11,880 x 16,820 mm

16,820 x 23,780 mm

1 = 10

2,100 x 2,970 mm

2,970 x 4,200 mm

4,200 x 5,940 mm

5,940 x 8,410 mm

8,410 x 11,890 mm

1 = 5

1,050 x 1,485 mm

1,485 x 2,100 mm

2,100 x 2,970 mm

2,970 x 4,205 mm

4,205 x 5,945 mm

Return to www.markcad.com.

Last updated 17-Jan-2000 by
mark@markcad.com

Drawing Scale	8-1/2" x 11" (Letter size)	11" x 17" (Ledger size)	24" x 36" (Arch. D)	30" x 42" (Arch. E)	36" x 48" (Large E)
1/16" = 1’-0"	136’ x 176’	176’ x 272’	384’ x 576’	480’ x 672’	576’ x 768’
1/8" = 1’-0"	68’ x 88’	88’ x 136’	192’ x 288’	240’ x 336’	288’ x 384’
1/4" = 1’-0"	34’ x 44’	44’ x 68’	96’ x 144’	120’ x 168’	144’ x 192’
1/2" = 1’-0"	17’ x 22’	22’ x 34’	48’ x 72’	60’ x 84’	72’ x 96’
3/4" = 1’-0"	11’-4" x 14’-8"	14’-8" x 22’-8"	32’ x 48’	40’ x 56’	48’ x 64’
1" = 1’-0"	8’-6" x 11’	11’ x 17’	24’ x 36’	30’ x 42’	36’ x 48’
1-1/2" = 1’-0"	5’-8" x 7’-4"	7’-4" x 11’-4"	16’ x 24’	20’ x 28’	24’ x 32’
3" = 1’-0"	2’-10" x 3’-8"	3’-8" x 5’-8"	8’ x 12’	10’ x 14’	12’ x 16’

Drawing Scale	210 x 297 mm (A4)	297 x 420 mm (A3)	420 x 594 mm (A2)	594 x 841 mm (A1)	841x 1189 mm (A0)
1 = 200	42,000 x 59,400 mm	59,400 x 84,000 mm	84,000 x 118,800 mm	118,800 x 168,200 mm	168,200 x 237,800 mm
1 = 100	21,000 x 29,700 mm	29,700 x 42,000 mm	42,000 x 59,400 mm	59,400 x 84,100 mm	84,100 x 118,900 mm
1 = 50	10,500 x 14,850 mm	14,850 x 21,000 mm	21,000 x 29,700 mm	29,700 x 42,050 mm	42,050 x 59,450 mm
1 = 20	4200 x 5940 mm	5940 x 8400 mm	8400 x 11,880	11,880 x 16,820 mm	16,820 x 23,780 mm
1 = 10	2,100 x 2,970 mm	2,970 x 4,200 mm	4,200 x 5,940 mm	5,940 x 8,410 mm	8,410 x 11,890 mm
1 = 5	1,050 x 1,485 mm	1,485 x 2,100 mm	2,100 x 2,970 mm	2,970 x 4,205 mm	4,205 x 5,945 mm