Guides · Reviewed 25 September 2026

How to prepare an STL for 3D printing

A print-ready STL is the right size in millimetres, watertight (no holes in the mesh), and has walls thick enough to print. Get those right and most models print cleanly. Our quote tool checks your file as you upload it and flags common problems, but here's what to look for first.

1. Size and units

STL files don't store units, so export in millimetres and double-check the overall dimensions in your CAD tool before exporting. A model that should be 50 mm but was exported in inches or centimetres will come out the wrong size. When you upload, the quote card shows the measured dimensions — if they look wrong, re-export at the correct scale. A single part must fit our 250 × 250 × 250 mm build volume.

2. Watertight (manifold) geometry

The mesh should be a single closed surface with no holes, gaps or stray faces — “watertight” or “manifold”. Non-watertight meshes can confuse the slicer. The good news: we can usually repair minor issues automatically at no charge, and the quote page tells you if yours needs it. For bigger problems, most CAD and mesh tools have a “repair” or “make solid” function.

3. Wall thickness

Very thin walls may not print reliably. As a rule of thumb, keep walls at least around 1 mm (thicker for parts that need to be sturdy). Text, fine ribs and small pins are the usual casualties of walls that are too thin.

4. Holes, fits and tolerances

FDM printing has real-world tolerances, so holes and mating parts print slightly tighter or looser than the exact CAD dimension. If a part needs to fit a bolt, shaft or another component, add a little clearance (often a few tenths of a millimetre) rather than modelling an exact fit. For anything critical, a quick test print first is worth it.

5. Orientation — leave it to us

You don't need to worry about how the part sits on the print bed. We choose the print orientation and add any supports as part of preparing your order, based on strength, finish and reliability. Just send the model the right way up in your own CAD sense; the on-screen 3D preview is there for you to inspect the shape, not to set how it prints.

Related: STL vs 3MF · Where to find 3D models