Article 2 in a series

Units are Key in AutoCAD

To be on a first-name, good-buddy basis with AutoCAD, I believe that you need to thoroughly understand units. 

A fisherman, to be outstanding at fishing, needs to understand the nature of water, lakes, and the sea. It’s the same with AutoCAD. All of our created objects swim in a sea of units.

We’ll focus on length units in this article. Angular units will be discussed in a future post.

3-D Grid

Illustration of the 3-D Grid used by AutoCAD

In the first article of this series, we discussed the absolutely central role of units in AutoCAD. I’ve linked that article HERE if you’d like to review it.

We saw how AutoCAD organizes its universe into an imaginary three-dimensional grid. All of the objects placed in it are organized by their x, y, and z coordinates. Those coordinates relate back to the origin, which is at the 0, 0, 0 location where all of AutoCAD’s internal measurements begin. Here’s the animated video I made of the grid: STOMP!

17th-century map of the universe

17th-Century map of the universe with Earth at the center

This origin point is literally the center of the universe to AutoCAD, even though, as a designer, I may usually be unaware of its location relative to my work.

AutoCAD doesn’t much care what real-world units that you (the designer) have in mind for your project. Do you like inches? Millimeters? Cubits? Furlongs?Light years? Whatever, says ACAD. To AutoCAD, it’s all about units.

It’s our job as designers to be clear and consistent about units as they’re used in our project.

The actual real-world length measurement of a unit is only of secondary importance to the software. It’s really up to us, as designers, to be clear, and to stay consistent, on the nature of the units as they’re used in our project.

Two Places to Set Up Units

There are two critical places in an AutoCAD project where we need to carefully set up the units:

  1. Drawing Utilities and
  2. Dimension Styles.

These two roles, while related, are really quite separate. The setting of one has no effect whatsoever on the other. What works well in the first may not be appropriate in the second, and vice versa.

The two roles are aimed at different audiences. The designer sees the results of the unit settings made in the Drawing Utilities dialog box. The builder or manufacturer who receives the drawings sees the results of the unit settings that the designer makes in the Dimension Styles dialog box.

Drawing Utilities is where units get defined for the direct, two-way communication between the AutoCAD software and us (the designers). The Properties dialog box is the main place that AutoCAD software “talks” to us, the designers. When you check the properties of a line or other object, how does the length appear? Does it show up with feet, inches and fractions? Or does it appear as simply a number, possibly with decimals?

The LIST command’s output is another place where the unit settings in Drawing Utilities affect the display of lengths. The Area and Volume displays are affected by the type of length unit setting. 

Very importantly, this Drawing Utilities setting controls the default unit which is assumed by AutoCAD when you type in a number (like 36) for a distance or length.

Dimension Styles is where we control how units appear in our dimensions. These dimensions appear in the printed documents and tell the builder what sizes we expect them to produce in the physical objects.

I’ll address setting up units in Dimension Styles in an upcoming post.

Choosing Unit Type and Precision Settings

Within the definition of the project’s units, there are two main settings for us to consider: 

  1. Type and
  2. Precision

You say you want your house’s walls laid out using meters as a measurement system? Great. We’ll choose Decimal units as the type. It’s important to note that AutoCAD will not automatically append “cm” (meaning centimeters) or “mm” (millimeters) onto the units’ display. Again, it’s up to the designer to keep track of the type of metric unit that she intends to be the default unit in the drawing.

If you choose Architectural units as your type, AutoCAD will help you out by displaying an apostrophe or double quotation marks after the unit quantity in the Properties.

Now tell me — how many decimal places do you want to see in the properties box? In the dimensions? And do you want the building’s concrete foundations laid out—and dimensioned—to the same level of precision as the kitchen cabinetry? Probably not.

These are the kind of unit-related considerations that the designer needs to address when setting up units for a project. Ideally, the designer will have a custom AutoCAD template (DTE file) where all of these decisions will be stored for use in the next project. Of course, even the best template file needs to be reviewed and updated periodically.

Menu to set Project Units

Setting the project’s units in Drawing Utilities

The first place where we need to specify the type of units that we want to use for a project is in the Drawing Utilities. Again, this is where we control how the units will appear in the Properties dialog, the LIST command’s output, and other internal locations. The settings can be found by clicking the Big Red A (Application Browser) button and then following the menu to Units.

Drawing Units dialog box

Drawing Units dialog box

Opening the Drawing Units dialog box shows you various controls used to specify the Length and Angle units (image at left). Here we will focus on length.

Expanding the Length Type selector gives you the following options:

  • Architectural (meaning feet, inches, and fractions). Mainly used in the United States. Also known as “Imperial” units.
  • Decimal (which can mean meters, centimeters, millimeters, or many other types of units).
  • Engineering (combines feet and decimal inches)
  • Fractional (inches and fractions, with no feet. Footless.)
  • Scientific (using exponents)

Let’s look at these one at a time.

Using the Architectural Unit Type

When you select Architectural units, the software translates lengths and distances into feet, inches, and fractions in its output to the designer. An apostrophe designates feet, and double quotation marks indicate inches.

The format of Architectural units that we input to AutoCAD is not the same as the output units’ format. In AutoCAD’s output to the designer, a sample length might be:   12′-3 1/2″. To input that same length into AutoCAD, the syntax changes. The designer is expected to type that length like this:  12’3-1/2. Go figure.

Weird Syntax

This makes some sense when you consider that AutoCAD from Version 1.0 has always used the [SPACEBAR] key to be equivalent to the [ENTER] key. So typing that space between the inch number and the fraction (in 12′-3 1/2″) would throw you out of the input part of the process before you could get the fraction typed. That’s how I justify the weird syntax required by AutoCAD for its input of Architectural unit quantities. 

Using Architectural units, also known as Imperial units, the default unit that AutoCAD assumes for designer input is inches. Say you’re executing a COPY or MOVE command. Type in 36 for the move distance, with no unit type specified, and AutoCAD moves your object 36 inches, not feet.

Fractions are rounded to whatever the precision level is set to be. The AutoCAD options are:

1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, or 1/256 of an inch.

Using Decimal, Engineering, Fractional, or Scientific Unit Type

With Decimal unit type chosen, the default unit could be meters, centimeters, or just about anything imaginable (other than feet, inches, and fractions).

In civil engineering and surveying workplaces in the U.S., the default unit would be decimal feet. In a U.S. machine shop, it would usually be decimal inches. In Europe it could be either meters, centimeters, or millimeters, depending on the industry that you’re working in.

Engineering unit type: I don’t know of any industry that uses a combination of feet and decimal inches. Seems strange. Almost as strange as Architectural units. A sample:   30′-2.375″.

Fractional unit type: The cabinet-building industry in the United States uses what AutoCAD calls Fractional units (inches and fractions, but no feet). A sample:  36 1/2″.

The Scientific unit type might be useful for very large or very small work. Maybe for astronomical-scale projects, or designing microscopic nanobots. A sample:  10.1467E+01.

By the way, I’m told that companies that design and build microchips use mils as the standard unit for their design drawings and manufacturing. A mil equals one-thousandth of an inch. In SI (International Standard) units, 1 mil is equal to 2.54E-5 meters.

This all may seem to be basic stuff, and for many experienced users it is. Still, I think it’s helpful to think about the way that AutoCAD sees the universe. By doing this, we can align our thinking with the internal processes of the software.

This understanding prepares us to respond effectively to special or unexpected challenges that arise.

Thanks for reading! Please share in the comments about your experience with unit-related matters. And as always . . .

Keep on CADDing!🙂


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