Bolt Torque Calculator
Free Bolt torque Calculator for materials specifications projects. Enter dimensions to get material lists and cost estimates.
Formula
T = K x D x F
Torque (T) in inch-pounds equals the torque coefficient (K, typically 0.15-0.25) times the nominal bolt diameter (D) in inches times the desired clamping force (F) in pounds. Divide by 12 for foot-pounds. The torque coefficient accounts for all friction in the threads and under the nut bearing face.
Worked Examples
Example 1: Grade 5 Flange Bolt
Problem: Calculate the required torque for a 3/4-inch Grade 5 bolt to achieve 75% of proof load with dry steel surfaces (K=0.20).
Solution: Tensile area = 0.3340 sq in\nProof load = 85,000 x 0.3340 = 28,390 lbs\nTarget preload = 75% x 28,390 = 21,293 lbs\nTorque = 0.20 x 0.75 x 21,293 = 3,194 in-lbs = 266 ft-lbs
Result: Torque of 266 ft-lbs for 21,293 lbs clamp force
Example 2: A490 Structural Bolt
Problem: Find the torque for a 1-inch A490 bolt at 70% proof load with lubricated threads (K=0.15).
Solution: Tensile area = 0.6057 sq in\nProof load = 120,000 x 0.6057 = 72,684 lbs\nTarget = 70% x 72,684 = 50,879 lbs\nTorque = 0.15 x 1.0 x 50,879 = 7,632 in-lbs = 636 ft-lbs
Result: Torque of 636 ft-lbs for 50,879 lbs clamp force
Frequently Asked Questions
How is bolt torque related to clamping force?
Bolt torque and clamping force are related by the simplified formula T = K x D x F, where T is torque, K is the torque coefficient (nut factor), D is the nominal bolt diameter, and F is the desired clamping force. The torque coefficient K typically ranges from 0.15 to 0.25 depending on lubrication and surface conditions. Only about 10 to 15 percent of the applied torque actually produces clamp force; the rest is consumed by friction under the nut face and in the threads.
What torque coefficient (K factor) should I use?
The torque coefficient K depends on the friction conditions of the bolt and nut surfaces. Typical values are 0.20 for plain dry steel, 0.18 for zinc plated, 0.15 for lubricated or cadmium plated, and 0.10 to 0.12 for anti-seize compounds or molybdenum disulfide. Using the wrong K factor can result in significantly incorrect preload. For critical joints, torque-angle or ultrasonic bolt stretch measurement provides more accurate preload control than torque alone.
Why is bolt lubrication important for accurate torquing?
Lubrication dramatically affects the relationship between applied torque and achieved clamping force. A dry bolt with K=0.20 produces about 60 percent less clamp force than a lubricated bolt with K=0.12 at the same torque value. Conversely, applying the torque specification for dry bolts to lubricated bolts can overstress the bolt to failure. Always verify that the torque specification matches the actual bolt condition, and use consistent lubrication across all bolts in a joint.
What is the difference between horsepower and torque?
Horsepower measures how quickly work is done (sustained speed and acceleration at higher RPMs). Torque measures rotational force (pulling power and low-end acceleration). Diesel trucks emphasize torque for towing. Sports cars emphasize horsepower for top speed. HP = torque x RPM / 5,252.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.