Your DXF imports at the wrong size? It is almost always the units.

You export a part that should be 100 mm wide. You import it into LightBurn, SheetCAM or your CAM tool, and it comes in at 2540 mm. Or at 3.9 mm. The geometry is fine. What broke is the unit declaration, and the exact scale factor tells you what happened.

Quick answer: a DXF file stores plain numbers with no physical unit attached. A header field called $INSUNITS is supposed to say what those numbers mean, but many exporters write nothing there, or write 0, which means "unitless". The importing program then has to guess. If it guesses inches for a millimetre file, everything comes in 25.4 times too big. The fix: set the import unit in your cutting software to match the file, or repair the file so it declares its unit explicitly.

Why a CAD format has no units

DXF dates back to 1982. Coordinates in it are bare numbers: a line from 0 to 100 is just that, one hundred of something. The header variable $INSUNITS (1 = inches, 4 = millimetres) was added much later and is optional. The old R12 dialect, which is still the default export in many tools because everything can read it, cannot store a unit at all.

So every DXF import is a small act of trust. If the file declares a unit, good software uses it. If it declares nothing, the importer falls back to a setting or a default, and that default may not match what the file was drawn in. Nothing is "corrupt" here. Two programs simply disagree about what the numbers mean.

The scale factor tells you the cause

Measure something in the imported file that you know the real size of, then divide. The factor is a fingerprint:

Imported result	Factor	What happened
Far too big	25.4	Numbers are millimetres, but they were read as inches.
Far too small	25.4	Numbers are inches, but they were read as millimetres.
Too small or too big	10	Centimetres vs. millimetres. Fusion 360's cloud export writes centimetres.
Too small	3.78	SVG pixels (96 per inch) carried into a DXF as if they were millimetres.
Too small	3.54	Old Inkscape DXF exporter (90 dpi era). Same story, different ratio.

If your factor is none of these, check whether the drawing was scaled in the source program itself. A factor like 2 or 5 is usually human, not a unit mismatch.

Fix it at import time

LightBurn

Open the settings (gear icon) and look for the file import settings. There you choose which unit DXF files are assumed to be in. Newer versions can auto-detect the unit, but that only works when the file actually declares one. If your supplier's files are always inches, set inches there and the problem disappears for good.

Fusion 360

How you export decides the unit. Right-click the sketch in the browser tree and choose "Save As DXF": that uses your document units, typically millimetres. The export from the Fusion web hub writes centimetres instead, which is where most 10x surprises come from. Export from the sketch, not from the hub.

Inkscape

In the DXF export dialog, set the base unit to mm (or whatever your machine software expects). While you are there: current Inkscape versions behave well, but files from Inkscape 0.91 and earlier carry the old 90 dpi scale and land at factor 3.54.

Any tool: measure and scale

If you cannot change the import settings, scale the geometry after import. Use the exact factor, not an eyeballed one: 25.4, 10, 3.7795 or 3.5433. A part scaled by "roughly 25" will be wrong by more than your kerf.

Fix the file, not the symptom

Import settings fix your machine. They do not fix the file, and the next program (or the next person you send it to) starts guessing again. The durable fix is a file that declares its unit: re-export from your CAD in a dialect newer than R12 with the unit set, and check that the export dialog actually has a unit field.

Check the file in seconds: drop your DXF into DXF Medic and it shows you which unit the file declares. If it declares none, you get a clear warning plus the unit it assumed, and you can override it and export a file that states its unit explicitly. That diagnosis is free and unlimited, and the file never leaves your browser.

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