Guide · Screws

Screw hole sizes for 3D printed parts (M2–M8)

Getting screw holes right is half of every functional print. Use the tables below as starting values, then add your printer's fit gap (printed holes come out undersized).

M3 clearance: 3.4 mm
M3 self-tapping in plastic: ≈ 2.6–2.8 mm
M3 heat-set insert: ≈ 4.0–4.2 mm (check the insert's datasheet)
M3 nut: 5.5 mm across flats, 2.4 mm thick

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Clearance holes and nuts (ISO metric)

Clearance = medium series (ISO 273). Add about half your fit gap for printed holes. Nut sizes per ISO 4032.
Thread	Clearance hole	Self-tap in plastic	Nut across flats	Nut thickness	Countersink head Ø
M2	2.4 mm	1.7–1.8 mm	4.0 mm	1.6 mm	≈ 4.4 mm
M2.5	2.9 mm	2.1–2.2 mm	5.0 mm	2.0 mm	≈ 5.5 mm
M3	3.4 mm	2.6–2.8 mm	5.5 mm	2.4 mm	≈ 6.7 mm
M4	4.5 mm	3.5–3.7 mm	7.0 mm	3.2 mm	≈ 9.0 mm
M5	5.5 mm	4.4–4.6 mm	8.0 mm	4.7 mm	≈ 11.2 mm
M6	6.6 mm	5.3–5.5 mm	10.0 mm	5.2 mm	≈ 13.4 mm
M8	9.0 mm	7.1–7.3 mm	13.0 mm	6.8 mm	≈ 17.9 mm

Practical rules

Nut traps: across-flats + 0.2–0.3 mm, and remember a hexagon pocket's corner-to-corner size is across-flats ÷ cos 30°.
Horizontal holes above ~8 mm: use a teardrop shape so they print without supports.
Countersinks print best on the top surface; on the bed side, use a counterbore or sacrificial bridge layer.
Heat-set inserts need about 1.5–2 mm of wall around them.
Wood screws for wall mounting: through hole ≈ screw Ø + 0.5 mm, 90° countersink of about 2 × screw Ø.

Frequently asked questions

What size hole for an M3 screw in a 3D print?

3.4 mm for a clearance hole (plus a little extra for printing), about 2.6–2.8 mm if the screw should cut its own thread, and about 4.0–4.2 mm for a typical M3 heat-set insert.

How do I make a nut trap?

Make a hexagonal pocket with the nut's across-flats size plus 0.2–0.3 mm and the nut's thickness plus 0.4 mm, aligned with the screw hole.

Updated 2026-10-18 · Generated parts are a starting point — always test-fit before relying on a part.