SVG for laser cutting: what the machine actually needs

A laser follows a toolpath, not a picture. It needs geometry it can travel in one colour per operation, in real units, with no gaps that break a contour into fragments.

Three properties decide whether an SVG is ready: closed contours, a stroke or fill that the software maps to a cut or engrave operation, and a document size in millimetres.

Not for the cut itself. A laser follows the centre of the stroke, so a 0.1 mm line and a 2 mm line cut the same path — unlike a vinyl blade, where stroke width is the physical width of material removed.

Stroke width does matter for engraving and for previews: some programs rasterise thick strokes as fills, which changes an engrave into a cut. Set cut layers to a hairline stroke and keep fills for engraving.

The laser removes material — typically 0.1–0.3 mm depending on the machine, lens and material. Every cut is that much wider than the path you drew.

For decorative work nobody notices. For parts that must fit together, offset each contour by half the kerf, outward for holes and inward for outer edges, and cut a test joint on scrap of the same material.

An SVG with only a viewBox has no physical size. Laser software then applies its own assumption — 96 dpi in most tools — and a part designed at 100 mm opens at 26 mm or 378 mm.

Two reliable fixes: export the SVG with an explicit width and height in millimetres, or convert to DXF with a declared width in millimetres. Never trust "it looks right on screen".

Both work; they fail differently.

·SVG keeps curves as curves and carries fills, colours and layers. Good for LightBurn and Glowforge.

·DXF is geometry only, no curves in the R12 flavour, but every laser program opens it and units are explicit.

If a file is going to another person with an unknown machine, DXF in millimetres is the safer handoff. If you are cutting it yourself in LightBurn, SVG is more convenient.

·Open the file and check the reported size in millimetres against your design.

·Run the cut on cardboard or scrap of the same thickness at low power.

·Check that the smallest feature survives and that contours are continuous.

·Only then cut the real material, with air assist and the correct focus.

A trace of a photo will fail this procedure at step three every time; a silhouette or a generated flat artwork will pass.