How do I save from Inkscape so my design imports at the right size with nothing missing?

Why sizes and shapes change

An SVG stores its real size in the root width and height (for example in inches or millimeters) together with a viewBox. Everything inside the file is in unitless user units. The Inkscape wiki explains that a pixel is fixed at 96 per inch, and that Inkscape 0.91 and earlier used 90 per inch. When a file was made with one assumption and read with the other, the drawing can come in about 6 to 7 percent off. The software's own manual says imported vectors are read in at the size they were created, so the size has to be right in the source file. A file drawn in pixels with no real units has no dependable physical size at all.

Text and special shapes are the other risk. Inkscape keeps extra information for its own tools (stars, spirals, layers) in its own namespaces, and live text depends on the font.

Steps

1. Open File > Document Properties and set the units to inches or millimeters, with the display units and the document units matching. Draw at the real finished size.
2. Select all text and shapes and choose Path > Object to Path (Shift+Ctrl+C). Text becomes a group with one path per letter and the font can no longer be changed, so keep an editable copy.
3. If a line must carve at its drawn width, choose Path > Stroke to Path (Ctrl+Alt+C). The stroke becomes a filled outline made of two parallel edges. The line must have a width above zero.
4. Choose File > Save As and pick Plain SVG. The Inkscape wiki describes Inkscape SVG as plain SVG plus extra Inkscape-only data. Plain SVG runs the drawing through the exporter and leaves that data out, which is why other makers of cutting tools recommend it.
5. Import the file, then measure one known dimension before toolpathing.

If the size is still off

Select everything and correct it with Set Size using the known dimension (see the entry on scaling a drawing to a real measurement). Then fix the source file so the next export is right.

Pro tips

• Files made in very old Inkscape versions carry the 90 per inch assumption. Open and resave them in a current version with real units set.
• Flowing curves survive SVG. The official FAQ notes that DXF holds only arcs and straight lines, so do not pick DXF for script lettering.
• Tracing a picture inside Inkscape is a separate job (see the entry on bitmap tracing).