Bolt Torque Calculator
Tightening torque for metric (M6–M36, classes 8.8 / 10.9 / 12.9) and SAE (Grade 5 / 8) bolts using T = K·F·d. Pick a K-factor preset for your lubrication, target a percentage of proof load or an exact clamp force, and get torque in N·m, ft·lb and in·lb — 100% free, runs in your browser.
Calculate tightening torque
Required tightening torque
Calculation
How the formula works
Tightening torque is estimated with the short-form torque–tension equation:
T = K · F · d
- T — tightening torque (N·m when F is in newtons and d in metres)
- K — nut (friction) factor: 0.20 dry steel, 0.15 oiled, 0.13 waxed, 0.12 anti-seize
- F — target clamp (preload) force
- d — bolt nominal diameter
The proof load is the clamp force at which the bolt starts to permanently deform: tensile stress area × proof stress (580 MPa for class 8.8, 830 MPa for 10.9, 970 MPa for 12.9 per ISO 898-1; 85 ksi for SAE Grade 5, 120 ksi for Grade 8). Designers typically target 75% of proof load, because torque control has roughly ±25% preload scatter — 75% keeps the whole scatter band below yield.
Worked example — M12, class 8.8, dry, 75% of proof
Target clamp = 0.75 × 48,875 = 36,656 N
T = 0.20 × 36,656 N × 0.012 m = 88.0 N·m = 64.9 ft·lb
Lubricate the same bolt with oil (K = 0.15) and the required torque drops to 66.0 N·m for the identical 36,656 N clamp load — using the dry torque on an oiled bolt would over-tension it by about 33%.
Standard torque chart — metric bolts
Torque at 75% of proof load, dry threads (K = 0.20). Coarse pitch series M6–M36.
| Bolt | Pitch (mm) | Stress area (mm²) | 8.8 (N·m) | 10.9 (N·m) | 12.9 (N·m) |
|---|---|---|---|---|---|
| M6 | 1 | 20.1 | 10.5 | 15.0 | 17.6 |
| M8 | 1.25 | 36.6 | 25.5 | 36.5 | 42.6 |
| M10 | 1.5 | 58.0 | 50.5 | 72.2 | 84.4 |
| M12 | 1.75 | 84.3 | 88.0 | 125.9 | 147.1 |
| M16 | 2 | 156.7 | 218.1 | 312.1 | 364.7 |
| M18 | 2.5 | 192.5 | 301.4 | 431.3 | 504.1 |
| M20 | 2.5 | 244.8 | 425.9 | 609.5 | 712.4 |
| M22 | 2.5 | 303.4 | 580.7 | 831.0 | 971.2 |
| M24 | 3 | 352.5 | 736.0 | 1053.3 | 1230.9 |
| M27 | 3 | 459.4 | 1079.1 | 1544.3 | 1804.8 |
| M30 | 3.5 | 560.6 | 1463.1 | 2093.8 | 2447.0 |
| M36 | 4 | 816.7 | 2558.0 | 3660.6 | 4278.0 |
Standard torque chart — SAE bolts
Torque at 75% of proof load, dry threads (K = 0.20), UNC series.
| Bolt | Grade 5 (ft·lb) | Grade 5 (N·m) | Grade 8 (ft·lb) | Grade 8 (N·m) |
|---|---|---|---|---|
| 1/4-20 | 8.5 | 11.5 | 11.9 | 16.2 |
| 5/16-18 | 17.4 | 23.6 | 24.6 | 33.3 |
| 3/8-16 | 30.9 | 41.9 | 43.6 | 59.1 |
| 7/16-14 | 49.4 | 67.0 | 69.8 | 94.6 |
| 1/2-13 | 75.4 | 102.2 | 106.4 | 144.3 |
| 5/8-11 | 150.1 | 203.5 | 211.9 | 287.3 |
| 3/4-10 | 266.5 | 361.4 | 376.3 | 510.2 |
| 7/8-9 | 373.7 | 506.7 | 606.0 | 821.7 |
| 1-8 | 560.3 | 759.7 | 908.6 | 1231.9 |
Nut factor (K) reference
| Condition | K factor |
|---|---|
| Dry steel threads | 0.20 |
| Zinc plated, dry | 0.20 |
| Oiled threads | 0.15 |
| Waxed / dry-film lube | 0.13 |
| Anti-seize compound | 0.12 |
K is a statistical average — actual friction varies ±20% or more. For critical joints, verify preload with a load cell or ultrasonic measurement instead of relying on torque alone.
Bolt torque FAQs
What is the bolt torque formula?
The short-form torque–tension equation is T = K × F × d: torque equals the nut factor K times the clamp force F times the nominal bolt diameter d (in metres for N·m). Example: an M12 class 8.8 bolt, dry (K = 0.20), tightened to 75% of proof load (36,656 N clamp), needs T = 0.20 × 36,656 × 0.012 = 88.0 N·m, which is 64.9 ft·lb.
How much torque does an M12 class 8.8 bolt need?
At 75% of proof load with dry threads (K = 0.20), an M12 class 8.8 bolt needs 88.0 N·m (64.9 ft·lb). Oiled (K = 0.15) it needs only 66.0 N·m, and with anti-seize (K = 0.12) just 52.8 N·m for the same 36,656 N clamp load. A class 10.9 M12 needs 125.9 N·m dry, and a class 12.9 needs 147.1 N·m dry.
What are typical torque values for M6, M8 and M10 bolts?
For class 8.8 bolts with dry threads (K = 0.20) at 75% of proof load: M6 needs 10.5 N·m, M8 needs 25.5 N·m, and M10 needs 50.5 N·m. These scale with K, so oiled threads need 25% less torque for the same clamp force.
Why does lubrication change the required torque so much?
About 90% of tightening torque is consumed by friction (threads and under-head), not by stretching the bolt. Lubrication lowers the nut factor K: dry steel is K = 0.20, oiled is 0.15, anti-seize is 0.12. For an M12 8.8 bolt at 36,656 N clamp, that means 88.0 N·m dry, 66.0 N·m oiled, or 52.8 N·m with anti-seize. Applying the dry torque value to a lubricated bolt over-tensions it by roughly 33–67%.
What is proof load, and why target 75% of it?
Proof load is the clamp force at which the bolt begins to take a permanent set — tensile stress area times proof stress (580 MPa for class 8.8, 830 MPa for 10.9, 970 MPa for 12.9; 85 ksi for SAE Grade 5, 120 ksi for Grade 8). 75% is the standard design target because torque control has about ±25% preload scatter; at 75% the entire scatter band stays below yield while the joint keeps high clamp.
How much torque for a 1/2 inch Grade 5 bolt?
A 1/2-13 UNC Grade 5 bolt at 75% of proof load with dry threads (K = 0.20) needs 75.4 ft·lb (102.2 N·m). Oiled, it needs 56.5 ft·lb. A Grade 8 1/2-13 bolt needs 106.4 ft·lb dry (144.3 N·m).
What happens if I exceed 100% of proof load?
The bolt yields: it stretches permanently and loses its elastic spring-back, so clamp force actually drops after the overload is removed. A yielded bolt must be replaced — it will not hold the designed preload and is prone to fatigue failure. Stay at or below 90% of proof for any reusable fastener.
How much more torque does a class 12.9 bolt need than 10.9?
For an M16 with dry threads (K = 0.20) at 75% of proof: class 10.9 needs 312.1 N·m, class 12.9 needs 364.7 N·m — about 17% more. Note that 12.9 bolts are harder but more brittle and susceptible to hydrogen embrittlement; many engineers cap them at 75% of proof and avoid them in shock or corrosive service.
What tightening sequence should I use for multiple bolts?
Use a star (criss-cross) pattern in at least three passes: roughly 30%, then 60%, then 100% of final torque, and finish with a full clockwise verification pass. This evens out gasket or flange compression so no single bolt ends up carrying the whole joint.
How do I convert N·m to ft·lb?
Multiply N·m by 0.7376 to get ft·lb. For example, 88.0 N·m × 0.7376 = 64.9 ft·lb. To go the other way, multiply ft·lb by 1.3558. Small torques are often given in in·lb: 1 N·m = 8.8508 in·lb.
Is this the same as electric motor torque?
No — they share the word torque but nothing else. This calculator covers fastener tightening torque (T = K·F·d, a static clamping force problem). Electric motor torque is rotational output power, computed from horsepower and RPM. For motors, use our Electric Motor Torque Calculator.
Does thread pitch change the required torque?
Yes, slightly. A finer pitch has a larger tensile stress area, so it carries more proof load and needs more torque at the same percentage. An M12 fine (1.25 mm pitch) class 8.8 at 75% of proof needs 96.1 N·m dry versus 88.0 N·m for the standard 1.75 mm coarse pitch, because its stress area is 92.1 mm² instead of 84.3 mm².
How much torque for an M20 bolt, 8.8 vs 10.9?
With dry threads (K = 0.20) at 75% of proof load: an M20 class 8.8 needs 425.9 N·m, and a class 10.9 needs 609.5 N·m. That is a real-world difference between a 1/2 in drive torque wrench and a heavy 3/4 in drive or multiplier setup.
Should I retorque bolts after installation?
Yes for critical joints. Embedment and gasket creep relax preload within the first hours of service — a retorque after 24 hours, or after the first thermal cycle, typically restores 5–10% of lost clamp. Never loosen first; just re-apply the final torque in sequence.
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