Bolt Torque Calculator
A bolt torque calculator determines the tightening torque required to achieve a specified clamping force (preload) in a bolted joint. Proper bolt preload is critical for joint integrity: too little and the joint may slip or the bolt may loosen under vibration; too much and the bolt may yield or fracture. The standard torque formula T = K x F x D relates the applied torque to the resulting clamp force through the torque coefficient K, which depends on lubrication and surface condition. This calculator allows you to set a target preload as a percentage of proof load for the selected bolt grade, computes the required torque, and outputs values in both lb-ft and Newton-meters.
Bolt torque formula
Stress Area = 0.7854 x (D - 0.9743/TPI)^2 [ASME B1.1]
Proof Load (lbs) = Proof Stress (psi) x Stress Area
Clamp Force = Proof Load x (Target%/100)
T (lb-in) = K x Clamp Force x D
T (lb-ft) = T (lb-in) / 12
T (Nm) = T (lb-ft) x 1.3558
Frequently asked questions
How is bolt tightening torque calculated?
The standard torque formula is T = K x F x D, where T is torque (lb-in or Nm), K is the torque coefficient (also called the nut factor), F is the desired clamp force (bolt preload) in pounds or Newtons, and D is the nominal bolt diameter. The K factor accounts for friction under the nut and in the threads.
What is the torque coefficient K?
The K factor (nut factor) ranges from about 0.10 for waxed or lubricated bolts to 0.20 for unlubricated zinc-plated bolts, and 0.15-0.17 for typical dry bolts. Using the wrong K value can result in significant under- or over-tightening. K = 0.20 is the standard assumption for dry, unlubricated steel bolts.
What is bolt preload?
Bolt preload is the tensile clamping force created in a bolt when it is tightened. For structural bolts, the design preload is typically 70-75% of the bolt's proof load (the tensile load at the yield point). Higher preload increases joint stiffness and resistance to loosening.
What is proof load for a bolt?
Proof load is the specified minimum tensile load a bolt must support without permanent deformation. For SAE Grade 5 (1/2-inch to 1-inch): proof load = 85,000 psi x stress area. For SAE Grade 8: proof load = 120,000 psi x stress area. ASTM A325 and A490 bolts have similar specifications for structural connections.
What happens if a bolt is over-torqued?
Over-torquing stretches the bolt beyond its yield point, reducing its clamping ability and fatigue life. The bolt may fracture during tightening or in service. Under-torqued bolts may loosen under vibration or cyclic loading, causing joint slippage or fatigue failure. Always use a calibrated torque wrench and follow the specification.
Official sources
- ASME: ASME B1.1 Unified Inch Screw Threads (Stress Area Formula).
- SAE International: SAE J429 Mechanical and Material Requirements for Externally Threaded Fasteners.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.