Why DXF files often don't say what units they use
Here's the uncomfortable truth about DXF: in many files, the numbers have no units attached. A line from 0 to 100 is just… a line from 0 to 100. One hundred what? Newer DXF versions have a header field that can declare units — but plenty of exporters never fill it in, and the old R12 format — often preferred when maximum machine and software compatibility is the priority — has no dependable units field at all.
So every program that opens the file has to make an assumption. Your CAD app assumed millimetres when it wrote the file; LightBurn assumed inches when it read it — or the other way round — and your part just changed size by a factor of exactly 25.4. The file didn't break. Two programs disagreed about what a number means.
Read the scale factor: the diagnosis table
A units bug is rarely random — the error is systematic, so the ratio carries information. Divide the imported size by the size you expected, and the result usually gives you a strong clue to the culprit:
| Imported result | Imported ÷ expected | To correct | Likely cause |
|---|---|---|---|
| 25.4× too small (100 mm → 3.937) | ≈ 0.0394 | Multiply by 25.4 | Inch coordinates were read as millimetres. Set the import units to inches for this file — or better, re-export in millimetres. |
| 25.4× too big (100 mm → 2,540) | 25.4 | Divide by 25.4 | Millimetre coordinates were read as inches. Set LightBurn's DXF units to millimetres. |
| 10× off (either way) | 10 or 0.1 | ÷ or × 10 | Often a centimetre/millimetre mismatch — but round powers of ten can also come from manual scaling or a drawing/export scale setting, so check both. |
| 1,000× off (either way) | 1,000 or 0.001 | ÷ or × 1,000 | Metres vs millimetres, common in architectural exports — or, again, a scale setting. |
| ≈ 3.78× or ≈ 0.26× | 96 ÷ 25.4 | ÷ or × 3.7795 | Pixel units (96 per inch) were read as millimetres — more common in SVG and graphics-app workflows than in proper CAD exports. |
| ≈ 2.83× or ≈ 0.35× | 72 ÷ 25.4 | ÷ or × 2.8346 | Point units (72 per inch) were read as millimetres — same graphics-app territory. |
One caution: don't “fix” a units problem by eyeballing a scale-up. If you rescale by 25 instead of 25.4, every hole in the part is 1.6% undersized — enough to ruin press-fits and joints. Use the exact factor, or better, re-import correctly.
The permanent fix, in five steps
Step 1 — Measure the damage. Select everything (Ctrl+A) and read the width in LightBurn's toolbar. Imported ÷ expected = your ratio; the table above usually points to the cause.
Step 2 — Check LightBurn's DXF import settings. Open LightBurn's Settings (the gear icon) and go to the import/export section. Enable Auto-detect units if possible, then set the fallback DXF unit to the unit your files' coordinate values actually represent — for most of us, millimetres. The fallback is what LightBurn uses when a file carries no usable unit information.
Step 3 — Re-import the original file. Re-importing with the right unit interpretation is the cleanest route, because it removes all uncertainty. If re-importing or re-exporting isn't possible, exact uniform scaling is valid too — use the precise factor (25.4, not 25), lock the aspect ratio, select the entire design, and verify a known dimension afterward.
Step 4 — Verify against a known dimension. Pick one feature you know cold — a 5 mm hole, a 100 mm edge — and confirm it. Ten seconds. It has saved me more material than any other habit I own.
Step 5 — Fix it at the source. In your design app's DXF export dialog, set the units to millimetres explicitly, every time. A matched pair — export in mm, import as mm — ends this bug for the rest of your life.
Per-app export notes
Fusion: right-click the sketch and choose Export DXF; in the export dialog, explicitly select millimetres as the output units and confirm the geometry being exported — don't rely on the document's default units alone.
Inkscape: set the document and page units to millimetres, and pick the right base unit in the DXF export options where offered. Inkscape's native environment is screen-based, so always verify one known physical dimension after export — changing only the visible ruler units doesn't guarantee a correctly scaled DXF.
Illustrator: set document units to millimetres for convenient design work, but the setting that matters is the DXF/DWG export dialog's Scale Units — it controls how lengths are written into the file. Re-import and verify one known dimension.
LibreCAD / QCAD: set the drawing unit in the drawing preferences before creating the geometry. When exporting to a unit-ambiguous format such as R12, make sure the receiving program assumes the same unit — and verify a known dimension.
And if a file reaches you from someone else — a customer, a forum, a marketplace — you don't control how it was exported. That's where DXF Studio's Cleanup & Repair helps: it reads any unit metadata the file carries, shows you what it detected, and lets you confirm or override the source units before converting — then you inspect the corrected result, with its measurements, before paying anything. (No software can divine intent from a truly unitless file; that's why it shows you what it found and asks.)
Does a DXF file store what units it was drawn in?
Not reliably. Newer DXF versions have a units header, but many exporters leave it unset, and the widely-used R12 format has no dependable units field at all. The numbers in the file are just numbers — the importing program has to assume what they mean.
What does the scale factor tell me?
It usually gives a strong clue. A ratio of 25.4 in either direction suggests a millimetres/inches mix-up; 10x suggests centimetres or a scale setting; 1,000x suggests metres vs millimetres. Ratios near 3.78 or 2.83 suggest pixel (96 per inch) or point (72 per inch) units read as millimetres — more common in graphics-app workflows. Round powers of ten can also come from manual scaling, so verify before correcting.
Should I just scale the shape up in LightBurn?
As a one-off rescue, fine: select everything, lock the aspect ratio, and type the correct width. But it's a patch, not a fix — set LightBurn's DXF import units and export in explicit millimetres so every future file lands right.
Which units should I export from Fusion 360 for laser work?
Millimetres, explicitly, in the export dialog — and keep your LightBurn DXF import units set to millimetres to match. One matched pair of settings ends the problem for good.
Why does the same DXF open at different sizes in different programs?
Because each program makes its own assumption when the file doesn't declare units. One assumes millimetres, another inches, a third reads a units header the others ignore. The file isn't changing — the assumptions are.