Bolt Torque Calculator
Work out the tightening torque and clamp force for metric and UNC/UNF bolts by grade and lubrication, from 4.6 to 12.9 and SAE Grade 2, 5 and 8.
New: please check your results.
For guidance only. This is not a substitute for a qualified engineer, and important designs should be checked by one.
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- Tightening torque
- 50.5 N·m
- Clamp force (preload)
- 25.2 kN
- Proof load
- 33.6 kN
- Tensile stress area
- 58 mm²
What it does
Find how tight to do up a bolt. Choose the thread, size and grade, and how the threads are lubricated, and see the tightening torque and the clamp force it gives. It covers metric property classes 4.6 to 12.9, A2 and A4 stainless, and SAE Grades 2, 5 and 8 for UNC and UNF bolts.
How to use it
- Thread type
Most metric bolts are coarse thread. UNC and UNF are American inch threads.
- Bolt size
The thread diameter: an M10 bolt is 10 mm across the thread (usually a 16 or 17 mm spanner).
- Bolt size
The diameter and threads per inch.
- Bolt size
The diameter and threads per inch.
- Grade
Stamped on the bolt head, for example 8.8 (the most common high-tensile bolt), 10.9, or 12.9 (socket cap screws). 4.6 is mild steel. Stainless bolts are marked A2 or A4 with a number such as 70.
- Grade
SAE grade: count the radial lines on the head: none for Grade 2, 3 for Grade 5, 6 for Grade 8.
- Thread condition
Dry means plain or zinc-plated steel as supplied (nut factor K 0.2); lightly oiled K 0.15; anti-seize or moly paste K 0.12. Lubricated threads reach the same clamp force with less torque. Using a dry figure on an oiled bolt over-tightens it, which can stretch or snap it.
- Nut factor (K)
From the fastener or lubricant maker. Typical values: 0.2 dry steel, 0.15 to 0.18 oiled, 0.1 to 0.13 with anti-seize, 0.25 to 0.3 for dry stainless.
- Clamp force (% of proof load)
75% is the usual target for bolts that may be taken apart; up to 90% for permanent joints. The proof load is the most a bolt can hold without stretching permanently.
Common uses
- Setting a torque wrench for bolts with no torque figure in the manual.
- Choosing between bolt grades for a bracket, frame or machine.
- Checking a torque chart for oiled or anti-seized threads.
- Drilling and tapping the holes. Try the Tap Drill Size Calculator
- Laying out holes on a flange. Try the Bolt Circle Calculator (PCD)
The formula
Proof load = proof stress × tensile stress areaClamp force F = proof load × % ÷ 100Torque T = K × F × dWhere:
- K
- nut factor: friction in the threads and under the head (about 0.2 dry)
- d
- the bolt's nominal diameter
- Stress area
- metric: π/4 × (d − 0.9382 × pitch)²; inch: π/4 × (d − 0.9743 ÷ TPI)²
A guide based on typical friction. Always use the manufacturer's torque figure where there is one (cars, engines, wheels, structural steelwork), and remember torque wrenches are only accurate to about ±25% in clamp force because friction varies.
Worked example
An M10 (1.5 mm pitch) bolt, grade 8.8, dry, tightened to 75% of its proof load:
- Tensile stress area: 58 mm²
- Proof load: 580 MPa × 58 mm² = 33.6 kN
- Clamp force: 75% of that = 25.2 kN
- Torque: 0.2 × 25.23 kN × 10 mm = 50.5 N·m
Questions people ask
What torque for an M10 8.8 bolt?
About 50 N·m dry (37 lbf·ft), or about 38 N·m lightly oiled, for a clamp force of about 25 kN (75% of its proof load). An M8 8.8 is about 25 N·m dry and an M12 8.8 about 88 N·m.
Why does lubrication change the torque so much?
Around 90% of the torque you apply is lost to friction in the threads and under the head; only about 10% stretches the bolt. Oil or anti-seize cuts that friction, so the same torque gives far more clamp force. If a torque figure is for dry threads, using it on oiled threads can over-tighten the bolt.
What do the numbers on a bolt head mean?
On metric bolts the first number × 100 is the tensile strength in MPa, and the second × 10 is the yield as a percentage of it: an 8.8 bolt has about 800 MPa tensile strength and yields at 80% of that. Inch bolts show radial lines: three for Grade 5, six for Grade 8.
Last checked 3 November 2026 · Last rev. 31 October 2026 · 6051