Two dimensions sitting on top of one another are unreadable at plot size but look perfectly fine on screen at working zoom. They routinely survive all the way to the printed sheet, where the contractor cannot read either figure. DIMCLASH finds them before that happens.
What DimClash does
This is a pre-issue QA check. Press Enter at the prompt and the whole drawing is checked — every layout and every block definition, so dimensions buried inside blocks are found too. Or select specific dimensions to check just those.
Offending dimensions are moved to a DIMOVERLAP layer, created red and continuous, so they are immediately obvious.
Nothing is deleted and nothing is moved in space. Only the layer changes, so a single UNDO reverses the whole check — you can run it on a drawing you are about to issue without any risk of leaving it altered.
How to use it
- Load the routine with
APPLOAD, or put it on your support file search path. - Type
DIMCLASH. - Press Enter to check the whole drawing, including every layout and block definition, or select specific dimensions.
- Look at what landed on the red
DIMOVERLAPlayer and fix it. - Undo the check when you are done, or delete the layer.
How it works
Each dimension is reduced to the line its dimension text sits on — not its extension lines, and not the points it measures. That line is derived from the dimension line point (DXF 10) and the definition points (13 and 14) by projecting perpendicular from the measured run.
That choice is what makes the check useful. Extension lines cross each other constantly on a normal drawing and mean nothing; it is the text colliding that makes a sheet unreadable.
Two dimensions clash when those lines lie along one another and their extents overlap. Both the case of shared endpoints and the case of one line running through another are tested, which is what lets it catch dimensions overlapping from either side rather than only the straightforward head-on case.
Notes and limits
- Linear and aligned dimensions only. Radial, angular and ordinate dimensions have no comparable dimension line and are excluded by the selection filter.
- It only changes layers. Geometry, positions and values are untouched, and one undo puts everything back.
- Press Enter to check everything, including inside block definitions — which is where the overlaps nobody has noticed usually are.
- Remember to clear the layer before issuing, or the red dimensions plot red.
Download and details
| Name | DimClash — find dimensions that overlap and flag them on their own layer |
| Download | DimClash.lsp — 16.4 KB, plain AutoLISP source. Downloads directly; no zip to unpack. |
| Type | AutoCAD AutoLISP routine |
| Command | DIMCLASH |
| Requires | the ActiveX layer (loaded automatically by the routine with vl-load-com). |
| Source size | 16.4 KB, 401 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
- DIMTATTLE LISP — the other pre-issue dimension check: overridden text
- Dimensioning to metric standards — the conventions a checked drawing should meet
- More free AutoLISP routines on this site
- Wisey’s Steel Shapes — free structural steel section drawing program