3D print thread calculator: metric thread sizes with print clearance

The formula

ISO 724 gives every basic diameter of a metric thread from its nominal diameter and pitch:

H  = 0.866025 × P
D2 = d2 = D − 0.649519 × P     pitch diameter
D1 = D − 1.082532 × P          minor diameter of the nut (flat crest)
d3 = d − 1.226869 × P          minor diameter of the bolt (rounded root)

With a radial print clearance c on the part you model:

bolt (external):  d − 2c,  d2 − 2c,  d3 − 2c     model the rod at d − 2c
nut (internal):   D + 2c,  D2 + 2c,  D1 + 2c     model the bore at D1 + 2c

• D and d are the nominal (major) diameter of the nut and the bolt, the number after the M.
• P is the pitch, the distance from one thread crest to the next.
• H is the height of the 60° triangle the profile is drawn on. The nut’s thread height is H1 = 0.541266 × P and the bolt’s is h3 = 0.613435 × P.
• c is the radial clearance this part takes: the gap between the mating threads measured along a radius, on each side of the axis. The profile moves by c, so every diameter changes by 2c. A clearance quoted on the diameter is halved first. When both parts are printed, each can take half of the total.

The two threads overlap radially by H1 minus the total gap. The calculator shows what is left, because the same gap costs a fine or small thread a bigger share of its grip.

How much clearance do 3D printed threads need?

Start with 0.1 to 0.2 mm of radial clearance per side (0.2 to 0.4 mm on the diameter). Sovol and SelfCAD both publish that range as the starting point for filament (FDM) printing. The calculator defaults to 0.2 mm per side, the top of that range. Print one short test pair, as Sovol advises, before the real part.

Sovol adds that no clearance is universal: it depends on nozzle diameter, line width and layer height. Raise the clearance if the test pair binds, and lower it if the pair wobbles.

Other published figures use other conventions, so convert them before you compare. The last column puts each one in this calculator’s terms: radial clearance per side.

Source	Published figure	What it measures	Radial, per side
Sovol (printer maker)	0.10 to 0.20 mm	Radial, per side: 0.20 to 0.40 mm on the diameter	0.10 to 0.20 mm
SelfCAD (CAD software)	0.1 to 0.2 mm	Each side: 0.2 to 0.4 mm total on the diameter	0.1 to 0.2 mm
JLC3DP (print service)	0.5 mm minimum for FDM parts that assemble, which its thread rule points to	Not stated	0.25 mm if on the diameter, 0.5 mm if per side
Fusion-360-FDM-threads (Fusion add-in)	Classes 0 to 0.5 mm in 0.025 mm steps; a 0.1 bolt in a 0.1 nut leaves a 0.2 mm gap	An offset normal to the thread faces: class 0.1 moves the major and minor diameters 0.2 mm and the pitch diameter 0.4 mm	2 × the class at the flanks, per part
ISO Metric Thread Profiles for 3D Printing (Fusion app by Arroway)	0.1 to 1.0 mm in 0.1 mm steps, with a test print file	Not stated	Not stated
BOSL2 (OpenSCAD library)	$slop, 0 by default, set from the library’s own test print	Adds 4 × $slop to the internal thread’s diameter, 2 × $slop each side	2 × $slop

M6, M8, M10 and M12 ready to model

The diameter to type in CAD at 0.1 and 0.2 mm per side, with all of the clearance on the printed part:

Thread	Bolt rod (major)	Nut bore (minor)	Overlap left
M6 × 1	5.8 / 5.6 mm	5.117 / 5.317 mm	82% / 63%
M8 × 1.25	7.8 / 7.6 mm	6.847 / 7.047 mm	85% / 70%
M10 × 1.5	9.8 / 9.6 mm	8.576 / 8.776 mm	88% / 75%
M12 × 1.75	11.8 / 11.6 mm	10.306 / 10.506 mm	89% / 79%

Worked example: a printed M8 nut for a steel bolt

1. Basic size from ISO 724: D1 = 8 − 1.082532 × 1.25 = 6.647 mm, D2 = 8 − 0.649519 × 1.25 = 7.188 mm.
2. Clearance: 0.2 mm per side, all of it on the nut, so each nut diameter grows by 0.4 mm.
3. The bore to model is 6.647 + 0.4 = 7.047 mm. The thread is cut out to 8.4 mm, with a pitch diameter of 7.588 mm.
4. Overlap left: H1 = 0.541266 × 1.25 = 0.677 mm, and 0.677 − 0.2 = 0.477 mm, 70% of the thread height.
5. Layer height: 1.25 ÷ 8 to 1.25 ÷ 6 is 0.16 to 0.21 mm.

Where the clearance goes: bolt, nut or both

• A printed nut or threaded hole on a metal bolt: the nut takes all of it. Model the bore at D1 + 2c and the thread out to D + 2c. Sovol says internal threads need a positive size adjustment, in CAD or as slicer compensation.
• A printed bolt into a metal nut or a tapped part: the bolt takes all of it. Model the rod at d − 2c.
• Both parts printed: Snapmaker keeps the internal thread at its true size and shrinks only the external one. The Fusion-360-FDM-threads add-in shrinks one and enlarges the other (0.1 mm each in its example). The total gap is what matters, so pick “each part takes half” to split it, or keep “this part takes all” and model the other part at basic size.

If your CAD thread tool only draws the standard profile, offset the thread faces instead. A 60° flank moves radially twice as far as the face offset, so offset by half the radial clearance (0.1 mm for 0.2 mm per side). The crest and root move only by that half.

What changes the clearance a thread needs

• Pitch and size. The clearance comes out of the thread height, which scales with pitch. At 0.2 mm per side, an M8 keeps 0.477 of its 0.677 mm overlap (70%), but an M3 keeps 0.071 of 0.271 mm (26%).
• Smallest printable size. JLC3DP gives M6 as the smallest thread to print. Sovol calls M6 and up, usually with a 1 mm pitch or coarser, the conservative range for a 0.4 mm nozzle. SelfCAD advises against going below M5 and says pitches under 0.5 mm are hard to print with FDM.
• Fine pitch. A fine thread has the same major diameter and a shallower profile. M8 × 1 has an H1 of 0.541 mm against 0.677 mm for M8 coarse, so 0.2 mm per side leaves it 63% of its overlap instead of 70%.
• Nozzle and layer height. Extruded plastic comes out slightly wider than the nozzle, and that squish closes small gaps (Snapmaker). Sovol keeps the layer height at no more than 1/6 to 1/8 of the pitch, and separately sets 0.12 to 0.16 mm layers for a standard 0.4 mm nozzle. SelfCAD says layers of about 0.1 mm or less capture the thread form better. The calculator shows pitch ÷ 8 to pitch ÷ 6 for your thread.
• Orientation. Sovol, SelfCAD and Snapmaker all start with the thread axis vertical, which keeps each layer round. A 60° thread printed upright has an overhang angle of 90 − 60 ÷ 2 = 60°, by the rule in the Fusion-360-FDM-threads README. The README adds that standard 60° threads print satisfactorily with a lower print speed or more cooling. Sovol notes that printing the axis horizontal makes a bolt stronger along its length but needs support under the overhang.
• The first layers. When a screw binds in the first turns of a printed hole, Sovol traces it to first-layer squish at the entry. Its fix is elephant’s foot compensation at 0.15 to 0.20 mm in the slicer, with a 45° chamfer at the hole’s mouth.

What are the pitch and diameters of metric threads?

A metric thread is named by its major diameter and pitch: M8 is 8 mm with a 1.25 mm coarse pitch, M8 × 1 a fine one. Per ISO 724, the pitch diameter is D − 0.649519 P, the nut’s minor diameter D − 1.082532 P and the bolt’s d − 1.226869 P.

Coarse pitches and tap drills come from Gühring’s ISO metric (DIN 13) chart. Diameters follow ISO 724, in mm, before any clearance.

Thread	Pitch	Pitch diameter D2	Nut minor D1	Bolt minor d3	Nut thread height H1	Tap drill
M2	0.4	1.740	1.567	1.509	0.217	1.6
M2.5	0.45	2.208	2.013	1.948	0.244	2.05
M3	0.5	2.675	2.459	2.387	0.271	2.5
M4	0.7	3.545	3.242	3.141	0.379	3.3
M5	0.8	4.480	4.134	4.019	0.433	4.2
M6	1	5.350	4.917	4.773	0.541	5.0
M8	1.25	7.188	6.647	6.466	0.677	6.8
M10	1.5	9.026	8.376	8.160	0.812	8.5
M12	1.75	10.863	10.106	9.853	0.947	10.2
M14	2	12.701	11.835	11.546	1.083	12.0
M16	2	14.701	13.835	13.546	1.083	14.0
M18	2.5	16.376	15.294	14.933	1.353	15.5
M20	2.5	18.376	17.294	16.933	1.353	17.5
M22	2.5	20.376	19.294	18.933	1.353	19.5
M24	3	22.051	20.752	20.319	1.624	21.0
M27	3	25.051	23.752	23.319	1.624	24.0
M30	3.5	27.727	26.211	25.706	1.894	26.5

Fine pitches

The fine sizes in the calculator, from Gühring’s ISO metric fine (DIN 13) list, with ISO 724 diameters in mm:

Thread	Pitch diameter D2	Nut minor D1	Bolt minor d3	Nut thread height H1	Tap drill
M6 × 0.75	5.513	5.188	5.080	0.406	5.2
M8 × 1	7.350	6.917	6.773	0.541	7.0
M10 × 1.25	9.188	8.647	8.466	0.677	8.8
M10 × 1	9.350	8.917	8.773	0.541	9.0
M12 × 1.5	11.026	10.376	10.160	0.812	10.5
M12 × 1.25	11.188	10.647	10.466	0.677	10.8
M14 × 1.5	13.026	12.376	12.160	0.812	12.5
M16 × 1.5	15.026	14.376	14.160	0.812	14.5
M18 × 1.5	17.026	16.376	16.160	0.812	16.5
M20 × 2	18.701	17.835	17.546	1.083	18.0
M20 × 1.5	19.026	18.376	18.160	0.812	18.5
M22 × 1.5	21.026	20.376	20.160	0.812	20.5
M24 × 2	22.701	21.835	21.546	1.083	22.0
M27 × 2	25.701	24.835	24.546	1.083	25.0
M30 × 2	28.701	27.835	27.546	1.083	28.0

Other sizes and inch threads

For another size, pick “Other” and type the diameter and pitch. An inch thread works the same way. Unified Thread Standard (UTS) threads share the ISO basic profile, and the pitch is 25.4 mm ÷ threads per inch: a 1/4-20 is 6.35 mm with a 1.27 mm pitch.

Tapping the thread instead of printing it

For a size too small to print, print a plain hole and cut the thread with a tap. The hole is the tap drill in the tables, about D − P for metric threads. Gühring’s chart rounds a few: M8 is 6.8 mm rather than 6.75 and M12 is 10.2 mm. For a size off the chart, the calculator gives D − P.

If your printer’s holes come out small, correct the hole first with the hole compensation calculator. For a plain clearance hole, or a pilot hole for a self-tapping screw, use the screw hole size calculator. For pins, hinges and other unthreaded fits, the tolerance calculator covers the gap.