Placing dimensions in AutoCAD takes ten minutes to learn. The trouble starts afterwards: the numbers print the size of ants, extension lines end at ragged heights, the text drifts away from where you left it, or the value keeps changing when you did not want it to.
Those four complaints have four different causes, and each lives in exactly one box. Nothing about dimensions in AutoCAD is guesswork once you know which box owns which symptom. This article names that box. For the whole drafting and documentation workflow, see the AutoCAD for Construction course.
What a dimension is made of
Fixing the right thing means naming it correctly. A dimension has five parts, and their English names are exactly the labels you will meet in the settings dialog.
| Part | Name in the software | How it usually goes wrong |
|---|---|---|
| The number | Dimension text | Too small, or drifted away from its place |
| Dimension line | Dimension line | Stacked rows sit on top of each other |
| Extension line | Extension line | Ragged lengths across the drawing |
| Arrowhead | Arrowhead | Filled arrows where ticks belong |
| Origin point | Definition point | Association lost, so the value is wrong |
Autodesk groups dimensions into five basic types: linear, radial, angular, ordinate and arc length. Each is described on the About the Types of Dimensions page.
Every setting below lives in one place. Type D and press Enter to open Dimension Style Manager, pick the style, then click Modify.
Change dimension text size in AutoCAD with one box
This is the most searched question of the lot, and the answer is often confused with the SCALE command. They do different jobs. SCALE enlarges the geometry, while what you want is larger text and arrowheads with the geometry left alone.
A dimension carries six sizes that all have to follow the plot scale: text height, arrowhead size, extension line offset, extension line overshoot, spacing between stacked rows, and the gap between text and line. Multiplying each by hand is tedious and easy to get wrong.
AutoCAD collects all six into one box. On the Fit tab, in the Scale for dimension features group, set Use overall scale of to the denominator of your plot scale.
| Plot scale | Set Use overall scale of to |
|---|---|
| 1:50 | 50 |
| 1:100 | 100 |
| 1:200 | 200 |
That box is the DIMSCALE variable. Its DIMSCALE page states two limits worth remembering. First, it "does not affect measured lengths, coordinates, or angles" — the measured value stays put, only the lettering grows.
The second one costs people an afternoon. Autodesk writes that DIMSCALE "does not affect multileader objects created with the MLEADER command". So you scale the dimensions, and every leader note on the sheet stays microscopic. Those follow a separate variable called MLEADERSCALE. Building leader notes is covered in how to draw an arrow in CAD.
Dimensions on a sheet that carries several scales
The method above only holds when the whole sheet prints at one scale. Real documents often put a 1:100 plan next to a 1:20 detail on the same sheet.
In that case do not enter a number at all. Tick Annotative, sitting just above in the same Scale for dimension features group. The program then multiplies by each viewport's own scale, so one dimension reads correctly in both places.
The tidiest arrangement for an office is three ready-made styles, TCVN-50, TCVN-100 and TCVN-200, differing in exactly one box, saved into the template. From then on you pick the style that matches the sheet.
One more setting is rarely used. The DIMLAYER variable puts every new dimension on a layer of your choosing, whatever layer you happen to be working on. Dimensions then never mix into your linework, and the whole set toggles off in one click. Layer housekeeping is covered in layers in AutoCAD.
Fixed length extension lines, or trimming the legs
Draughtsmen often want every extension line the same length instead of each one chasing the geometry. Many go looking for a LISP routine for it, while AutoCAD does it with one checkbox.
| Number | Group | What it covers |
|---|---|---|
| 1 | Seven tabs | Each tab holds one family of settings, the open one is Lines |
| 2 | Dimension lines | Colour, linetype, and Baseline spacing between stacked rows |
| 3 | Extension lines | Extension lines, including the fixed-length boxes on the right |
| 4 | Preview pane | The sample redraws as you change each box |
Tick Fixed length extension lines and put a number in Length. The two boxes are the DIMFXLON and DIMFXL variables. The DIMFXL page defines it as the total extension line length, measured from the dimension line back toward the origin.
Here is the trap that makes the box look broken. The About Controlling Extension Lines page states that "The extension line offset distance from the origin will never be less than the value specified by the DIMEXO system variable". The gap between the object and the foot of the line is always kept, however small you set Length. To go shorter you have to lower Offset from origin as well, directly above it.
Remember that both numbers are multiplied by Use overall scale of too. A Length of 8 at scale 100 gives extension lines 800 units long in the model.
When dimension text moved out of place
Text that drifts is usually text you gave a position of its own, whether deliberately or by catching a grip with the cursor. It is not a fault in the software.
The quickest cure is built in but little known. Select the dimension, hover over the blue grip on the text and do not click. A menu of seven entries appears, and the last one is what you need.
| Menu entry | What it does |
|---|---|
Stretch | The default behaviour when you drag the grip |
Move with Dim Line | Moves the text and the dimension line together |
Move Text Only | Moves the text while the dimension line stays |
Move with Leader | Moves the text away and links it back with a leader |
Above Dim Line | Places the text above the dimension line |
Center Vertically | Places the line through the vertical centre of the text |
Reset Text Position | Returns the text to where the style says it belongs |
Reset Text Position is the rescue. Autodesk describes it as putting the text back to "its default (or "home") position based on the active dimension style". Select a whole run of crooked dimensions and one pass straightens the sheet.
To stop it recurring, stop dragging the text grip freehand. When text has to move, use the matching entry above, because each entry records a different kind of move.
When the value no longer matches: associative dimensions
This kind of drift is the dangerous one: the number sits perfectly still while no longer describing the geometry. The link between dimension and object has broken.
The About Associative Dimensions page sets out three levels, chosen with the DIMASSOC variable.
DIMASSOC | What the dimension becomes | When geometry is edited |
|---|---|---|
| 2 | Fully associative, the default | The value follows automatically |
| 1 | Non-associative but still one object | The value stays put |
| 0 | Exploded into loose lines, text and arrows | No longer a dimension at all |
Association also breaks for two reasons the vendor names. One is an associated object modified significantly or deleted. The other is a block redefined so that the edge carrying the dimension moves.
AutoCAD ships an alarm for exactly this, called the Annotation Monitor. Switch it on and every dimension that has lost its link wears a small badge; clicking the badge opens a menu that calls the repair command directly. Almost no guide mentions it, yet it is the only way to learn about the problem before hunting by hand.
The repair is DIMREASSOCIATE: run the command, select the dimension, then click the point on the geometry again. A bare cross marker means that point is unassociated, a cross inside a box means it is associated.
One limit is worth knowing before you spend time on it. Associative dimensions do not support several object types, among them hatches, images, underlays, multilines and 2D solids. Dimensioning a hatched region will never give you a self-updating number, and hatching itself is covered in the HATCH command in CAD.
How to lock a dimension, and when not to
Sometimes you want the opposite: the value held fast no matter how the geometry changes. The genuine case is an as-built survey where the numbers come from the tape, not from the linework.
The command for it is DIMDISASSOCIATE. Run it, select the dimensions to freeze, press Enter. From then on they ignore the geometry.
The DIMDISASSOCIATE page names two cases the command skips, and that is why people think it did nothing. Dimensions on a locked layer are filtered out. So are dimensions living in a different space from the one you are in. The command prints both counts at the command line, how many were filtered and how many were disassociated, so reading that line settles it.
Think before freezing a whole sheet. Once frozen you lose the greatest benefit of associative dimensions, which is a drawing that reports on itself when someone edits geometry and forgets the number. Freeze only what has a reason to be frozen.
Frequently asked questions
Why did old dimensions keep their old size after I changed the scale?
Those dimensions carry a different style, or carry overrides sitting on top of the style. Select them and reassign the corrected style in the Properties palette, or use MATCHPROP to copy properties from one that already looks right.
Fixed length is on, so why do the extension lines still stand off the object?
That gap is Offset from origin, the DIMEXO variable. The program always keeps it however small Length is set. Lower Offset from origin as well to go shorter.
How do I put dimension text back after it jumped?
Select the dimension, hover over the blue grip on the text and choose Reset Text Position. It works on a whole selection at once.
I scaled the dimensions, so why are the leader notes still tiny?
Multileader objects ignore the dimension scale variable. They follow MLEADERSCALE, which has to be set separately.
Should I use Use overall scale of or the Annotative box?
One scale on the sheet means entering the number, which is simpler and easier to audit. Several scales on one sheet means Annotative, so you avoid maintaining a style per scale.