What Units Should a DXF File Use for Laser Cutting?
A DXF for laser cutting should use millimetres, with the header variable INSUNITS set to 4, and be drawn at real size in 2D. A DXF does not carry units in the geometry itself, so a file drawn in inches or without a unit flag can arrive 25.4 times too large or too small.
The file says 4 and you wanted 100. You import a DXF, the preview shows a dot the size of a grain of rice, and you spend ten minutes looking for the zoom. The part is not tiny; it was drawn in inches and read as millimetres, or the other way round. That is the whole story of DXF units.
A DXF is a CAD interchange file, and its coordinates are plain numbers. Number 100 means nothing until a unit is attached, and the unit lives in the file header as a variable named INSUNITS (and sometimes MEASUREMENT).
The unit codes you will meet
| INSUNITS value | Unit | Notes |
|---|---|---|
| 0 | Unitless | Software guesses; frequent source of errors |
| 1 | Inches | Common for US CAD |
| 2 | Feet | Rare in laser work |
| 4 | Millimetres | Recommended for laser |
| 5 | Centimetres | Occasionally used |
| 6 | Metres | Architecture files |
If INSUNITS is 0 or missing, laser software has to guess, often millimetres, but some programs show a prompt or use a default set in preferences. Never rely on this; check by measuring.
Why millimetres is the best choice
- Nearly all laser software, material thicknesses and kerf values are discussed in millimetres.
- Our files are in millimetres, so the numbers on screen match the numbers on the parts: 90 × 124 mm for the reindeer, a 4 mm hanging hole.
- Fewer conversions means fewer mistakes: the error between inches and mm is exactly 25.4.
How wrong units show up
| What happened | Symptom | Factor |
|---|---|---|
| Drawn in inches, read as mm | Part about 25 times too small | 1 ÷ 25.4 = 0.039 |
| Drawn in mm, read as inches | Part 25.4 times too big | 25.4 |
| Drawn in cm, read as mm | 10 times too small | 10 |
| Drawn in mm, read as cm | 10 times too big | 0.1 |
| Unitless, read as the default | Anything | Check the known size |
A quick checklist for exporting a DXF for the laser
- Set the drawing units to millimetres in your CAD or design program.
- Draw at 1:1, real size. Do not use paper space scale tricks.
- Keep the drawing 2D, with all entities on Z = 0.
- Put cut paths on a layer named CUT and engraving on ENGRAVE, as our files do.
- Export as an ASCII DXF in a common version such as R12 or 2000, because newer binary or exotic versions are a frequent source of import trouble.
- Remove text, dimensions and hatches or convert them to outlines.
Pro tip: draw a 100 mm square at the corner of the drawing. After import it tells you the unit situation in one glance.
Worked example
Our reindeer ornament is 90 × 124 mm. Suppose you import a DXF of it and LightBurn reports 3.54 × 4.88 mm. That is 90 ÷ 25.4 and 124 ÷ 25.4: the file was drawn in inches (3.54 in) but read as millimetres, so scale by 25.4 and you are back at 90 × 124 mm. If instead it reports 2286 × 3150 mm, the file was drawn in mm but read as inches, so divide by 25.4. The tell is that dividing or multiplying by exactly 25.4 gives round, expected numbers. If no clean factor appears, the problem is not units; look for a stray object far from the part.
Common mistake: scaling the imported part by eye until it fits the sheet. If a unit mismatch is the true cause, your arcs, holes and slots all end up at slightly wrong sizes and nothing mates later. Find the exact factor and apply it.
Related file-format notes
SVG files handle size differently, through width, height and DPI; see why an SVG is the wrong size in LightBurn. For a comparison of both formats, read SVG vs DXF for laser cutting. If gaps in outlines are the problem after importing, read fixing open contours in DXF files. The box generator exports both SVG and DXF in millimetres.
Questions and answers
What unit is best for a laser DXF?
Millimetres. It matches how materials, kerf and laser settings are described and avoids the 25.4 times conversion error with inches.
What is INSUNITS in a DXF?
A header variable that tells software what unit the drawing uses. A value of 4 means millimetres and 1 means inches. Zero means unitless.
Why did my DXF import tiny or huge?
Because the unit flag is missing or different from what the software assumed. The error is usually 25.4, 10 or 1,000 times.
Does the laser software read the unit flag?
Many programs do, but some ignore it or default to a setting in preferences. Always measure a known feature after import.
Can I fix a DXF unit error by scaling?
Yes. Scale by the exact conversion factor, 25.4 for inches and mm, instead of dragging by eye.
Should I export 3D DXF files for a laser?
No. Use 2D geometry only. 3D solids and surfaces are usually ignored or imported incorrectly.