Thread & Tap Size Calculator

Metric Thread Quick Reference

Size	Pitch	Tap Drill	Close	Normal	Loose	Insert
M2	0.4	1.6	2.2	2.4	2.6	3.2
M2.5	0.45	2.05	2.7	2.9	3.1	4
M3	0.5	2.5	3.2	3.4	3.6	4
M4	0.7	3.3	4.3	4.5	4.8	5.6
M5	0.8	4.2	5.3	5.5	5.8	6.4
M6	1	5	6.4	6.6	7	8
M8	1.25	6.8	8.4	9	10	9.7
M10	1.5	8.5	10.5	11	12	12
M12	1.75	10.2	13	13.5	14.5	14

Tap drill and clearance holes per ISO 261 / ISO 273. All values in millimeters. Heat-set insert pilot holes based on Ruthex drill set (M2-M6) and CNC Kitchen recommendations (M8-M12); check your insert's datasheet for exact values.

Socket Head & Nut Dimensions

Size	Head ∅	Head Height	Hex Key	Nut Flats	Nut Height
M2	3.8	2	1.5	4	1.6
M2.5	4.5	2.5	2	5	2
M3	5.5	3	2.5	5.5	2.4
M4	7	4	3	7	3.2
M5	8.5	5	4	8	4.7
M6	10	6	5	10	5.2
M8	13	6.8	6	13	6.8
M10	16	8.4	8	16	8.4
M12	18	10	10	18	10.8

Socket head cap screw dimensions per DIN 912. Hex nut dimensions per ISO 4032. When modeling counterbores or nut traps in your CAD, add 0.3-0.5 mm clearance to the head diameter and nut width for FDM parts.

Which approach should you use?

There are four common ways to add screw holes to a 3D printed part. Each has trade-offs in strength, reusability, and effort. Here is when I reach for each one.

Clearance hole + nut

The easiest and most common option. The screw passes through a clearance hole in one part and threads into a nut (or nut trap) on the other side. Works every time, no special tools needed, and the connection can be taken apart and reassembled without wearing out. Use this as your default for enclosures, mounts, and general assembly.

Heat-set inserts

A brass insert gets pushed into a pilot hole with a soldering iron (or a dedicated insert tip for cleaner results). The knurled outside melts into the plastic and locks in place, giving you proper metal threads. The connection handles repeated assembly/disassembly cycles without stripping. Worth the extra cost for anything that gets opened regularly (electronics housings, jigs, product prototypes).

Tapping (cutting threads)

Print a hole at the tap drill diameter, then cut threads with a hand tap. This works better than most people expect, especially in PLA and PETG at M3 and above. The threads can strip under high torque or repeated use, so it's best for low-stress fastening or one-off fixes where you don't have inserts on hand.

Printed threads

You can model threads directly in your CAD file, but this only works well for coarse threads (M6+) with fine layer heights (0.12-0.20 mm). Good for bottle caps, decorative lids, or non-structural threaded connections. Below M6, the thread profile is too small for FDM to resolve cleanly.

Tips for screw holes in 3D prints

Print a test block

Before printing your full part, print a small test block with the hole size you plan to use. Try the screw or insert in it. A 10-minute test print can save you hours of reprinting. Every printer and filament combo produces slightly different hole sizes.

Orient holes vertically

Holes printed along the Z axis (standing upright) come out rounder and more accurate than holes printed on their side. Horizontal holes suffer from layer stepping on the overhanging top portion, which makes them slightly oval. If your design allows it, orient the part so screw holes face up.

Minimum wall thickness

Keep at least 1× the screw diameter of wall material around a screw hole. For heat-set inserts, aim for a wall thickness of at least 2× the insert's outer diameter. Thin walls crack when you tighten screws or push inserts in. This is easy to overlook in compact designs.

Counterbore depth

For flush-mount socket head cap screws, the counterbore depth should equal the head height plus 0.5 mm. The extra half millimeter ensures the head sits fully below the surface even with minor layer inconsistencies. Add 0.3-0.5 mm clearance to the counterbore diameter too.

Thread Size

Hole Type

FDM Print Compensation

Hole Size