Clearance & Fit: The Four Numbers That Make Printed Parts Fit Together

Lids that jam, hinges that fuse, dowels that rattle — almost every “my parts don't fit” problem comes down to one missing habit: designing a deliberate clearance between mating surfaces. Learn four names and four numbers — the fit ladder — and your parts start fitting on the first try.

What clearance actually means

Clearance is the gap you deliberately leave between two surfaces that face each other. Model a 10 mm pin and a 10 mm hole, and in CAD they mate perfectly — on a printer they jam, because a well-tuned FDM machine still varies by roughly ±0.1–0.2 mm in XY, holes print 0.1–0.3 mm undersized (the nozzle rounds inside corners on segmented arcs), and the seam adds a small bump on every perimeter. The gap has to absorb all of that and leave room for the fit you want.

Every number in this guide is the gap per side — per pair of facing surfaces. For a pin in a hole that means the hole diameter grows by twice the clearance:

The fit ladder: press → snug → free → loose

Mechanical engineers have a whole ISO system for this (ISO 286 — H7/g6 and friends), but on an FDM printer four fits cover practically everything you will ever design. The numbers form a ladder that is easy to remember: 0.1 → 0.2 → 0.3 → 0.4.

The nozzle rule is the fastest way to remember the middle of the ladder: with a standard 0.4 mm nozzle, half a nozzle width (0.2 mm) gives a snug slide, a full nozzle width (0.4 mm) gives a loose fit. Bigger nozzle, bigger clearances — the rule scales with it.

Try it: the clearance visualizer

Drag the slider and watch the gap between a 10 mm pin and its hole — drawn to scale on the left, zoomed 10× on the right so you can actually see what 0.1 mm means.

Find YOUR printer's numbers: the 15-minute test coin

The ladder is the map — your printer is the terrain. Two printers with the same slicer profile can easily differ by 0.1 mm, which is a whole rung. So calibrate the ladder to your machine once:

1. Model a gauge: one plate with a 10 × 15 mm pin standing on it, and five holes: Ø 10.2, 10.4, 10.6, 10.8 and 11.0 mm (= 0.1–0.5 mm clearance per side). Mark each hole with embossed text. Any CAD tool can do it in ten minutes — or search for “tolerance test” on a model site.
2. Print it flat in the filament you actually build with, using your normal profile.
3. Test the pin in each hole and write down what you feel: first hole it presses into = your press fit; first it slides with friction = snug; first it moves freely = free; first with visible play = loose.
4. Keep the coin. Re-print it when you change nozzle, filament brand or printer — it is the cheapest insurance in 3D printing.

If your measured ladder sits higher than 0.1/0.2/0.3/0.4, your printer over-extrudes slightly or the holes shrink more than average — run a flow ratio calibration first, then re-test. A well-calibrated machine lands close to the standard rungs.

Adjustments the cheat sheet can't skip

• Material: ABS and ASA shrink noticeably as they cool — add ~0.1 mm to every rung. PETG tends to ooze a little fatter — add ~0.05 mm. TPU squishes, so press fits can go 0.05 mm tighter. PLA is the reference: use the ladder as-is.
• Vertical holes print small: add ~0.2 mm to the diameter of any hole printed lying down (axis pointing up), on top of the fit clearance. Slicer arc segmentation and perimeter squeeze both steal from the inside.
• The seam eats clearance: the Z-seam is a small bump on every perimeter. For rotating or sliding parts, put the seam on a non-mating face (paint it in the slicer) or use a scarf seam — otherwise budget an extra 0.05–0.1 mm.
• Elephant foot ruins the first 0.3 mm: the bottom edge flares outward. Chamfer every bottom edge of a mating surface 0.3–0.5 mm × 45°, or your measured clearance disappears where parts meet the plate. (Deep-dive: fixing elephant foot.)
• Printed threads live on the top rung: model threads coarse (M8 and up, or custom trapezoid) with 0.4–0.6 mm clearance. Smaller than M6, skip printing threads — use a heat-set insert or a nut pocket instead.
• Chamfer the entry: a 0.5 mm chamfer on the leading edge of any press or snug fit makes parts self-align instead of shaving each other on the way in.

Download the cheat sheet

One A4 page with the ladder, the nozzle rule, the hole formula and the material adjustments — print it and hang it next to the printer.

Download the cheat sheet (PDF)