About the Bolt Torque Calculator
This bolt torque calculator estimates how hard to tighten a bolt to reach a target clamp load (preload). Pick inch (UNC/UNF) or metric coarse threads, the bolt size and its strength grade, the percentage of proof load you want to reach and the nut factor for the thread condition. It returns the recommended torque in lb-ft and N·m, the preload in pounds-force and kilonewtons, the tensile stress area and the bolt’s proof load, plus a full torque chart for that grade.
It is meant for mechanics, fabricators, maintenance technicians and engineering students who need a sensible starting torque when a manufacturer’s specification is not available. A common target for reusable joints is 75% of proof load; permanent structural connections are often taken to around 90%.
Torque is only an indirect way of setting preload: roughly 90% of the applied torque is lost to friction under the head and in the threads, so lubrication changes the result dramatically. A lubricated bolt tightened to a dry-bolt torque can be stretched past its proof load. Proof strengths follow SAE J429 (Grade 5 drops from 85 to 74 ksi above 1 in; Grade 2 from 55 to 33 ksi above 3/4 in) and ISO 898-1:2013 (class 8.8 is 580 MPa up to M16 and 600 MPa above). Always use the vehicle, equipment or joint designer’s specified torque when one exists.
How to use the bolt torque calculator
- 1Choose inch or metric threads, then the bolt size (and UNC or UNF for inch bolts).
- 2Select the bolt grade or property class marked on the head.
- 3Set the target preload — 75% of proof load is a typical choice for reusable joints.
- 4Enter the nut factor for your thread condition: 0.20 for dry steel, lower when oiled or coated.
- 5Read the torque in lb-ft and N·m, and use the chart to compare other sizes.
Formula and method
The short-form torque equation T = K·D·F relates tightening torque to preload through the nut factor K, an empirical number that lumps together thread friction, under-head friction and thread geometry. For dry, unplated steel with friction coefficients around 0.15, K is close to 0.20; lubricants and coatings lower it.
The target preload F is a chosen fraction of the bolt’s proof load, which is the minimum proof strength for the grade (SAE J429 for inch bolts, ISO 898-1 for metric classes) multiplied by the tensile stress area. The stress area uses the standard thread formulas with n threads per inch or pitch P in millimetres. Inch torque comes out in lb-in (divided by 12 for lb-ft) and metric torque in N·mm (divided by 1,000 for N·m).
- T
- Tightening torque
- K
- Nut factor (≈ 0.20 dry steel)
- D, d
- Nominal bolt diameter (in or mm)
- F
- Target preload (clamp load)
- S_p
- Minimum proof strength of the grade
- A_t, A_s
- Tensile stress area
- n, P
- Threads per inch, or thread pitch in mm
Worked examples
1/2"-13 Grade 5 bolt, dry, 75% of proof
A 1/2-13 bolt has a stress area of 0.7854 × (0.5 − 0.9743/13)² ≈ 0.1419 in². At Grade 5’s 85,000 psi proof strength the proof load is about 12,061 lbf, and 75% of that is a 9,047 lbf preload. Torque = 0.20 × 0.5 × 9,047 ≈ 905 lb-in, or about 75 lb-ft.
M12 class 10.9 bolt, lightly oiled
An M12 × 1.75 thread has a stress area of 0.7854 × (12 − 0.9382 × 1.75)² ≈ 84.3 mm². Class 10.9’s proof stress of 830 MPa gives a 69.9 kN proof load; 75% is 52.5 kN. With K = 0.15 the torque is 0.15 × 12 × 52,457 ÷ 1,000 ≈ 94 N·m.
3/8"-24 Grade 8 fine thread to 90%
A 3/8-24 thread has a stress area of about 0.0878 in². At 120,000 psi proof strength the proof load is 10,539 lbf, and 90% of that is 9,485 lbf. Torque = 0.20 × 0.375 × 9,485 ≈ 711 lb-in, or about 59 lb-ft.
Frequently asked questions
How much torque does a 1/2 inch grade 8 bolt need?+
With dry, unplated threads (K = 0.20) and a 75% of proof target, a 1/2-13 Grade 8 bolt needs about 106 lb-ft (144 N·m). The same bolt in Grade 5 needs about 75 lb-ft. Lubricated threads need roughly 25% less.
What is the nut factor K?+
K is an experimentally measured factor that converts torque into preload. It captures friction in the threads and under the head; about 90% of applied torque is lost to friction. Typical values are 0.20 for dry steel, 0.15–0.18 for oiled threads and 0.10–0.15 with anti-seize or moly paste.
Should I torque bolts dry or lubricated?+
Use whichever condition the torque specification was written for. If a spec assumes dry threads and you lubricate them, the same torque produces much more preload and can yield the bolt. When you lubricate, lower the nut factor in this calculator so the torque drops accordingly.
What percentage of proof load should I tighten to?+
A common guideline is 75% of proof load for reusable connections and about 90% for permanent connections. Higher preload resists loosening and joint separation, but leaves less margin against yielding during tightening.
What is the difference between proof load and tensile strength?+
Proof load is the force a bolt must withstand without permanent stretch, roughly 88–96% of its minimum yield strength depending on grade (Grade 5: 85 vs 92 ksi; class 10.9: 830 vs 940 MPa). Tensile strength is the force at which it breaks. Preload is set as a percentage of proof load so the bolt stays elastic and can be reused.
How do I identify bolt grade?+
Inch bolts are marked with radial lines on the head: none for Grade 2, three lines for Grade 5 and six lines for Grade 8. Metric bolts carry the property class number, such as 8.8, 10.9 or 12.9, stamped on the head.