Bolt torque calculator: preload, hole size and inserts for printed parts
Pick a thread, a strength class and the friction and get the tightening torque, the preload, the clearance hole and the engagement depth, plus the sizes of a heat-set insert for a printed part. The calculation runs in your browser.
Opens without an account 3 times a day. A free account removes the limit.
- 10 threadsfrom M3 to M24
- 6 classes4.6 to 12.9 and A2-70
- 2.2 timestorque at μ 0.20 against 0.08
- 6 insertsM2 to M6
What bolt torque and preload mean
The bolt torque calculator from EduFacturing is a free online tool. It calculates the tightening torque and the preload (the force with which the bolt clamps the joint) of a metric bolt and shows the clearance hole, the engagement depth and the edge distance.
It starts from the force, not the torque: you choose a share of the yield strength (20 to 95%) and the friction, and the tool returns the torque for the wrench.
Bolt torque calculator: what you get
Calculates torque and preload
For 10 threads from M3 to M24 and 6 classes: you choose a share of the yield strength and get the torque in N·m and the force in kN.
Shows what friction does
Thread friction and head friction are set separately, from μ 0.08 to 0.16, and a table shows how the torque changes at the same preload.
Gives the hole and depth for the drawing
ISO 273 clearance hole, engagement depth from 1×d to 2.5×d, at least 1.5×d from the hole centre to the edge. The result downloads as CSV.
Sizes a thread in a printed part
Insert M2 to M6: hole, depth, boss and nut pocket, and a pull-out estimate as a range, checked against a published test.
When to use a bolt torque calculator
Torque is easy to measure, but a design needs force.
A designer or mechanic
How much to tighten an M10 class 10.9 bolt and what hole to drill.
Anyone printing enclosures
Hole, depth and boss for an insert and the nut pocket, before the part is modelled.
How many bolts you need
An M8 class 8.8 bolt carries 14.1 kN in shear, so 20 kN needs 2.
How bolt torque is calculated
Three steps: the stress area, the preload and the torque.
Stress area
An M8 gives 36.6 mm² under ISO 724. The yield strength for 4.6 to 12.9 follows ISO 898-1, for example 640 MPa for 8.8, and A2-70 is the stainless one, at 450 MPa.
Preload
The yield strength times the chosen share is the stress, and the stress over the area is the force. The target from the EDi Brain rules is 60 to 75%.
Torque
Three parts: work along the pitch, thread friction and head friction. On an M8 8.8, friction takes 85% of the torque when lightly oiled and 89% when dry.
| Applies to | Formula | Meaning |
|---|---|---|
| Stress area | A_s = (π/4)·(d − 0.9382·P)² | ISO 724; an M8 gives 36.6 mm² |
| Preload | F = R_p·u·A_s | R_p is the yield strength of the class, u is the share (0.20 to 0.95); an M8 8.8 at u = 0.7 gives 16.4 kN |
| Torque | T = F·(0.16·P + 0.58·d₂·μ_t + (D_km/2)·μ_h) | VDI 2230; d₂ = d − 0.6495·P, D_km is the diameter under the head; divide by 1000 for N·m |
| Pull-out from a printed part | F = π·D·L·(R_m/√3)·0.65·f | D and L are the insert size, f is the infill, 0.65 is a calibration factor, R_m is the tensile strength of the plastic (PLA 60, PETG 50, ABS/ASA 42 MPa); the range runs from 0.6·F to 1.4·F |
A bolt preload calculator with friction you set
Friction is a field, not a hidden constant
You choose it for the thread and the head separately and see how the torque moves between μ 0.08 and 0.20.
The class uses the guaranteed minimum
420 MPa for 5.8, 940 for 10.9 and 1,100 for 12.9 under ISO 898-1, not the nominal one from the marking.
The torque is a calculation under the conditions you set, not a table value. Fatigue, self-loosening under vibration and joint stiffness are not calculated. The bolt count is a guide: shear strength 0.6 of the bolt's tensile strength, and a factor of 1.25.
ISO 273 holes exist only for M3 to M10: for M12 to M24 the tool says there is no verified value. Insert sizes follow common series, not a standard. The 0.65 factor in the pull-out estimate is calibrated on M3 tests at 100% infill, the infill dependence comes from an M6 test, and the plastic strengths do not come from the EDi Brain rules. For a load-bearing joint, test your own print.
Questions about bolt torque and heat-set inserts
How to calculate bolt torque?
The yield strength of the class times the chosen share and the stress area gives the force, and with friction the torque follows. For an M8 class 8.8 at 70%, lightly oiled thread and head and 11.2 mm under the head: 22.5 N·m for 16.4 kN. With dry thread and head (μ = 0.16): 28.9 N·m.
What clearance hole does an M8 bolt need, and how deep should the thread go?
The ISO 273 medium-series hole: M3 3.4 mm, M4 4.5, M5 5.5, M6 6.6, M8 9, M10 11. Engagement depth: 1×d in steel, 1.5×d in cast iron, 2×d in aluminium, 2.5×d in a soft alloy or plastic. Keep at least 1.5×d from the hole centre to the edge.
What hole size and thread for a heat-set insert in a printed part?
A heat-set insert or a captive nut if it will be taken apart again. A tapped thread and a screw in the plastic are for one-time assembly. For an M3 insert of Ø4 × 5.7 mm: a Ø4.25 mm hole, 6.7 mm deep, in a boss at least Ø8 mm.
How much does an M3 insert hold in PLA?
The estimate at 100% infill is 99 to 230 kg, and a published CNC Kitchen test gives 181 kg (0.15 mm layers, installed at 210 °C). At 30% infill the estimate is 30 to 69 kg.
Is the bolt torque calculator free, and do I need an account?
Yes, it is free. Without an account the tool opens 3 times a day. A free account removes the limit.
The bolt torque calculators is free. Pro opens the rest
Pro costs 15 EUR a month and opens 45 Pro tools.
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