Convert tightening torque into bolt preload, then check preload stress, service stress, safety factor against yield and thread strip margin. Covers ISO metric coarse threads M6 to M30 in property classes 4.6 through 12.9 and stainless A2-70.
Torque control relies on the short-form torque–preload relationship. The applied torque is resisted by thread friction, under-head friction and the useful work of stretching the bolt. Those three are bundled into the empirical nut factor K:
T tightening torque (N·m) ·
K nut factor ·
d nominal diameter (mm) ·
p pitch (mm) ·
Fi preload (N) ·
Fa external axial load (N) ·
At tensile stress area (mm²) ·
Sy yield strength (MPa) ·
SF safety factor against yield
An M12 property class 8.8 bolt is tightened dry to 80 N·m and carries a 10 kN external axial service load.
The tensile stress area of 84.27 mm² agrees with the 84.3 mm² tabulated in ISO 898-1 for M12 coarse thread. The strip-to-applied torque ratio of 1.50 sits at the lower edge of the usual 1.5 guideline, so thread engagement length and nut material should be confirmed for this joint.
| Quantity | Symbol | Unit | Accepted range |
|---|---|---|---|
| Tightening torque | T | N·m or lb·ft | > 0 |
| Nominal diameter | d | mm | > 0 |
| Thread pitch | p | mm | > 0 |
| Yield strength | Sy | MPa | > 0 |
| Nut factor | K | dimensionless | 0.05 – 0.50 |
| External axial load | Fa | kN | ≥ 0 |
| Preload | Fi | kN | output |
| Tensile stress area | At | mm² | output |
| Strip torque | Tstrip | N·m | output |
Torque entered in lb·ft is converted using 1 lb·ft = 1.35582 N·m before calculation. All internal work is done in SI.
| Class | Sy (MPa) | Typical use |
|---|---|---|
| 4.6 | 240 | General low-duty steel fasteners |
| 8.8 | 660 | Most common structural and machine bolt |
| 10.9 | 940 | High-strength alloy, preloaded joints |
| 12.9 | 1100 | Highest standard class, socket head screws |
| A2-70 | 450 | Austenitic stainless, corrosion service |
K is an empirical coefficient that bundles thread friction, under-head friction and thread geometry into a single number. Typical values are 0.20 for dry as-received steel, 0.18 for lightly oiled, 0.15 for waxed or lubricated, 0.12 for PTFE coated and 0.10 for molybdenum disulphide. K is the largest source of scatter in torque control: a change from 0.20 to 0.15 raises preload by about 33 % for the same applied torque.
Preload depends on torque, nut factor and bolt diameter through Fi = T / (K·d). Two bolts of the same size tightened to the same torque will reach different preloads if their surface condition differs, because K changes. Field scatter of ±25–30 % on preload is normal with torque control alone. Angle control, bolt elongation measurement or ultrasonic methods give tighter control.
This calculator flags a joint as adequate when the safety factor against yield is at least 1.2 and the service stress stays below the yield strength. Many design codes target higher margins for fatigue-loaded or safety-critical joints. The safety factor here compares yield strength against the greater of the von Mises stress during tightening and the direct stress under preload plus external load.
Strip torque is the estimated torque at which the threads shear rather than the bolt stretching. The ratio of strip torque to applied torque should generally exceed 1.5 so that a bolt yields in tension before the threads fail, which is the safer and more detectable failure mode. A low ratio suggests insufficient thread engagement or a nut or tapped-hole material weaker than the bolt.
Yes. The tensile stress area is computed from the pitch diameter and minor diameter of the ISO metric thread as At = (π/4)·((d2+d3)/2)². For an M12 coarse thread this returns 84.27 mm² against the ISO 898-1 tabulated value of 84.3 mm².