3D models are often assembled from small solids: columns and beams, pile caps and piles, walls and pilasters. To make them one solid, so volumes are not counted twice, you use UNION. Its sibling INTERSECT does the opposite: it keeps only the part where the solids overlap. This article covers union and intersect in AutoCAD 2027 with the acadiso.dwt template, using two 1000 cubes and reading the volume after every step.
Subtracting solids is covered in holes in 3D solids; combining 2D areas is in the region command. For 3D modelling taught properly, the Institute runs an AutoCAD course for construction.
Starting the two commands
| Command | Alias | Menu | Toolbar |
|---|---|---|---|
UNION | UNI | Modify → Solid Editing → Union | Modeling → Union |
INTERSECT | IN | Modify → Solid Editing → Intersect | Modeling → Intersect |
Locations and aliases come from the 2027 acad.cuix and acad.pgp files. Both ask a single question, Select objects:; select the solids and press Enter. Unlike SUBTRACT, the order does not change the geometry. The Autodesk page on UNION describes the union command as combining two or more solids, surfaces or 2D regions into one.
UNION: the overlap counts once
Two 1000 × 1000 × 1000 boxes, the second shifted by 500, so they share 500 × 1000 × 1000. Separately they total 2 000 000 000.
| Step | Objects | Volume |
|---|---|---|
| Before | 2 | 1 000 000 000 each |
UNION | 1 | 1 500 000 000 |
The result is a single solid whose volume is the total minus the overlap: 2 000 000 000 − 500 000 000. That is why you merge before taking quantities: adding separate solids counts the overlap twice.
Solids that only touch face to face also merge: two boxes placed side by side become one solid of 2 000 000 000. There is no overlap to remove in that case, so the volume is simply the sum; the gain is that the pair now behaves as one object when you move, copy or measure it. A row of wall segments drawn separately and merged this way reports one total volume instead of a list you have to add up yourself.
The INTERSECT command
With the same two overlapping boxes, the intersect command keeps the overlap: one solid of 500 000 000. Both originals disappear; only the common part remains. Use it to find where two members overlap, such as a pile embedded in a cap or a beam running into a column, before you count volumes.
Footing, column, beam: overlaps and the resulting layer
A closer-to-life example. A 2000 × 2000 × 800 footing on layer DAI, a 400 × 400 column 3100 high embedded 100 into the footing on layer COT, and a 300 × 4000 × 400 beam passing through the top of the column on layer DAM.
| Volume | |
|---|---|
| Footing | 3 200 000 000 |
| Column | 496 000 000 |
| Beam | 480 000 000 |
| Three separate solids added | 4 176 000 000 |
After UNION | 4 112 000 000 |
The 64 000 000 difference is the overlap: the column embedded in the footing, 400 × 400 × 100 = 16 000 000, and the beam through the column, 300 × 400 × 400 = 48 000 000. Adding the three by hand overcounts the concrete by exactly that.
Which layer does the merged solid go on? AutoCAD uses the layer of the first solid selected:
| Selected first | Result on layer |
|---|---|
| Column | COT |
| Beam | DAM |
If your drawing separates members by layer for filtering or quantities, select the member whose layer you want to keep first, or change the layer right after merging.
INTERSECT needs more care still: it keeps what is common to all the selected solids. Select footing, column and beam, where footing and beam do not touch, and the common part of all three is empty, so AutoCAD deletes all three. Select only footing and column and you keep the embedded part, 16 000 000, on layer DAI of the first selection.
Trap one: separate solids still merge into one
Put the two boxes far apart, the second at X = 3000, not touching, and run UNION:
| Objects | Volume | |
|---|---|---|
After UNION | 1 | 2 000 000 000 |
AutoCAD does not refuse. The two distant boxes are now one object in two pieces. If that was a mistake, split them with SOLIDEDIT:
| Step | The prompt | What you do |
|---|---|---|
| 1 | Enter a solids editing option [Face/Edge/Body/Undo/eXit] <eXit>: | B |
| 2 | Enter a body editing option [Imprint/seParate solids/Shell/cLean/Check/Undo/eXit] <eXit>: | P |
| 3 | Select a 3D solid: | Select the two-piece solid |
| 4 | Two option prompts | Enter, then X |
Afterwards the solid is back to two objects of 1 000 000 000 each: that is how you get separate solids from a merged one whose pieces do not touch.
Trap two: INTERSECT on separate solids deletes both
With the same two distant boxes, run INTERSECT. They have nothing in common, and the 2027 release says:
Null solid created - deletedResult: no objects left. AutoCAD deletes both originals because the intersection is empty. On a large model, including one stray solid in the selection loses it. Type U straight away and both boxes come back, 1 000 000 000 each. The same undo works for the reversed subtraction: a cylinder subtracted by a slab it sits in leaves the drawing two objects short, and a single U brings back the cylinder of 6 283 185 and the slab of 800 000 000 unchanged. The lesson is to read the command line after every Boolean operation; the message appears once and is easy to miss while you move on to the next step. The same happens with a reversed selection in SUBTRACT; see holes in 3D solids.
Solids with 2D regions: refused
Select a box and a circular region and run UNION; AutoCAD refuses:
At least 2 solids, surfaces, or coplanar regions must be selected.Nothing changes: the box stays a solid and the region stays a region. UNION joins like with like: solids with solids, surfaces with surfaces, or regions on the same plane. To add a region to a solid, extrude it first with the extrude command.
A worked order for concrete quantities
Putting the results together, here is a safe order for getting the concrete volume of a group of members:
- Copy the solids to a temporary layer before merging, because
UNIONreplaces the originals with one new solid. - Select the group, picking first the member whose layer you want to keep, then
UNI. - Read the new solid's volume in Properties. Compared with the separate total, the difference is the overlap that has been removed.
- If the group contains a detached piece, the result is one solid in several pieces. The volume is still right, but to select pieces individually later, split them with
SOLIDEDIT→B→P. - Use
INTERSECTonly when you are sure every selected solid overlaps all the others. If you seeNull solid created - deleted, typeUat once.
| Job | Command | Result in the example |
|---|---|---|
| Volume of a group without double counting | UNION | 4 112 000 000 |
| Overlap between two members | INTERSECT on two solids | 16 000 000 |
| Split a mistaken merge | SOLIDEDIT seParate | Two solids of 1 000 000 000 |
Used this way, union and intersect in AutoCAD give volumes you can put straight into a concrete estimate, with the overlaps between members counted exactly once.
Three things often repeated that are not true
| Often repeated | What holds on the 2027 release |
|---|---|
| UNION only merges solids that touch | Solids far apart become one object in two pieces |
| INTERSECT on separate solids does nothing | It reports Null solid created - deleted and deletes both |
| A solid can be merged with a region | AutoCAD asks for at least two solids, surfaces or coplanar regions |
Frequently asked questions
How do I merge solids in AutoCAD?
UNION, alias UNI. Select all the solids you want to combine and press Enter; they become one solid on the layer of the first one selected.
How do I keep only the overlap of two solids?
INTERSECT, alias IN. Select the two solids; only the overlap remains, on the layer of the first one selected.
I merged two separate solids by mistake. How do I split them?
SOLIDEDIT, B, P, select the solid, then exit. It splits into its separate pieces, each with its own volume again.
Why did INTERSECT delete everything?
The solids have no common part, so AutoCAD reports Null solid created - deleted and deletes them. Type U at once.
Does the selection order matter?
Not for the shape made by UNION and INTERSECT, only for the layer of the result. For SUBTRACT, the order decides what is kept.
Why merge before measuring volume?
Adding separate solids counts overlaps twice. In the footing, column and beam example, the separate total is 4 176 000 000; merged, it is 4 112 000 000.