Understanding Units in AutoCAD

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Often, we get questions from panicked clients afraid that they are drawing in the “wrong units” or that they cannot understand the lengths or dimensions they are seeing. This is due to misunderstanding the AutoCAD drawing space and how it works. Let’s try to clear that up!

Drawing Units

Generally you will draw in either:

  • Metric (mm, m, km etc.)
  • Imperial (inches, ft etc.)

These will be different depending on the region you’re in. Most of the world uses metric, while North America and sometimes UK prefers Imperial.

In AutoCAD, you draw in AutoCAD units. These could be anything, i.e 1 unit could be metres, feet or even lightyears!

During drawing setup, you choose settings for length units (for measuring linear objects and distances) and angle units (for measuring angles between nonparallel objects or points on arcs or circles) in the Drawing Units dialog box. AutoCAD’s length unit types are as follows:

  • Architectural units are based in feet and inches and use fractions to represent partial inches: for example, 12′3 1/2″. The base unit is the inch unless otherwise specified, so if you enter a number like 147.5, then AutoCAD will understand it to be 12′3 1/2″.
  • Decimal units are unitless — that is, they’re not based on any particular real-world unit. With decimal units, each unit in the drawing could represent an inch, a millimeter, a parsec, a furlong, a fathom, a cubit (if you’re into building arks in case that rainy day should come), or any other unit of measure you deem suitable, from Danish alens to the Swiss zoll. An example would be 15.5.
  • Engineering units are based in feet and inches and use decimals to represent partial inches: for example, 12′3.5″.
  • Fractional units, like decimal units, are unitless and show values as fractions rather than decimal numbers: for example 15 1/2.
  • Scientific units, also unitless and show values as exponents, are used for drawing really tiny or really large things. If you design molecules or galaxies, this is the unit type for you. Examples would be 15.5E+06 (which is 15,500,000) and 15.5E–06 (which is 0.0000155).

 

AutoCAD’s angle unit types are as follows:

  • Decimal Degrees show angles as decimal numbers and are by far the easiest to work with, if your type of work allows it.
  • Deg/Min/Sec is based on the old style of dividing a degree into minutes and minutes into seconds. Seconds aren’t fine enough to display AutoCAD’s precision capabilities, though, so seconds can be further divided into decimals. There is no degree symbol on a standard keyboard, so AutoCAD uses the lowercase letter d. An example would be 45d30′10.7249″. One nautical mile (6,076 feet) is approximately 1 minute of arc of longitude on the equator. David Letterman once said that the equator is so long that it would reach once around the world.
  • Grads and Radians are mathematically beautiful but are not widely used in drafting. There are 400 grads, and 2*pi (6.2831…) radians, in a circle.
  • Surveyor’s Units type is similar to Deg/Min/Sec but uses quadrants (quarter circles) rather than a whole circle. An angle in Deg/Min/Sec might measure 300d0′.00″, while the same angle in Surveyor’s Units would be represented as S 30d0′0.00″ E.

The only setting that you might need to specify is the one that controls the display style of the units that you plan to use. For example, the Architectural units style displays symbols for feet and inches, while the Decimal units style does not.

So where do I change these?

You can change them in the drawing units dialogue box in AutoCAD. Simply type “units” into the command line and you will see the following menu.

Drawing Units dialog box

Note: There is also a sample output area which shows you a preview of how dimensions will display with the settings you’ve selected.

Of course that means you can draw you entire floorplan in the units you want to draw, and if you realize you would like to change them later on, simply use the above menu. No modification needed.

But what if, for example, you want to add in a block that is drawn in inches to a drawing that is done in mm? Well, here you would have to use a little math to scale the relationships in.

You can either use the scale tool in the “insert block” dialogue or you can place the block and scale it manually.

You can also do this to the entire drawing:

  1. Click Home tab  Modify panel  Scale Find
  2. At the Select Objects prompt, enter all.
  3. Enter a base point of *0,0.

    Scaling will be relative to the world coordinate system (WCS) origin and the location of the drawing origin will remain at the WCS origin.

  4. Enter the scale factor. To convert from inches to centimeters enter 2.54 (there are 2.54 centimeters per inch). Or, to convert from centimeters to inches enter 0.3937 (the inverse of 2.54 centimeters per inch).

Take note that you need to know the ratio and math behind the conversion.

Changing dimension styles

You can edit the dimension style by going to the Annotate tab, Dimensions panel.

  1. Click Home tabAnnotation panelDimension Style Find 
  2. In the Dimension Style Manager, click New.
  3. In the Create New Dimension Style dialog box, enter a name for the new dimension style, and click Continue.
  4. In the New Dimension Style dialog box, click each tab, and make any changes for the new dimension style.
  5. Click OK and then Close to exit the Dimension Style Manager.

Here you will find a range of fully customizable options to change everything from the size and colour of the text, the type of arrows and how far the extension lines start from the object.

If you would like to maintain these settings in all your drawings, you should create a new style in your template and save it. This will save you the hassle of having to constantly set it each time you create a new drawing.

Drawing templates

New drawings start from either a default drawing template file or a custom drawing template file that you create. Drawing template files store default settings, styles, and additional data.

This includes the above mentioned units and dimension styles.

CAD managers often create, maintain, and distribute drawing template files to maintain consistent standards and styles across an organization. Among the settings specified are

  • Units of measure and measurement style (UNITS)
  • Drafting settings (DSETTINGS)
  • Layers and layer properties (LAYER)
  • Linetype scale (LTSCALE)
  • Dimension styles (DIMSTYLE)
  • Text styles (STYLE)
  • Layouts with layout viewports and scales (LAYOUT)
  • Plot and publishing settings (PAGESETUP)

You can easily create your own template by creating a new drawing, based on an already existing template, then saving it as a template itself.

To do this:

       

Create a new template from an existing drawing or drawing template.

After changing the settings of any drawing file, you can create a template to quickly create new drawings with the same conventions and default settings.

    1. Erase all of the objects in the drawing that you do not want to keep.
    2. Click the Application button  Save As  AutoCAD Drawing Template.

      DWT files must be saved in the current drawing file format. To create a DWT file in a previous format, save the file in the desired DWG format, and then rename the DWG file using a DWT extension.

    3. In the Save Drawing As dialog box, File Name text box, enter a name for the drawing template and click Save.
    4. Enter a description for the drawing template and click OK.

The new template is saved in the template folder.

 

 

 

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