The EXTRUDE command in AutoCAD: from 2D shape to 3D solid — taper, path, and why an open shape only makes a surface

Most 3D models in construction start from a 2D shape: a wall plan, a beam section, a footing outline. The extrude command in AutoCAD pushes that flat shape up to a height to make a solid. It is the most used modelling command, and it has details that confuse newcomers: why one shape becomes a solid and another only a thin surface, and why the profile disappears after extruding.

This article tries EXTRUDE in AutoCAD 2027 with the acadiso.dwt template and reads the volume of every result against a hand calculation. Spinning a shape around an axis is covered in the revolve command. For 3D modelling taught properly, the Institute runs an AutoCAD course for construction.

Starting EXTRUDE

Locations and aliases come from the 2027 acad.cuix and acad.pgp files. The Autodesk documentation on EXTRUDE says it "Creates a 3D solid from an object that encloses an area, or a 3D surface from an object with open ends."

Type EXT and the 2027 release reports and asks:

Closed shapes become solids

A 2000 × 1000 rectangle drawn with RECTANG, extruded 500: the result is a solid (3dSolid) of 1 000 000 000, exactly 2000 × 1000 × 500, spanning heights 0 to 500.

Type a negative height, -500, and the solid goes down: from −500 to 0, same volume. Handy for footings drawn at ground level.

After extruding, the original rectangle disappears: the object count stays at 1. That is the DELOBJ variable, 3 by default, deleting the profile once the solid exists. To keep the profile, copy it first or change DELOBJ; the variable is covered in the revolve command.

Open shapes become an extruded surface

An open two-segment polyline extruded 500 gives no solid but an extruded surface (ExtrudedSurface), like a wall with no thickness. And the original polyline remains: the object count is 2.

This is the commonest mistake: walls drawn with LINE or an open polyline extrude to surfaces, not solids, and there is no volume to measure. Close the shape first, for example with the BOUNDARY command, which makes a closed polyline from loose lines.

Which profiles extrude

Each shape below was extruded 300:

For a self-intersecting curve, such as a bow tie whose sides cross, the 2027 release says:

Cannot sweep or extrude a self-intersecting curve.
1 object removed from selection set.

The shape is dropped from the selection and the command waits for another. Split the bow tie into two triangles, or redraw the outline so it does not cross itself.

You can extrude several shapes at once. A 1000 × 1000 square, a circle of radius 500 and a 2000 × 500 rectangle, selected together and extruded 300, give three separate solids of 300 000 000, 235 619 449 and 300 000 000. They are not merged; join them afterwards if you need one solid. Each keeps its own volume, which is convenient for quantities: select one solid and you read that element alone, without subtracting anything. The circle and the region of the same radius give exactly the same volume, 235 619 449, so it makes no difference to the result whether a round column is drawn as a circle or converted to a region first.

Taper angle

The T (Taper angle) option slopes the sides inwards. The same 2000 × 1000 rectangle, T, angle 5, height 500, gives a tapered solid of 935 659 213.

By hand: each side moves in by 500 × tan 5° ≈ 43.74, so the top face is 1912.5 × 912.5. The frustum formula V = h/6 × (base + 4 × middle + top) gives about 935 660 000, matching AutoCAD's figure to the thousands. In numbers: the base is 2 000 000, the middle section at height 250 is about 1956.3 × 956.3 ≈ 1 870 700, and the top is about 1912.5 × 912.5 ≈ 1 745 200. So a 5° taper on a 500-high block removes roughly 6.4 % of the volume, a figure worth knowing before you order concrete for a batch of tapered pile caps. Tapered extrusions suit sloped footings, pile caps and machine plinths.

Extrude along a path

The P (Path) option pushes the profile along an existing curve instead of straight up. A circle of radius 50 along a three-segment 3D polyline, each segment 1000 long:

The result is a pipe of 23 561 945, exactly π × 50² × 3000: circle area times total path length. Note that with DELOBJ at 3, both the circle and the path are deleted, leaving only the pipe. If you need the path again, copy it first.

The D (Direction) option extrudes along two points you give. Extruding the rectangle from 0,0,0 towards 0,500,500 gives a slanted solid, 500 high, still 1 000 000 000.

MOde: solid or surface

The line Closed profiles creation mode = Solid tells you the current mode. Type MO at the first prompt, then SU (Surface), and even a closed rectangle becomes a surface, with the profile kept. Type MO, SO to go back to solids. If a closed shape keeps producing surfaces, read that line first.

Walls from a floor plan: a worked order

Putting the results together, here is a safe order for turning a plan drawn with loose lines into 3D walls:

1. Copy the plan to its own layer first, because the profiles are deleted after extruding.
2. Use BO and pick inside each wall segment to get closed polylines; loose lines will not extrude to solids.
3. Check the information line when you start EXT: it must say Closed profiles creation mode = Solid.
4. Select all the wall polylines together and type the storey height, for example 3300. Each wall segment becomes its own solid.
5. If AutoCAD says Cannot sweep or extrude a self-intersecting curve., find the wall whose outline crosses itself and redraw it.
6. For footings, do the same with a negative height so they go down below floor level.

When you are done, each solid's volume equals its plan area times its height, so the model is also usable for a first estimate of masonry or concrete quantities.

The same order works for columns, beams drawn in section, and slabs drawn in plan. The only change is the direction: slabs and footings usually go down from a reference level, walls and columns go up. Keeping the profiles on a copied layer also means you can re-extrude at a different height if the storey height changes, instead of redrawing the plan.

Three things often repeated that are not true

Frequently asked questions

What does the extrude command in AutoCAD do?

It pushes a 2D shape up to a height, along a direction or along a path, to make a 3D solid or surface.

Why did I get a surface instead of a solid?

The shape is not closed, or MOde is set to Surface. Close the shape, or type MO, SO.

Why did my original shape disappear?

DELOBJ at 3 deletes the profile, and the path too if you used Path. Copy them first if you need them.

How do I extrude downwards?

Type a negative height, for example -500: the solid runs from −500 to 0.

How do I make a pipe along a route?

Draw the circular section and the route, type EXT, select the section, type P, and select the route.

How do I make a tapered footing?

At the height prompt type T, enter the taper angle, then the height. A 5° taper on a 2000 × 1000 block 500 high gives 935 659 213.