Drawing units in AutoCAD: change the display, not the geometry

The biggest misunderstanding about drawing units in AutoCAD sits in the word units. People assume changing units changes the size of the geometry, so they either never touch the setting or panic when the numbers start looking different.

The measured truth is short: the UNITS command changes how a number is written, and the geometry does not move by a thousandth. This article proves it with figures taken on AutoCAD 2027 at the Institute. For setting up a drawing environment properly, see the AutoCAD for Construction course.

One line, five different numbers

We drew five identical lines, each exactly 1234.5678 drawing units long, and dimensioned each with a different unit display setting.

Five numbers, five notations, one line. We read the true length back from the endpoint coordinates after every change, and the answer was 1234.567800 every single time.

That is the whole story. Changing units does not touch the geometry.

Changing units does not rescale the drawing

This is worth repeating because it saves whole afternoons.

Your drawing is in millimetres and a wall is 3600. Switch the display to metres and that wall is still 3600 drawing units, not 3.6. AutoCAD has no idea whether one of your drawing units is a millimetre, a metre or a mile. It only counts units.

Which means converting a drawing from millimetres to metres for real takes SCALE with a factor of 0.001. The UNITS command does not do it.

Autodesk's UNITS page says exactly that in one line: "Controls the precision and display formats for coordinates, distances, and angles." Precision and display format. Not size.

The five system variables behind the dialog

The UNITS dialog looks simple, and behind it are five system variables. Knowing their names lets you set them from the command line, and more importantly lets you set them in a template.

The sample drawing we measured had LUNITS 2, LUPREC 1 and INSUNITS 0. That zero means the drawing declares no unit at all, which is the state worth fixing in any office template.

Working out what unit an unfamiliar file uses

A common situation: a client sends a file, the numbers are there, and nobody knows what they mean. Three ways to find out in seconds.

First, measure something you know. Type DIST and snap across an ordinary door. Around 800 means millimetres, 0.8 means metres, about 31 means inches.

Second, read the variable. Type INSUNITS and press Enter. A 4 is millimetres, 6 is metres, 1 is inches, and 0 means the drawing declares nothing. Plenty of files sit at 0, so this only helps when the previous drafter set it.

Third, look at the title block. A scale of 1:100 on an A3 sheet is all but certain to be a millimetre drawing, because that is the convention in Vietnamese construction documentation.

Knowing the unit is what decides whether you have to scale the file when inserting it into your own. Getting that factor wrong by a thousand happens every week in design offices.

Angle units: the same 30 degrees read five ways

Angles confuse people more than lengths, especially on infrastructure work.

We drew a line at exactly 30 degrees and read the angle back through each value of AUNITS.

The last row causes the most trouble. The same line, the same angle, and the surveyor's convention reads it as N 60d0'0" E. The reason is that bearings are measured from north rather than from the X axis, so 30 degrees from X becomes 60 degrees from north.

The radian value is worth noting too: 0.5236 is exactly 30 degrees in radians. Reading the true angle through AutoLISP gave 0.523599 radians on all five measurements.

Dimension units are a separate setting

This is the trap that wastes the most time, and it is not in the UNITS dialog at all.

Change LUNITS and the command line coordinates change, the Properties palette changes, the measuring functions change. But the dimension text does not, because dimensions use their own variable, DIMLUNIT.

We measured it: setting LUNITS to 4 and running REGENALL left the dimension text in decimal format until DIMLUNIT was set to 4 as well.

Remember that pair and you will never again change units and then wonder why the drawing looks untouched. Dimensioning in full is covered in our article on dimensions in AutoCAD.

Unit precision: how many decimal places is right

The short answer for millimetre construction drawings: LUPREC 0, and DIMDEC 0 as well.

Unit precision is a readability decision, not an accuracy one — the stored coordinates keep full precision whatever you display. A millimetre is already small enough. A wall reading 3600 is clearer than one reading 3600.00, and short numbers keep dimension chains from crowding. Metre drawings are the opposite and need two places, because 3.6 metres and 3.65 metres are not the same wall.

There is one case for more places even on millimetre drawings: checking. Set LUPREC to 4 temporarily and measure a few lines, and lines that are out by 0.0001 show themselves immediately. Set it back to 0 afterwards.

Unit display is per drawing, not per machine

One consequence catches people out when they move between offices. Open a colleague's file and the numbers look different from yours, even though nothing is wrong with either drawing. The display settings travelled with the file.

That is also why a drawing you send out may look different on the client's screen. If the numbers matter — a survey schedule, a setting-out table — put them in text or in a dimension with an explicit override, rather than relying on the reader's display settings matching yours.

The same applies to angles. A file set to surveyor's bearings shows every angle measured from north, and a reader whose drawing is set to decimal degrees sees a different number for the same line. Neither is wrong; they are two notations for one angle.

Set it once in the template

Everything in this article is stored per drawing, not per machine. Open a new file and it reverts to whatever the template says.

So the place to set it is not the drawing in front of you but the office .dwt template. Set all five variables once, save, and every new drawing starts right. A short checklist, in order.

1. UNITS set to decimal, with the number of places your office prefers.
2. INSUNITS set to 4 for millimetre drawings.
3. DIMLUNIT and DIMDEC set to match, saved into the dimension style rather than left as variables.
4. Save with SAVEAS and choose the Drawing Template type.

Step three deserves emphasis. Setting the DIMLUNIT variable lasts only until the drawing closes; saving it into the dimension style travels with the documents. Managing text and annotation is covered in our article on text in AutoCAD.

Three things often repeated that are not true

Frequently asked questions

Where do I change drawing units in AutoCAD?

Type UNITS and press Enter to open the Drawing Units dialog, or set the variables directly: LUNITS, LUPREC, AUNITS, AUPREC.

I changed the units and nothing happened. Why?

Because the command only changes how numbers are written. To change the real size, use SCALE — for example a factor of 0.001 to turn millimetres into metres.

Why did the dimension text not follow?

Dimensions use their own variable. Set DIMLUNIT to match LUNITS, then run REGENALL.

How many decimal places should I use?

Zero for millimetre drawings, two for metre drawings. Four temporarily while checking, to catch lines that are fractionally out.

Does it matter that INSUNITS is 0?

A drawing that declares no unit gives AutoCAD nothing to work from when content is inserted from another file. Set INSUNITS to 4 for millimetre drawings and save it into the template.