AutoCAD Scale Factor Chart and Calculator
Pick the scale you are plotting at and this page gives you the scale factor, the model space text height, the viewport zoom value and the dimension setting that goes with it. Architectural, engineering and metric scales are all covered, and every table below says where its numbers come from.
We draft millwork shop drawings for a living, so this started as an internal cheat sheet. Our millwork shop drawing services run on these numbers every day. It is published here because the charts floating around the web repeat a few things that stopped being true years ago.
Contents:
- Scale factor calculator
- How the scale factor is worked out
- Architectural scale factors
- Engineering scales and the 12x trap
- Metric scales
- Text height on the sheet
- What to scale and what to leave alone
- Millwork scales under AWI 100
- Where scale factors go wrong
- Scale factor FAQ
Scale factor calculator
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How the scale factor is worked out
The scale factor is how many times bigger the real object is than its drawn size. Nothing more complicated than that. In metric it is obvious, because the scale is already a ratio: at 1:20 the factor is 20.
Imperial notation hides the arithmetic, because a scale like 1/4″ = 1′-0″ mixes two units. Convert the foot into inches first and the ratio appears.
So a quarter inch drawing is 1:48, three eighths is 1:32, and an inch and a half is 1:8. Engineering scales run through the same formula. At 1″ = 20′ you are compressing twenty feet into one inch, which is 240 inches into one, so the factor is 240.
Architectural scale factors
Text heights below are for text that is not annotative. Multiply the height you want on paper by the scale factor and you get the height to type in model space.
| Scale | Ratio | Scale factor | Text 3/32″ plotted | Text 1/8″ plotted | Viewport zoom |
|---|---|---|---|---|---|
| 12″ = 1′-0″ | 1:1 | 1 | 0.0938″ | 0.125″ | 1xp |
| 6″ = 1′-0″ | 1:2 | 2 | 0.1875″ | 0.25″ | 1/2xp |
| 3″ = 1′-0″ | 1:4 | 4 | 0.375″ | 0.5″ | 1/4xp |
| 1-1/2″ = 1′-0″ | 1:8 | 8 | 0.75″ | 1″ | 1/8xp |
| 1″ = 1′-0″ | 1:12 | 12 | 1.125″ | 1.5″ | 1/12xp |
| 3/4″ = 1′-0″ | 1:16 | 16 | 1.5″ | 2″ | 1/16xp |
| 1/2″ = 1′-0″ | 1:24 | 24 | 2.25″ | 3″ | 1/24xp |
| 3/8″ = 1′-0″ | 1:32 | 32 | 3″ | 4″ | 1/32xp |
| 1/4″ = 1′-0″ | 1:48 | 48 | 4.5″ | 6″ | 1/48xp |
| 3/16″ = 1′-0″ | 1:64 | 64 | 6″ | 8″ | 1/64xp |
| 1/8″ = 1′-0″ | 1:96 | 96 | 9″ | 12″ | 1/96xp |
| 3/32″ = 1′-0″ | 1:128 | 128 | 12″ | 16″ | 1/128xp |
| 1/16″ = 1′-0″ | 1:192 | 192 | 18″ | 24″ | 1/192xp |
| 3/64″ = 1′-0″ | 1:256 | 256 | 24″ | 32″ | 1/256xp |
| 1/32″ = 1′-0″ | 1:384 | 384 | 36″ | 48″ | 1/384xp |
The method and the 1/4 inch example are documented by Autodesk. The rest is arithmetic from the same formula.
Engineering scales and the mistake that costs an afternoon
Engineering scales carry a trap that architectural scales do not. The viewport value depends on what one drawing unit means in your model, and site drawings are often built in feet while building drawings are built in inches.
The 12x error. At 1″ = 20′ the geometric factor is 240. If one model unit is an inch, the viewport is 1/240xp. If one model unit is a foot, the same scale needs 1/20xp. Get it wrong and the view lands twelve times too big or too small. This is the most common complaint about engineering scales in Autodesk’s own forums.
| Scale | Ratio | Scale factor | Viewport, inch model | Viewport, foot model |
|---|---|---|---|---|
| 1″ = 10′-0″ | 1:120 | 120 | 1/120xp | 1/10xp |
| 1″ = 20′-0″ | 1:240 | 240 | 1/240xp | 1/20xp |
| 1″ = 30′-0″ | 1:360 | 360 | 1/360xp | 1/30xp |
| 1″ = 40′-0″ | 1:480 | 480 | 1/480xp | 1/40xp |
| 1″ = 50′-0″ | 1:600 | 600 | 1/600xp | 1/50xp |
| 1″ = 60′-0″ | 1:720 | 720 | 1/720xp | 1/60xp |
| 1″ = 80′-0″ | 1:960 | 960 | 1/960xp | 1/80xp |
| 1″ = 100′-0″ | 1:1200 | 1200 | 1/1200xp | 1/100xp |
| 1″ = 200′-0″ | 1:2400 | 2400 | 1/2400xp | 1/200xp |
Metric scales
ISO 5455 sets the preferred series: 1:1, 1:2, 1:5, 1:10, 1:20, 1:50, 1:100 and their multiples. With the model in millimetres and the sheet in millimetres, the factor is simply the denominator.
| Scale | Scale factor | Text 2.5 mm plotted | Text 3.5 mm plotted | Viewport zoom |
|---|---|---|---|---|
| 1:1 | 1 | 2.5 mm | 3.5 mm | 1xp |
| 1:2 | 2 | 5 mm | 7 mm | 1/2xp |
| 1:5 | 5 | 12.5 mm | 17.5 mm | 1/5xp |
| 1:10 | 10 | 25 mm | 35 mm | 1/10xp |
| 1:20 | 20 | 50 mm | 70 mm | 1/20xp |
| 1:25 | 25 | 62.5 mm | 87.5 mm | 1/25xp |
| 1:50 | 50 | 125 mm | 175 mm | 1/50xp |
| 1:100 | 100 | 250 mm | 350 mm | 1/100xp |
| 1:200 | 200 | 500 mm | 700 mm | 1/200xp |
| 1:500 | 500 | 1250 mm | 1750 mm | 1/500xp |
Lettering sizes come from ISO 3098-1: 1.8, 2.5, 3.5, 5, 7, 10, 14 and 20 mm. The standard fixes the sizes, not what each size is used for, so those roles belong in your own template.
Watch the 2.4 against 2.5 millimetre confusion. The US National CAD Standard treats 2.4 mm as the soft conversion of 3/32 inch, while ISO 3098 lists 2.5 mm as a nominal lettering size. Close, but they come from different places and mean different things.
Text height on the sheet
There is no national rule in the United States fixing a height for every kind of text. What exists is a common range and a lot of agency standards picking inside it. Autodesk calls 3/32 inch and 1/8 inch extremely common for dimension text, and even two districts of the Army Corps of Engineers disagree: Mobile sets notes and dimensions at 3/32″, Jacksonville sets normal text at 1/8″.
Treat the height as an office decision, write it into the template once and stop relitigating it per project. The same goes for which CAD file format you hand over at the end of the job. Consistency across a sheet matters more than which of the two you picked.
What to scale, and what to leave alone
Old scale factor charts tell you to set everything to the drawing scale factor. That advice is half right, and the other half causes real problems.
| Variable | What to do | Status |
|---|---|---|
| DIMSCALE | Set it to the scale factor for dimensions placed in model space. An annotative dimension style forces it to 0 and the annotation scale takes over. | Documented by Autodesk |
| LTSCALE | A global linetype factor. There is no rule that ties it to the scale factor. Keep a calibrated value in your template. | Legacy habit |
| PSLTSCALE | Set to 1 so dashed patterns plot the same through viewports at different scales. | Autodesk default |
| MSLTSCALE | Set to 1 so linetypes on the Model tab follow the current annotation scale. | Documented by Autodesk |
| Hatch scale | Depends on the pattern and on how dense the material should read. Make the hatch annotative if it must hold its paper look. | Not a formula |
The short version: dimensions in a legacy model space workflow do follow the scale factor. Linetypes and hatches never did in any published Autodesk rule. Hatch spacing depends on the pattern definition and on how dense you want the material to read, which is a drawing decision rather than a calculation.
Millwork shop drawings: the scales AWI requires
Architectural woodwork submittals have their own floor. If the term itself is fuzzy, we sorted out what millwork actually covers separately. AWI 100, Submittals, section 3.2.3 sets a universal minimum scale for each view, and it is larger than what an architect would use for the same room on a contract set.
| View | AWI metric | AWI imperial | Scale factor | Viewport zoom |
|---|---|---|---|---|
| Reference plan | 1:50 | 1/4″ = 1′-0″ | 48 | 1/48xp |
| Plans and elevations | 1:20 | 3/8″ = 1′-0″ | 32 | 1/32xp |
| Sections | 1:10 | 1-1/2″ = 1′-0″ | 8 | 1/8xp |
| Details | 1:4 | 3″ = 1′-0″ | 4 | 1/4xp |
| Trim and scribe profiles | 1:1 | 12″ = 1′-0″ | 1 | 1xp |
Two things catch people out here. The metric and imperial columns are not conversions of each other: AWI lists 1:20 beside 3/8″ = 1′-0″, which is 1:32. Take the column that matches the measurement system of the contract documents instead of converting one into the other. And the full size requirement is narrower than it sounds, because it covers trim and scribe profiles rather than every joinery or hardware detail.
The same section fixes the sheet as well: no smaller than 11″ x 17″, no larger than 24″ x 36″, sequentially numbered and dated, with PDF submittals allowed. Worth a look at real millwork shop drawing examples to see how those scales land on a sheet. Source: AWI 100 Submittals, section 3.2.
Where scale factors go wrong
- The viewport was right, then somebody zoomed inside it. Scrolling in an unlocked viewport changes its scale. Set it, then lock it.
- Feet and inches mixed on one sheet. That is the 12x error above, and it usually shows up as a title block that suddenly looks tiny.
- Hatch scale far too small. The pattern fills solid, the file slows down and the plot goes muddy. Raise the hatch scale, not the drawing scale factor.
- An annotative object missing a scale. Text or a dimension vanishes in one viewport while it is fine in another, because the object does not carry that annotation scale yet.
- DIMSCALE used to fix a wrong measurement. It will not. DIMSCALE changes the size of arrows, text and gaps, never the measured length.
Scale Factor FAQ
What is the scale factor for 1/4″ = 1′-0″?
48. Twelve inches in a foot divided by a quarter inch on paper. The same arithmetic gives 96 at 1/8″ and 32 at 3/8″.
Do I still need scale factors if I use annotative objects?
For text and dimensions, no. You set the paper height once and annotation scales handle the rest. You still need the factor for viewport scaling, for older drawings you inherit, and for anything drawn at a scale in model space.
Should LTSCALE equal the drawing scale factor?
That is a habit from single scale model space drawings, not a rule. Autodesk defines LTSCALE as a global linetype factor. In a layout based drawing, keep a calibrated global value and set PSLTSCALE to 1 so dashes look the same through viewports at different scales.
What scale should millwork elevations be drawn at?
AWI 100 sets 3/8″ = 1′-0″ as the minimum for plans and elevations in woodwork shop drawings, with 1-1/2″ for sections and 3″ for details. Larger is allowed where the detail needs it.
Why does my drawing come out 12 times too big?
Almost always a feet against inches problem. Check what one unit means in the model, then apply the viewport value that matches it.
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Nathalie Paul
Blog Writer
Nathalie is a senior draftswoman at ArchiCGI. She loves the fine art of creating quality drawings. After work, Nathalie is passionate about Ishvara yoga, traveling, and strong bitter coffee.
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