# A Pre-Print Checklist for 3D Models > Nine checks that catch most failed prints before they start: scale, fit, watertightness, wall thickness, overhangs, orientation, clearances, small features and material estimate. Source: https://freestlviewer.com/blog/pre-print-checklist/ Published: August 5, 2026 · Updated: August 5, 2026 Most failed prints are decided before the printer moves. These are the checks worth running on any model — your own or a download — in the order that catches problems soonest. ## 1. Scale Read the bounding box and compare it with what you expect. STL carries no unit, so a file exported from an inch-based document opens 25.4 times too small. The [unit trap](https://freestlviewer.com/blog/stl-units-mm-vs-inches/) has a table of the exact ratios and what each one means. ## 2. Fit on the build plate Compare the bounding box against your printer's build volume, remembering that usable Z is often a few millimetres less than the specification. If the part does not fit, deciding *now* whether to scale it or split it is much cheaper than discovering it after slicing. ## 3. Watertightness Every edge should be shared by exactly two triangles. Holes and non-manifold edges make volume figures meaningless and can produce missing walls or unexpectedly hollow sections. Slicers repair small defects silently, so run the check yourself rather than assuming a successful slice means a clean mesh — see [watertight and manifold meshes](https://freestlviewer.com/blog/watertight-and-manifold-stl-files/). ## 4. Wall thickness Anything thinner than about two extrusion widths — roughly 0.8 mm on a standard 0.4 mm nozzle — either gets dropped by the slicer or prints as one fragile bead. This is the most common failure in models converted from decorative renders, where thin decorative surfaces have no thickness at all. The reliable test is the slicer preview: slice the model and look at a layer that cuts through the thin region. If the wall is missing there, it will be missing on the printer. ## 5. Overhangs and supports Check which surfaces slope more than about 45 degrees from vertical, and decide whether they need support or whether reorienting removes them. Bridges between two anchored points usually need nothing. [Overhang angle and supports](https://freestlviewer.com/blog/overhang-angle-and-supports/) covers where the rule holds and where it does not. ## 6. Orientation Orientation drives four things at once: overhangs, support contact on visible surfaces, print time, and strength. FDM parts are anisotropic — the bond between layers is weaker than the material within a layer — so a hook, bracket or clip should be oriented with the load running along the layers rather than pulling them apart. When those goals conflict, strength usually wins for functional parts and surface quality wins for display pieces. ## 7. Clearances For parts that mate, leave a gap. About 0.2 mm gives a snug press fit; 0.3 to 0.5 mm lets parts move freely. Holes print slightly undersized because of extrusion width and material shrinkage, so a 5 mm hole intended for a 5 mm rod generally needs to be drawn a few tenths larger. These numbers vary by printer and material — print a tolerance test once and reuse the result rather than guessing per project. ## 8. Small features Text, engraved detail and thin ribs cannot be finer than the nozzle can draw. Embossed text below roughly 0.5 mm line width, or recessed text shallower than a layer or two, disappears. Increasing resolution in the export does not help: the limit is the nozzle, not the mesh. ## 9. Material and time estimate Finally, sanity-check the cost. Mesh volume tells you the solid-material upper bound; the slicer tells you the real figure once walls and infill are set. If the slicer reports *more* material than the solid volume, something is wrong — usually supports, a brim, or a scaling mistake. See [volume and print weight](https://freestlviewer.com/blog/stl-volume-and-print-weight/) for the maths. ## Running the geometry checks quickly Items 1 through 5 are all geometry questions that can be answered before you open a slicer. [Mes3D](https://freestlviewer.com/) reports dimensions, triangle count, volume, surface area, watertightness with an open-edge count, and overhang ratio directly in the browser, with nothing uploaded. ## Frequently asked questions ### What should I check before 3D printing a model? Scale and fit on the build plate, watertightness, wall thickness against your nozzle diameter, overhangs and support needs, print orientation, clearances between mating parts, features smaller than the nozzle can resolve, and the material estimate. Most failed prints trace back to one of these rather than to the printer. ### What is the minimum wall thickness for FDM printing? Roughly two extrusion widths, so about 0.8 mm with a standard 0.4 mm nozzle. Thinner walls either get dropped by the slicer or print as a single fragile bead. Preview the sliced result rather than trusting the number, since the slicer decides what survives. ### How much clearance do I need between printed parts? About 0.2 mm for a snug press fit and 0.3 to 0.5 mm for parts that must move freely. The exact value depends on your printer, material and shrinkage, so print a small tolerance test once and reuse the result. ### Does print orientation affect strength? Significantly. FDM parts are anisotropic: layers bond to each other more weakly than the material within a layer, so a part loaded across the layer lines can be several times weaker. Orient the part so the main load runs along the layers rather than across them.