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

Bolt Standard

Bolt Size

Grade / Class

K-Factor (Nut Factor)

Flange Bolt Pattern

Number of Bolts

Bolt Circle Diameter (in)

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.