Types of Dimensions in AutoCAD

Concept:
This tutorial will explain the different types of dimensions which are commonly used in AutoCAD and help you understand their differences. A good AutoCAD drawing will convey the needed information clearly and accurately. Dimensioning is used to let the drawing reader know what size each length or curve is, without having to open the drawing and measure it. If your dimensions are inaccurate, you will not be working in CAD for very long, no matter how accurate your drawing is. Learn when each dimension is needed and how to use it.

If you need to modify the appearance of your dimension, please refer to the previous tutorial - 4-11.

Dim Type	Keystroke	Icon	Description
Linear	DIMLINEAR / DIMLIN		Dimensions for straight lines either horizontal or vertical to the UCS.
Aligned	DIMALIGNED / DIMALI		Dimensions for straight lines that are angled to the UCS and provides the line's length.
Angular	DIMANGLUAR / DIMANG		Dimensions between 2 points and gives the resulting angle measurement.
Arc Length	DIMARC		Dimensions the length of an arc or arc segment of a polyline.
Radius	DIMRADIUS / DIMRAD		Dimensions the radius of an arc, circle or arc segment of a polyline.
Diameter	DIMDIAMETER / DIMDIA		Dimensions the diameter of an arc, circle or arc segment of a polyline.
Center Mark	DIMCENTER		Places a center mark at the center point of an arc or circle
Rotated	DIMROTATED	-	A modifier to a dimension to make it parallel to a selected line.
Spacing	DIMSPACE		Aligns the selected dimensions to an even spacing.
Continue	DIMCONTINUE / DIMCONT		Continues dimensioning from a base dimension
Baseline	DIMBASELINE / DIMBASE		Continues dimension from a base dimension to give an overall dim.

Types of Dimensions in AutoCAD

You have most likely already used some of the common dimension types like 'linear' or 'radius'. This section will describe how each one works in more detail.

Linear Dimensions:
Linear dimensions are used to create a dimension that measures a line that is either horizontal or perpendicular to the UCS. This is the most commonly used dimension and is very easy to use. Below are some typical linear dimensions:

It's good practice (thought not always allowed by time) to have your dimensions at a consistent distance from the object. Make sure that it is not to close to overlap the object and not too far to confuse which object you dimensioned. To create a linear dimension, either use the icon or key in DIMLINEAR. Just pick the two endpoints of a line (or any two points) and then place the dimension line where you want it. Very quick, very simple.

Command: DIMLINEAR

Specify first extension line origin or <select object>:SELECT FIRST POINT
Specify second extension line origin:SELECT SECOND POINT
Specify dimension line location or
[Mtext/Text/Angle/Horizontal/Vertical/Rotated]: SELECT LOCATION OF DIM LINE
Dimension text = 5.00

Note that there are some options at the command line:

You'll also notice on the first prompt, you can <select object>, but press <enter> at this point and then you can just select the line instead of the two endpoints. This is a lot quicker and more accurate as you don't have to worry about picking the wrong Endpoint Osnap.

Since this is a common command, it is worth creating an alias for it (see next lesson).

Aligned Dimensions:
The Aligned dimension works just like the linear dimension. The big difference is that it works with lines or distances that are not square to the UCS. You can also use this on any 2 points that are also compatible with the linear dimension. It will create a dimension parallel to the angled line.

Command: DIMALIGNED

Specify first extension line origin or <select object>:SELECT FIRST POINT
Specify second extension line origin:SELECT SECOND POINT
Specify dimension line location or
[Mtext/Text/Angle]: SELECT LOCATION OF DIM LINE
Dimension text = 2.00

You'll see that command line prompts are almost the same. Of course, there are no options for horizontal or vertical placement. Also Rotate is missing to avoid confusion and errors. You can also select Angle for your text to match the angle of line (just select 2 points on the line you are dimensioning). You can also set your text to be aligned with the dimension line in your Dimension Style Manager.

Angular Dimensions:
Not every dimension gives the reader a length. Sometimes other measurements are need. Angular dimensions will display the angle between two objects. The measurement of the angle is defined by your dimstyle, but degrees is standard. The command works similar to the linear dimensions (see a pattern?).

Command: _dimangular
Select arc, circle, line, or <specify vertex>: <select first line>
Select second line: <select second line>
Specify dimension arc

The example above shows 3 common uses of this tool. To get the angle on the arc, just select anywhere on it. Getting the outside measurement (225° in this example) is done by using the prompt <select vertex>.

Command: DIMANGULAR
Select arc, circle, line, or <specify vertex>:

Specify angle vertex: <select point A>
Specify first angle endpoint: <select point B>
Specify second angle endpoint: <select point C>
Specify dimension arc line location or [Mtext/Text/Angle/Quadrant]:<place dim>
Dimension text = 225

Arc Length:
This is a fairly new command in AutoCAD and will not be available on some older versions. This command works simply, just start the command and select an arc or arc segment of a polyline. In the example below, I have shown the Angular (180°) dimension with the Arc length so that you can see the difference. Notice the symbol in front of the 3.14.

Command: _dimarc
Select arc or polyline arc segment: <select the arc>
Specify arc length dimension location, or [Mtext/Text/Angle/Partial/Leader]: 1.2
Dimension text = 3.14

Once again, you are offered some options when dimensioning. Mtext/Text/Angle are the same options for text that you saw earlier. Partial will allow you to dimension only part of the arc. This is useful for midpoints or nodes, when the arc is segmented. Leader will draw a leader from the dim text to the arc that it refers to.

Radius Dimension:
Another common and simple command. DIMRAD will dimension any arc or circle and give you the radius and place a center mark at the center of the selected object. A center mark is a cross the indicates the center point of an arc or circle.

Command: _dimradius
Select arc or circle: <select object>
Dimension text = 0.40
Specify dimension line location or [Mtext/Text/Angle]: <place the dim>

Once again, you have some options for the text at the command prompt.

Diameter Dimension:
The diameter and radius dimensions work the same, but give a different measurement. As a rule, I will use the diameter dim on things like a hole to be drilled or for the dimensions of an overall part and radius for filleted corners, etc.

As you see, the diameter and radius dimensions add a center mark (this can be turned off, or changed to a larger line (in DDIM). I will usually dimension the circles first so that I can select the center mark and have a small gap (it looks cleaner).

Command: _dimdiameter
Select arc or circle: <select the object>
Dimension text = 1.00
Specify dimension line location or [Mtext/Text/Angle]: <place the dim>

In the example image above, you can see that even a very simple drawing can have enough dimensions to clutter things up. Make sure that you have enough dimensions and don't duplicate or miss any. Make sure that they can be read from one or two angles when printed (usually from the bottom right corner).

Center Mark:
By now you should be getting the hang of how this works. The DIMCENTER command adds a center mark to a circle or arc. How it is displayed is set in the DDIM dialog box under the "Symbols and Arrows" tab. If you select "None" you will not be able to place a center mark using this command. The icon for center marks is in the sub menu on the dimension panel.

Sometimes you might use both in the drawing. Below. on the left, I used the center mark that was made with the diameter dimension, then I dimensioned from it. On the right, I used a Line center mark and extended one of the lines to the other side to indicate the the holes are aligned. Then I double-clicked on the diameter text and added a suffix to indicate that both holes are the same size.

But when you do this, any of the center marks made by the diameter or radius dimensions will change to the new style that you selected. It's best to start with one style and leave it. The size of your center mark is defined by the DIMCEN system variable.

Rotated Dimensions:
A rotated dimension is not a dimension that was rotated using the rotate command. Instead it allows you to create a linear dimension to two points that are not in a straight line. Look at the image below and see the difference between an aligned dimension and a rotated one.

First, look the dimension on the far right. See how it gives a true overall size of 10 units? On the right, the aligned dimension is not the overall size due to the 94° angle in the bottom left corner. Also in the top right, notice the rotated dimension with 2.00 units. I need to rotate it as the 2 points that I selected were not on the same axis. It's a little confusing at first, but try it a few times.

Command: dimlin
DIMLINEAR
Specify first extension line origin or <select object>: <first endpoint>
Specify second extension line origin: <first endpoint>
Specify dimension line location or
[Mtext/Text/Angle/Horizontal/Vertical/Rotated]: R <enter>

Specify angle of dimension line <0>: <select first point on axis>
Specify second point: <select second point on axis>
Specify dimension line location or
[Mtext/Text/Angle/Horizontal/Vertical/Rotated]: <place dim>
Dimension text = 2.00

When it comes to selecting points on the axis, pick the common parallel line (in this example, I click 2 points on the 10 unit dimension. You can also created a rotated dim by using the DIMROTATED command. In this case, you select the angle first.

Even Spacing of Dimensions:
Sometimes you might have a bunch of dimensions in one area and want to space them evenly. The easy way to do this is to place them quickly, then use the Adjust Space command, DIMSPACE. Look at the image below with the random spacing. Then mouse over it to see how this commands gives them even spacing.

Draw a similar shape and try this. Place your dims, but allow for enough room to fit them all (just don't worry about the spacing). Then start the DIMSPACE command.

Command: DIMSPACE
Select base dimension: <I selected the 2.00 dim in this example>
Select dimensions to space:
Specify opposite corner: 3 found <select the others>
Select dimensions to space: <enter>
Enter value or [Auto] <Auto>: <enter>

Fast and easy. You have the option at the end to enter a distance between each dim, but Auto should work in most cases. Just be careful to select the "Base" dimension you want first - try selecting other dimensions as your 'base' to see what results you get.

Continue Dimensions:
Another shortcut is to use is "Continued Dimensions". This is used when you have a series of linear dimensions that will all be on one side of an object. With this command, you can select a 'base' dimension and then have AutoCAD help by continuing the command so that you can just click and endpoint and continue to the next, click an endpoint, etc. Have a look at the drawing below:

When you have a series of dimensions all in one plane like this, just create the first one with DIMLIN and then use the DIMCONT command to create the rest of them. In this example, I created the dim on the left first.

Command: _dimcontinue
Specify second extension line origin or [Select/Undo] <Select>: <SELECT A>
Dimension text = 2.0000
Specify second extension line origin or [Select/Undo] <Select>: <SELECT A>
Dimension text = 1.0000
Specify second extension line origin or [Select/Undo] <Select>: <ENTER>

After doing this, the next time you start the DIMCONT command, it will return to this point and continue on. Try this out in your own drawing with a simple shape like the one above.

Baseline Dimensions
This command is very similar in function to the continue command, but instead provides an overall dimension based upon the first point you select in your first (base) linear dimension.

Command: _dimbaseline
Specify second extension line origin or [Select/Undo] <Select>: <SELECT A>
Dimension text = 3.0000
Specify second extension line origin or [Select/Undo] <Select>: <SELECT B>
Dimension text = 4.0000
Specify second extension line origin or [Select/Undo] <Select>: <ENTER>

Once again, I first needed to create a base dimension with the DIMLIN command. I again used the dim on the left (first point was on the left). Then I started the DIMBASE command and selected points A and B. Once again, try it out and see how this works. See what happens when you create your base dim from right to left.

Summary:
You have a lot of tools at your disposal when it comes to dimensions. The trick is to know when to use each one. It's also extremely important that you are accurate. Clicking on another dimension instead of a line's endpoint can be a huge mistake - especially in mechanical drafting where tolerances are small. Review your dimensions before sending a drawing off to someone else, or printing it- then check it again. Check for overlaps or other confusing areas. The commands themselves are easy to use, but accuracy is critical.

Video: Types of Dimensions in AutoCAD