ENGINEERING TOOL
Bolt Torque Calculator
Tightening torque range and target preload by property class and friction condition (K-factor method), fine threads, imperial UNC/UNF and stud bolts included
Custom K Factor (measured)
An accurate K can only be determined by testing with the identical coating/lubrication/washer stack as production. Range 0.08–0.40.
Suggested Torque (N·m)
Min
48 N·m
Typical
54 N·m
Max
64 N·m
Target Preload
26.6 kN (5.98 kip)
K
0.15–0.2
Rp0.2
450 MPa
As / At
84.3 mm²
Unit Conversion (typical)
54 N·m = 40 lbf·ft = 478 lbf·in = 6 kgf·m
Bearing Surface Pressure (preload / bearing area)
188 MPa
Bearing circle assumed at dw≈1.5d. Reference: clamped-part allowable bearing stress is on the order of 200 MPa for steel, 90 for aluminium, 60 for copper busbar, exceeding it can crush the clamped part; use a bigger washer or larger size.
Friction Scatter: preload band at the same typical torque
22.5–30 kN
Controlling friction (lubrication + proper washers) narrows the preload scatter at a given torque from ±20% to ±10%, often allowing a smaller bolt, smaller tool and lighter joint.
⚠ Friction is the dominant variable: the same fastener can require 30–50% different torque across lubrication states. Always follow the design specification or manufacturer torque table for actual assembly.
Output is a range, not a single number, the honest way to talk about torque
Check Your Torque
Full Torque Chart, A2-70 · Electro Zinc (dry)
Torque ranges for the full size series at the selected parameters. Current selection highlighted. Printable as an A4 wall chart.
Open the full M12 chart page (all classes & friction states) →| Nominal Diameter | As mm² | Target Preload kN | Min N·m | Typical N·m | Max N·m |
|---|---|---|---|---|---|
| M6 | 20.1 | 6.3 | 6 | 6 | 8 |
| M8 | 36.6 | 11.5 | 14 | 16 | 18 |
| M10 | 58 | 18.3 | 27 | 31 | 37 |
| M12 | 84.3 | 26.6 | 48 | 54 | 64 |
| M14 | 115 | 36.2 | 76 | 86 | 101 |
| M16 | 157 | 49.5 | 119 | 135 | 158 |
| M18 | 192 | 60.5 | 163 | 185 | 218 |
| M20 | 245 | 77.2 | 232 | 262 | 309 |
| M22 | 303 | 95.4 | 315 | 357 | 420 |
| M24 | 353 | 111.2 | 400 | 454 | 534 |
| M27 | 459 | 144.6 | 586 | 664 | 781 |
| M30 | 561 | 176.7 | 795 | 901 | 1060 |
| M33 | 694 | 218.6 | 1082 | 1226 | 1443 |
| M36 | 817 | 257.4 | 1390 | 1575 | 1853 |
Method & Data Sources
The K-factor (nut factor) method: T = K·D·F, where T is torque, D the nominal diameter, and F the target preload. Target preload follows the selected level: Reusable = 70% of Rp0.2 × stress area (≈77% of proof load); Near-yield = 90% of proof load. K follows published empirical ranges per friction condition, with a measured-K override. Imperial (UNC/UNF + SAE grades) follows the Machinery's Handbook convention: 75% of proof load by default.
Data sources: coarse-thread stress areas and ISO Rp0.2/proof values from the published ISO 898-1 / ISO 3506-1 property-class systems; fine-thread As from ISO 898-1 Table 6 (e.g. M8×1 = 39.2 mm², M14×1.5 = 125 mm²); imperial At by the UN formula π/4·(d−0.9382P)² with SAE J429 proof stresses (size-dependent); K ranges per Machinery's Handbook and Machine Design experimental reviews (waxed/MoS₂/PTFE ≈ 0.10, galvanized + wax ≈ 0.12).
Accuracy: torque-controlled assembly typically scatters preload by ±15–20% (K varying between 0.10 and 0.30 produces differences of this magnitude); hydraulic tensioning reaches ±5%. Results are for selection and budgeting, not assembly instructions.
Boundary: this is a simplified estimate, not a full VDI 2230 analysis, the latter accounts for joint resilience, embedment, assembly method and more. For structurally critical joints, run a full VDI 2230 analysis or consult an engineer.
FAQ
Why a range instead of a single value?
Because friction varies in the real world, surface condition, coating thickness and lubricant batches all shift it. A single value is false precision; a range is the honest engineering answer.
Why does lubrication lower the torque?
With better lubrication, less torque is lost to friction and more converts into preload. Reaching the same preload therefore requires significantly less torque.
What happens if torque exceeds the proof load?
Beyond proof load the bolt enters plastic deformation, clamp force no longer grows linearly with torque, and permanent elongation or fracture may follow. Use the torque check above to confirm your margin; run a VDI 2230 analysis for critical joints.
How different is fine-thread torque from coarse?
Fine threads have a larger stress area As (e.g. M14 fine = 125 mm² vs 115 mm² coarse), so preload and torque are higher at the same class; but fine threads gall more easily and demand cleaner assembly. The torque difference roughly follows the As ratio (≈ +9% for M14).
Why can K only be confirmed by testing?
K can range from 0.10 to 0.30 depending on coating, lubrication and washer stack, even batches from one supplier drift. Preset ranges are a first estimate; the accurate value must be measured on the production-identical stack (ISO 16047 method).
Why do stainless bolts gall easily?
Austenitic A2/A4 stainless is tough and work-hardens rapidly; frictional heating can locally weld thread flanks. Use anti-seize and follow the Greased/MoS₂ or Waxed/PTFE presets (K ≈ 0.10–0.12).
How do imperial and metric torque compare?
1 lbf·ft ≈ 1.356 N·m (built into this tool). But SAE grades and ISO 8.8/10.9 are different strength systems, never convert the unit without re-checking the grade.
Should anti-seize be counted in the torque?
Anti-seize drops K into the 0.10–0.12 band. Following a dry-state torque table after applying it overloads the joint, and vice versa. Always match the preset to the actual lubrication state.
When is a full VDI 2230 analysis required?
Fatigue-critical joints under cyclic loading, non-steel clamped parts, very large sizes, or applications with certified preload accuracy requirements.
Related References
Disclaimer: This is a simplified estimation tool. Results are for selection and budgeting reference only and do not constitute assembly instructions. Follow the design specification or manufacturer torque table for actual assembly.