SECTPROPS reports the full set of section properties an engineer needs for a built-up or cut-out cross section: area and centroid; Ix, Iy and Ixy about the origin and about the centroid; principal moments I1 and I2 with the angle of the principal axes; radii of gyration; and section moduli to the extreme fibres.
What SectionProps does
Holes are handled. Select the outer boundary and every hole together and the holes are subtracted automatically.
How to use it
- Load the routine with
APPLOAD, or put it on your support file search path. - Draw the section as closed curves — outline and holes.
- Type
SECTPROPS. - Select the outline and the holes together.
- Read the report.
How it works
Every selected closed curve is turned into a region. Regions are exact area primitives — AutoCAD computes their properties analytically, so a curved boundary is exact rather than approximated by short chords.
The region with the largest area is taken as the outer boundary and every other is subtracted from it. That single rule handles the normal case — one outline, any number of holes inside it — without asking you to nominate which is which.
Area, centroid, moment of inertia and product of inertia are then read straight off the resulting region, and the centroidal values derived with the parallel axis theorem:
Ix about centroid = Ix about origin - A × ŷ²
Iy about centroid = Iy about origin - A × x̄²
Ixy about centroid = Ixy about origin - A × x̄ × ŷ
with the principal values from the standard Mohr’s circle relations:
I1, I2 = (Ix+Iy)/2 ± √( ((Ix-Iy)/2)² + Ixy² )
angle = ½ atan2( -2Ixy, Ix-Iy )
atan2 rather than atan, so the angle lands in the correct quadrant — and because when Ix equals Iy, which happens on any square or circular section, a plain division divides by zero.
The temporary regions are erased. Your geometry is never modified and nothing is left behind, whether the routine finishes or errors out.
Notes and limits
- Closed curves only — closed polylines, circles, ellipses, closed splines. Open curves are ignored and the count of skipped objects is reported.
- The largest area is the outline. That rule is what makes hole subtraction automatic; a section where it does not hold needs separating first.
- Your geometry is not modified.
- Curved boundaries are exact, because regions are computed analytically.
Download and details
| Name | SectionProps — full section properties for a closed shape with holes |
| Download | SectionProps.lsp — 15.9 KB, plain AutoLISP source. Downloads directly; no zip to unpack. |
| Type | AutoCAD AutoLISP routine |
| Command | SECTPROPS |
| Requires | the ActiveX layer (loaded automatically by the routine with vl-load-com). |
| Source size | 15.9 KB, 345 lines |
| Error handling | Yes — a *error* handler restores every system variable it changed, on cancel as well as on error. |
| Undo | The whole operation is wrapped in a single undo group, so one U reverses all of it. |
| Compatibility | AutoCAD (any release with AutoLISP), BricsCAD, ZWCAD, GstarCAD, ProgeCAD and other IntelliCAD-based platforms. AutoCAD LT needs a LISP enabler. No .NET, no ObjectARX, no installer. |
| Author | YZ, August 2026 |
| Licence | Free to use, supplied “as is” with no warranty. |
References
- PARTWEIGHT LISP — weight rather than section properties, holes subtracted the same way
- Wisey’s Steel Shapes — standard sections drawn to published dimensions
- SUMAREA LISP — a live area total with subtraction
- More free AutoLISP routines on this site
- Wisey’s Steel Shapes — free structural steel section drawing program