Convert SVG to STL

Only a closed shape can be given a thickness

This is the one rule that decides whether your file will work. A shape has to be a complete loop before it encloses anything, and only something that encloses an area can be pushed out into a solid. A drawn line has no width in three dimensions. Neither does an arc, a sketch stroke, or an outline whose two ends stop a hair apart.

So closed paths become solid shapes, and open ones are left out. If any are left out you are told how many, right above the settings, before you download anything. If your whole drawing is open paths you get a message saying so rather than an empty file.

The fix is the same in every program. In Inkscape, select everything and use Path → Union (Ctrl+Shift+Plus) or close the paths with Ctrl+Shift+J. In Illustrator, Object → Path → Join. If you traced a bitmap, the trace usually produced closed paths already.

Holes stay holes

The middle of a letter O, the bore of a washer, the windows in a stencil: these are all shapes drawn inside other shapes, and a converter that fills them in has ruined the part. This one works out which outlines sit inside which, across the whole drawing at once, and turns the inner ones into holes.

It counts levels rather than trusting the file. A shape inside a hole becomes solid again — the dot inside a letter i, an island in the middle of a lake. That is what your eye expects, and it is what you get.

It also does not matter how the drawing was built. A letter O drawn as one path with two loops and a washer drawn as two separate circles both come out with a hole in the middle, because the nesting is what decides, not which element each ring belongs to.

How big it comes out

An SVG usually states its real size in its header — width="100mm" together with a viewBox — and when it does, that is the size you get. A 100 mm drawing becomes a 100 mm part. Most files out of Inkscape, Illustrator and Affinity Designer are like this.

Some are not. A file with no stated size is read at 96 pixels to the inch, which is the size a web browser would draw it, and a note appears above the settings saying so. Use the scale box to fix it, and watch the measurements beside the 3D view until they say what you wanted.

The thickness is separate and is always in millimetres. Three is a good starting point: thick enough for a keyring or a sign to survive being handled, thin enough to print in under an hour.

Text has to become outlines first

A <text> element in an SVG is not a shape. It is an instruction to draw some letters in a named font, and the font is on the machine that made the file, not in the file. There is nothing there to extrude.

Convert the text to paths before exporting: Path → Object to Path in Inkscape, Type → Create Outlines in Illustrator. The letters then become real outlines and convert like anything else. If you drop a file with live text in it, you are told that it was skipped rather than left to wonder where the words went.