Bolt Torque Calculator
The Bolt Torque Calculator computes tightening torque using T = K × D × F for SAE Grade 2/5/8 and Metric Class 8.8/10.9/12.9 bolts. It includes a flange bolt pattern calculator with interactive SVG circular bolt arrangement, K-factor adjustment for dry/lubricated conditions, and complete torque reference tables — free, no signup required.
Bolt Specification
Flange Bolt Pattern
Torque Specification Table — Grade 5
| Bolt Size | Thread Pitch | Torque (Dry) (ft-lbs) | Torque (Lubricated) (ft-lbs) | Clamp Force (lbs) |
|---|---|---|---|---|
| 1/4"-20 | 20 TPI | 8.4 | 6.3 | 2027 |
| 5/16"-18 | 18 TPI | 17.4 | 13.0 | 3341 |
| 3/8"-16 | 16 TPI | 30.9 | 23.2 | 4941 |
| 7/16"-14 | 14 TPI | 49.4 | 37.1 | 6777 |
| 1/2"-13 | 13 TPI | 75.4 | 56.5 | 9046 |
| 9/16"-12 | 12 TPI | 108.8 | 81.6 | 11603 |
| 5/8"-11 | 11 TPI | 150.1 | 112.6 | 14408 |
| 3/4"-10 | 10 TPI | 266.2 | 199.6 | 21293 |
| 7/8"-9 | 9 TPI | 429.5 | 322.1 | 29453 |
| 1"-8 | 8 TPI | 643.9 | 482.9 | 38633 |
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What is Bolt Torque?
Bolt torque is the rotational force applied to a fastener to achieve a specific clamping load. The relationship is T = K × D × F, where T is torque (N·m or ft·lb), K is the nut factor (0.20 dry, 0.15 lubricated), D is the nominal bolt diameter, and F is the desired clamp force. Proper torque is critical: under-torquing risks joint failure and leaks, while over-torquing causes bolt stretch, thread stripping, or breakage. SAE Grade 5 and Metric Class 8.8 are the most common general-purpose fastener grades.
How to Use This Calculator
- Select the bolt standard (SAE or Metric) and choose the bolt size from the dropdown
- Select the bolt grade (SAE Grade 2/5/8 or Metric Class 8.8/10.9/12.9)
- Choose the lubrication condition — this sets the K-factor that affects the required torque
- Click Calculate to see the recommended torque value, clamp force, and proof load
- Use the Flange Pattern section to calculate per-bolt torque for multi-bolt flanges
Frequently Asked Questions
Why does lubrication affect bolt torque?
Approximately 90% of input torque is lost to friction in the threads and under the bolt head. Lubrication reduces friction, so a lower torque achieves the same clamping force. A dry bolt (K=0.20) needs about 33% more torque than a lubricated bolt (K=0.15) for the same clamp load. Anti-seize compound (K=0.12) reduces it further.
What is the difference between SAE Grade 5 and Grade 8?
Grade 5 bolts have a proof load of 85,000 PSI and tensile strength of 120,000 PSI — suitable for most automotive and general applications. Grade 8 bolts have a proof load of 120,000 PSI and tensile strength of 150,000 PSI — used for high-stress applications like suspension, drivetrain, and structural connections.
What is the correct torque sequence for flange bolts?
Flange bolts should be tightened in a star (crisscross) pattern in 3–4 passes: first to 30% of final torque, then 60%, then 100%, with a final verification pass. This ensures even gasket compression. Never tighten in a circular sequence, as this causes uneven loading and potential leaks.