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

Around 2.5 mm if the screw threads directly into the plastic — that is roughly the core diameter of an M3. If you are fitting a brass heat-set insert instead, the hole is larger, about 4.0 mm for the common tapered inserts.

Why does my printed thread strip after a few uses?

The thread is held by layer adhesion, the weakest axis of an FDM part. Each assembly cycle shaves a little material away, so the joint usually survives the first one or two and fails on the third or fourth.

Can I just print the thread instead of cutting it?

You can, and for large coarse threads it works. For small fasteners the printed thread has the same layer-adhesion problem plus the resolution limits of the nozzle, so it wears faster than a tapped hole.

Screw Hole Size for 3D Printed Parts

2026-09-09 · VERTIPRESS Engineering

There are two different questions hiding behind this one, and mixing them up is why so many printed parts end up with stripped holes. Screwing into plastic and screwing into a metal insert want different hole sizes, because they fail in different ways.

Screwing directly into the print

For a self-tapping or wood screw driven straight into PLA or PETG, model the hole at roughly the screw's core diameter — the shaft measured between the threads, not across them. For a common M3 machine screw that lands near 2.5 mm; for a 3 mm self-tapper, near 2.4 mm. Too tight and the boss splits as the screw wedges the layers apart. Too loose and the thread has nothing to bite.

Print the boss with enough wall around it. Below about 1.6 mm of plastic on every side, the hoop stress from the screw finds the weakest layer line and opens it.

Why it strips anyway

A thread cut into thermoplastic is held by layer adhesion, and layer adhesion is the weakest axis of an FDM part. The first assembly usually feels fine. The third or fourth shaves the thread flat, because each cycle removes a little material and the screw seats deeper each time.

That is the real decision point. If a joint will be opened once, a direct screw is fine and costs nothing. If it will be opened repeatedly — a battery door, a serviceable enclosure, a jig that gets reconfigured — the plastic thread is a consumable, and it will fail during use rather than during assembly.

When an insert is cheaper

A brass heat-set insert moves the thread from plastic into metal. The plastic then only has to hold the insert's knurls, which it does across a much larger area and in compression rather than shear.

The trade is a slightly larger hole and one extra operation per hole. For anything that gets opened more than twice, that operation buys back more time than it costs.

Common questions

What size hole for an M3 screw in a 3D print?: Around 2.5 mm if the screw threads directly into the plastic — that is roughly the core diameter of an M3. If you are fitting a brass heat-set insert instead, the hole is larger, about 4.0 mm for the common tapered inserts.
Why does my printed thread strip after a few uses?: The thread is held by layer adhesion, the weakest axis of an FDM part. Each assembly cycle shaves a little material away, so the joint usually survives the first one or two and fails on the third or fourth.
Can I just print the thread instead of cutting it?: You can, and for large coarse threads it works. For small fasteners the printed thread has the same layer-adhesion problem plus the resolution limits of the nozzle, so it wears faster than a tapped hole.