A water tank, a services duct, a square hollow section. All three are a solid with the middle taken out, leaving a wall a few centimetres thick. You can build them by subtracting one solid from another, but it is slow and the inner dimensions are easy to get wrong.
The shell command in AutoCAD does the whole job in one pass. The catch is that typing the word SHELL gives you something else entirely — a trap people fall into constantly, and the right place to start. For the whole three-dimensional toolkit see our AutoCAD for Construction course.
What typing SHELL actually does
Type SHELL and press Enter on the 2027 release and this is what comes back:
Command: SHELL
SHELL OS Command:Nothing about picking a solid. SHELL is the command that reaches the operating system, and has no connection to three-dimensional geometry at all. Autodesk's SHELL page says it plainly: "Accesses operating system commands." It opens a prompt where you can type Windows commands without leaving AutoCAD.
So the English name of the operation collides with the name of an unrelated command. That is why so many people try it once and conclude their copy of AutoCAD does not have the feature.
Where to start it properly
Read from the acad.cuix interface file of the 2027 release, for the default Drafting & Annotation workspace.
| Way in | What to use |
|---|---|
| Command line | SOLIDEDIT, then Body → Shell |
| Alias | none |
| Ribbon | none |
| Classic menu | Modify → Solid Editing → Shell |
| Toolbar | Solid Editing → Shell |
The menu item and the toolbar button both exist, and both simply run the SOLIDEDIT → Body → Shell sequence rather than calling a command of their own. Bringing the classic menus back is covered in our article on the AutoCAD interface.
The steps on the command line
Copied verbatim from AutoCAD 2027.
Command: SOLIDEDIT
Solids editing automatic checking: SOLIDCHECK=1
Enter a solids editing option [Face/Edge/Body/Undo/eXit] <eXit>: b
Enter a body editing option
[Imprint/sePerate solids/Shell/cLean/Check/Undo/eXit] <eXit>: s
Select a 3D solid:
Remove faces or [Undo/Add/ALL]:
Enter the shell offset distance: 50
Solid validation started.
Solid validation completed.| Step | What it asks | What you do |
|---|---|---|
| 1 | Enter a solids editing option [Face/Edge/Body/Undo/eXit] <eXit>: | Type b |
| 2 | [Imprint/sePerate solids/Shell/cLean/Check/Undo/eXit] <eXit>: | Type s |
| 3 | Select a 3D solid: | Pick the solid |
| 4 | Remove faces or [Undo/Add/ALL]: | Pick faces to remove, or Enter to keep all six closed |
| 5 | Enter the shell offset distance: | Declare the wall thickness |
After step five two lines run by, Solid validation started. and Solid validation completed., and the prompt returns to where it was. The command has not finished: it takes two more presses of Enter to get back to a blank command line.
Step four is the one most often skipped. Building an open-topped tank, that is exactly where you pick the top face so the program removes it. Press Enter straight through and you get a hollow solid sealed on all six sides, indistinguishable from a solid one from outside.
The volume left can be worked out beforehand
This is the surest way to tell whether the wall came out at the thickness you asked for.
Take a box of 1000 × 600 × 400 and shell it to 50. The wall eats into both opposing faces, so every dimension loses 100 and the void left is 900 × 500 × 300:
| Value | |
|---|---|
| Solid volume to begin with | 240,000,000 |
| Void removed, 900 × 500 × 300 | 135,000,000 |
| Left over, by subtraction | 105,000,000 |
| Volume read off the solid afterwards | 105,000,000 |
| Difference | 0.000000% |
An exact match. The practical use: after shelling, select the solid, open the Properties palette and read the volume, and you know at once whether the wall is the thickness you declared — no need to cut a section and measure it. That palette is covered in our article on the Properties palette in AutoCAD.
A negative offset pushes the wall outwards
The prompt at step five accepts negative numbers, and the result is quite different.
Take the same 1000 × 600 × 400 box and declare −50:
| Value | |
|---|---|
| Bounding box to begin with | 1000 × 600 × 400 |
| Bounding box after declaring −50 | 1100 × 700 × 500 |
| Volume left | 145,000,000 |
The bounding box grew rather than shrank. A negative figure tells the program to build the wall outside the original faces, taking the shape you drew as the inner surface. The arithmetic agrees: an outer box of 1100 × 700 × 500 is 385,000,000, less a void of 1000 × 600 × 400 at 240,000,000, leaving exactly 145,000,000.
There is a real use for this in construction work. Where you have modelled the clear inside of a duct or a services box correctly and want to build the wall outwards so the clear dimension stays put, a negative figure does it in one pass, with no second solid to draw and subtract.
What shelling a solid is good for
The jobs it comes up on most in construction drawings:
Tanks, septic tanks, gullies. Model the solid to the overall dimensions, shell it to the wall thickness, then remove the top face at step four to open it up. For the capacity, take the overall volume less the volume of the shelled solid.
Services ducts, square hollow sections, riser boxes. Same method, except both end faces come off so the run is open through.
The envelope of a building for a quick massing model. Shelling one large box to the wall thickness gives four closed walls straight away, faster than drawing them one at a time — a solid model of the whole envelope in a couple of minutes.
What all three share: a thin wall of one single thickness across the whole solid. Where the outer wall is 220 and an inner one is 110, this will not do it in one pass and the model has to be split into several solids.
There is another advantage that gets overlooked. Built by subtraction, the drawing carries two solids throughout, and every change to the overall size means editing both. With the shell command in AutoCAD there is only ever one solid: change the overall size, run it again, and the wall thickness is still right. On a job that goes through many rounds of revision that difference adds up.
Solid editing: what else SOLIDEDIT does
Shelling is one entry on a longer list. The prompts lay out three branches:
| Branch | What is inside |
|---|---|
Face | Extrude, move, rotate, taper, delete, colour and copy faces |
Edge | Copy edges, colour edges |
Body | Imprint, separate solids, Shell, clean, check |
The one most worth knowing for construction work is Face → Extrude faces: pick a single face of a solid and pull it out, and the solid gets longer while every other face stays put. That is how you change the length of a member you have already modelled without rebuilding it.
The two lines reading Solid validation started. and completed. appear after every operation because the SOLIDCHECK variable is on, at 1. The program checks the solid after each edit to make sure no broken geometry was produced.
For building the solid in the first place, see our articles on the sweep command in AutoCAD and the loft command in AutoCAD.
Four things people say that are not true
| What people say | What the 2027 release actually does |
|---|---|
Typing SHELL hollows out a solid | SHELL opens the OS Command: prompt of the operating system |
| The hollowing operation has its own alias | It has none; the way in is SOLIDEDIT or the menu |
| The thickness has to be positive | A negative figure works, and the wall goes outwards: 1000 × 600 × 400 becomes 1100 × 700 × 500 |
| Shelling leaves the solid open | Pressing Enter at step four gives a hollow solid closed on all six faces; opening it means picking faces to remove |
Frequently asked questions
Why does typing SHELL ask for an OS Command?
Because that is what it is for. SHELL reaches the operating system. The hollowing operation lives in SOLIDEDIT, under Body → Shell.
Is there a quicker alias?
Not as supplied. The quickest route is the menu item Modify → Solid Editing → Shell, or defining your own alias in acad.pgp.
How do I make an open-topped tank?
At Remove faces or [Undo/Add/ALL]: pick the top face, then press Enter. Pressing Enter straight away gives a hollow solid closed on all six sides.
What does a negative thickness do?
The wall is built outside the original shape. A 1000 × 600 × 400 box at −50 gives a bounding box of 1100 × 700 × 500 and a volume of 145,000,000.
How do I check the wall came out right?
Select the solid, open the Properties palette and read the volume, then compare it with your own subtraction. A 1000 × 600 × 400 box with a 50 wall must give exactly 105,000,000.
Can different faces have different thicknesses?
No. One run takes one figure for the whole solid. Walls of differing thickness mean splitting the model into several solids and joining them.