A shape in AutoCAD is only a set of outlines. It has no inside. To add two shapes together, cut one out of another, or ask the program for the moment of inertia of a section, you first have to give it that inside. The region command in AutoCAD does exactly that.
This article measures every operation on AutoCAD 2027 at the Institute, and the four areas that come back agree with hand arithmetic to the last digit. For the full set of drawing tools see our AutoCAD for Construction course.
Where to start it, and why the outline must close
Read straight out of the acad.cuix interface file of the 2027 release on the machine.
| Command | Alias | Ribbon | Classic menu |
|---|---|---|---|
REGION | REG | Home → Draw → Region | Draw → Region |
UNION | UNI | none | Modify → Solid Editing → Union |
SUBTRACT | SU | none | Modify → Solid Editing → Subtract |
INTERSECT | IN | none | Modify → Solid Editing → Intersect |
BOUNDARY | BO, BPOLY | Home → Draw → Hatch Drop-down → Boundary | Draw → Boundary |
MASSPROP | none | none | Tools → Inquiry → Region/Mass Properties |
The three operators sit under Solid Editing, the group meant for 3D solids. Plenty of people therefore never think to use them on flat geometry. They work perfectly well on it.
Autodesk describes the main command on its REGION (Command) page for the 2027 release: "Converts objects that enclose an area into a 2D region object". And lists what qualifies: "Valid objects include polylines, lines, circular arcs, circles, elliptical arcs, ellipses, and splines".
The word enclose is the condition, and it is the single most common reason the command appears to do nothing at all. We tried an open polyline of just two segments. The program answered, verbatim:
0 loops extracted.
0 Regions created.And the object stayed exactly where it was, still an LWPOLYLINE. Repeating it with a closed 2000 × 1000 rectangle produced one region, whose area read back as exactly 2,000,000.
Autodesk is also clear about several shapes at once: "Each closed loop is converted into a separate region". Select two circles and the program prints 2 loops extracted. then 2 Regions created. — two separate regions, not one combined.
The union command: the overlap counts once
This is the operation people reach for most, and the one most often misunderstood.
We built two circles of radius 600 with centres 800 apart, so they overlap. Converted both to regions and read the areas: 1,130,973.4 each, which is π times 600 squared.
Running UNION and reading again gives 2,014,148.1. That is not the sum of 2,261,946.8. The overlapping part is counted once, exactly as a union should behave.
One claim in circulation deserved its own test. Several guides say two regions must intersect before they can be combined. We took two circles standing entirely apart, 1,400 units between them, and ran UNION. The result: two objects became one, with an area of 2,261,946.7, precisely the sum of the two circles.
So they combine, and the single region then holds two separate loops. Click one and you have both. That behaviour is genuinely useful on a drawing set: a floor plate broken into several wings can live as one region, so a single reading gives the whole area without adding anything up.
The subtract command and intersect: cutting and common ground
The other two operations use the same pair of circles, so all three results can be compared.
| Operation | Measured area | Checked by hand |
|---|---|---|
| One circle | 1,130,973.4 | π × 600² |
UNION | 2,014,148.1 | both circles less the overlap |
SUBTRACT | 883,174.8 | one circle less the overlap |
INTERSECT | 247,798.6 | the overlap itself |
Take a calculator to it: 1,130,973.4 times two minus 247,798.6 gives 2,014,148.2, matching the UNION figure. And 1,130,973.4 minus 247,798.6 gives exactly 883,174.8, matching SUBTRACT.
The four numbers close on each other. That is evidence the program is doing real set arithmetic rather than an approximation.
SUBTRACT differs from the other two in one way: it asks for two rounds of selection. The first is what you keep, then Enter, then what to cut away. Reverse the order and you get a completely different shape.
The massprop command gives the moment of inertia
This is the part worth most to anyone doing structures, and almost no guide mentions it.
Once a shape is a region, type MASSPROP and select it. The program prints a full report. We ran it on the 2000 × 1000 rectangular region and got:
| What the program prints | Value |
|---|---|
Area | 2000000.00 |
Perimeter | 6000.00 |
Centroid | X: 81000.00, Y: -39500.00 |
Principal moments | I: 166666666666.50 and J: 666666666666.00 |
The last line is the one that matters. The textbook formula for the second moment of area of a rectangle about its own centroid is b times h cubed, divided by twelve.
Put the numbers in: 2000 times 1000 cubed divided by 12 is 166,666,666,666.7. The program printed 166,666,666,666.50. They agree.
So for any section at all, including a compound one built from several shapes, you can draw it closed, convert it to a region, and let AutoCAD work out the moment of inertia instead of splitting it up by hand. At the end of the report it offers Write analysis to a file? [Yes/No] <N>: so the figures can go straight into a text file.
The boundary command: a region without picking every line
When a shape is built from a dozen separate lines, selecting them all is tedious. BOUNDARY, aliased BO, does it in one click.
Type BO, change Object type in the dialog from Polyline to Region, click Pick Points, then click a point inside the area you want. The program traces the boundary and builds the region.
That route has two advantages over selecting by hand. It is quicker, and more importantly it leaves the original lines alone, so the drawing is untouched. On a survey plan traced from many short segments, this is often the only practical way in.
Other ways of measuring area are covered in our article on calculating areas in AutoCAD.
The original shape disappears: the DELOBJ variable
This is the trap that alarms newcomers most. Convert a shape to a region and the outline you drew vanishes.
We measured it: DELOBJ reads 3, and after running REGION the original rectangle was no longer in the drawing. Autodesk says as much: "REGION deletes the original objects after converting them to regions unless the system variable DELOBJ is set to 0".
There are two ways to handle it.
The first is to set DELOBJ to 0, so the program keeps the original. Convenient when you only want a single measurement, though it stays changed for the rest of the session, so set it back afterwards.
The second, and safer: use COPY to duplicate the shape somewhere empty, convert the copy, measure, then erase the lot. The original drawing is never touched.
To reverse it, type X for EXPLODE. We tried it on a region made from two circles: it broke into ARC objects. So you get the outlines back, but as separate arcs rather than the circles you started with.
Joining those arcs back into one continuous outline is covered in our article on polylines in AutoCAD.
Three things people say that are not true
| What people say | What the 2027 release actually does |
|---|---|
| Regions must intersect before they can be combined | They need not. Two circles standing apart combined into one object whose area was the exact sum |
UNION adds the two areas together | Only when they do not overlap. Where they do, the overlap counts once |
| The three operators are for 3D solids only | They sit under Solid Editing but work on flat geometry, as the four figures above show |
Put simply, the region command in AutoCAD is the step that turns a drawing into something you can calculate with. After that, adding, cutting, taking common ground and reading a moment of inertia are each a single command.
Frequently asked questions
What is the alias?
REG. The three companions are UNI for UNION, SU for SUBTRACT and IN for INTERSECT.
Why does it report 0 Regions created?
The outline is not closed. An open polyline produces no region at all, and the original object stays exactly where it was.
Why did my shape disappear after running it?
Because DELOBJ is 3, so the original is deleted. Set DELOBJ to 0, or copy the shape elsewhere before converting.
Which shape do I select first with SUBTRACT?
The one you are keeping, then Enter, then the one to cut away. The other order gives a different result.
How do I get the moment of inertia of a section?
Draw the section closed, convert it to a region, type MASSPROP and select it. Read the Principal moments line.
Is there a way to make a region without picking every line?
Yes. Type BO, set Object type to Region, and click a point inside the area.