How to use the bolt torque calculator
- Choose metric or imperial, then the thread size and the bolt grade or property class.
- Pick the lubrication state (this sets the nut factor K, the biggest single influence on the result) and the preload as a percentage of proof load; 75% is the usual default.
- Read the tightening torque in N·m and lb·ft, the clamp force it produces, the bolt stress, and the minimum thread engagement for the material you are screwing into.
The torque–tension relationship
A bolt does its job by being stretched: the clamp force it puts on the joint is what resists loosening, carries shear by friction and keeps a gasket sealed. Torque is just the convenient way to produce that stretch. The two are linked by the short-form equation used in every engineering handbook:
T = K × F × d where T is torque, F the preload (clamp force), d the nominal diameter and K the nut factor.
With F in newtons and d in millimetres, dividing by 1,000 gives N·m; with F in lbf and d in inches the result is lb·in, divided by 12 for lb·ft. The preload is a chosen fraction of the proof load, which is the proof stress for the grade multiplied by the tensile stress area of the thread (the area of a circle whose diameter is the mean of the pitch and minor diameters; the values here are from ISO 898-1 and ASME B1.1).
Why K matters more than anything else
Roughly half the torque is spent overcoming friction under the bolt head or nut, about 40% on friction in the threads, and only around 10% actually stretches the bolt. So K is dominated by friction, and friction depends on plating, lubrication, surface roughness, whether a washer is used, and how many times the fastener has been tightened before. Published K values are averages with a scatter of ±25% or worse. This is why the same torque on an oiled bolt gives 30% more preload than on a dry one, why reused zinc-plated bolts behave differently from new, and why torque alone is a mediocre way to control preload in critical joints. In aerospace and pressure-vessel work the preload is measured directly, by bolt elongation, ultrasonic length change, or by tightening to a measured angle past snug.
Choosing the preload
75% of proof load is the conventional target for reusable joints in steel. Reasons to go lower: stainless fasteners (which gall; 60–65% with anti-seize is typical), soft or gasketed joints, aluminium threads, or when K is uncertain. Reasons to go higher, up to 90%: permanent joints, fatigue-loaded joints where a high preload keeps the bolt's stress range small, and structural bolting, where codes specify 70% of ultimate (which is above proof). Never exceed proof: the bolt yields, the clamp force stops increasing and the thread or shank may fail.
Grades and classes
Metric property classes are written as two numbers: the first is a tenth of the tensile strength in MPa, the second is the ratio of yield to tensile. So 8.8 means 800 MPa tensile and 640 MPa yield; 10.9 is 1,040 and 940; 12.9 is 1,220 and 1,100. SAE J429 grades for inch bolts: Grade 2 (74 ksi tensile), Grade 5 (120 ksi, three radial marks on the head), Grade 8 (150 ksi, six marks). Stainless A2-70 and 18-8 are roughly 700 MPa tensile but with a low, poorly defined yield around 450 MPa, which is why their torque values are between Grade 2 and Grade 5 despite the high tensile figure. A4-80 (316, cold-worked) is stronger.
Thread engagement
The minimum engagement is the length of internal thread needed so that the threads shear at a higher load than the bolt breaks. For equal-strength steels this is about 0.9–1.0 d, rounded up to 1 d. For weaker nut materials the engagement scales roughly with the ratio of the bolt's tensile strength to the nut material's shear strength: 1.5 d for cast iron and brass, 2 d for aluminium alloys, 3 d or more for engineering plastics. A high-strength 12.9 bolt into mild steel also needs more than 1 d. For aluminium parts that will be assembled more than a few times, a steel helical or key-locking insert is better than deep threads.
Worked example: M16 10.9 oiled
Stress area 157 mm², proof stress 830 MPa, 75% preload: F = 0.75 × 830 × 157 = 97,700 N (97.7 kN). Oiled, K = 0.15: T = 0.15 × 97,700 × 16 ÷ 1,000 = 234 N·m (173 lb·ft). The same bolt tightened dry at K = 0.20 would need 313 N·m for the same preload; applying 313 N·m to the oiled bolt would push the preload to 130 kN, which is 100% of proof and into yield.
General machine-design values. Use manufacturer or code torque for engines, wheels, structural steel, pressure flanges and anything safety-critical, and calibrate your wrench.
Questions and answers
What is the torque for an M12 8.8 bolt?
About 88 N·m (65 lb·ft) dry or zinc-plated at 75% of proof load, which gives roughly 37 kN of clamp force. Lubricated with oil the same preload needs only about 66 N·m; with moly or PTFE anti-seize, about 50 N·m. Using the dry torque on a lubricated bolt over-tightens it by 30–50%, which is the commonest way to strip a thread or snap a bolt.
What is the torque for a 1/2-13 Grade 5 bolt?
75 lb·ft (102 N·m) dry, producing about 9,000 lbf of clamp. Grade 8 is 106 lb·ft; Grade 2 is 49 lb·ft. For a 1/2-20 fine thread add about 12%, because the stress area is larger.
What is the K factor (nut factor)?
A dimensionless number that lumps together thread friction, under-head friction and the thread's own geometry. T = K × F × d. Typical values: 0.20 for plain or zinc-plated steel, dry; 0.15–0.18 oiled; 0.10–0.12 with moly or PTFE anti-seize; 0.16 for cadmium plating. Because about 90% of the torque goes into friction and only 10% into stretching the bolt, a small change in K changes the preload a lot.
Why 75% of proof load?
Proof load is the force a bolt can carry without any permanent stretch. Tightening to 75% of it leaves margin for the scatter in K (easily ±25%), for external load, and for the torsional stress the wrench adds while tightening. Permanent joints that will never be disassembled are sometimes taken to 90%; joints that see fatigue, or soft gaskets, are often lower. Aerospace practice uses measured preload (stretch or ultrasonic) rather than torque where it matters.
How much thread engagement do I need?
Enough that the internal threads do not strip before the bolt breaks. Into steel of similar strength, one bolt diameter is enough; into cast iron 1.5 diameters; into aluminium 2 diameters; into plastics 3 or more, or use an insert. These are rules of thumb that assume the nut material is not much weaker than the bolt; a 12.9 bolt in mild steel needs more.
Can I use this for wheel nuts, cylinder heads or structural steel?
For wheel nuts and engine fasteners use the manufacturer's figure, because they specify torque-to-yield, angle tightening, or sequence that this general formula cannot know. For structural bolts (ASTM A325/A490, EN 14399) use the turn-of-nut or calibrated-wrench method in the relevant code. This calculator is for general machine design and maintenance fastening.
Free to use, runs in your browser, nothing uploaded. Last updated 2026-10-07. Made by HandyToolkits; questions to [email protected].