Bolt Torque
Calculate bolt tightening torque and clamping force using T = K × F × d. Select bolt material, diameter, lubrication, and clamping force for the recommended torque.
About This Calculator
The Bolt Torque Calculator helps engineers, mechanics, and DIY enthusiasts determine the correct tightening torque for bolted joints. Proper bolt torque is essential for creating a secure connection that will not loosen under vibration or fail under load. This calculator uses the standard engineering formula T = K × F × d to compute the recommended torque value based on your bolt specifications and clamping requirements.
Bolt torque, also known as tightening torque, is calculated using the formula T = K × F × d × (1 − l/100), where T is the target torque in N·m, K is the nut factor (torque coefficient) that accounts for thread friction, F is the required clamping force in N, d is the nominal bolt diameter in m, and l is the lubrication reduction percentage. The nut factor K varies by bolt material — 0.20 for dry mild steel, 0.15 for lubricated or cadmium-plated steel, 0.22 for black finish, and 0.20 for zinc-plated steel. Lubrication reduces the torque required by up to 45% depending on the lubricant type.
Regional Notes
Worldwide (metric): Enter bolt diameter in millimeters and clamping force in Newtons to get torque in N·m. This is the standard for most global engineering applications including ISO and DIN standards.
United States: For imperial units, convert bolt diameter from inches to mm (multiply by 25.4) and clamping force from lbf to N (multiply by 4.448). Torque output in N·m can be converted to lbf·ft by multiplying by 0.738. Common US bolt grades include SAE Grade 5 and Grade 8.
United Kingdom: UK engineering follows metric ISO standards. Bolt grades are typically denoted as 8.8, 10.9, or 12.9 for high-strength applications. The calculator output in N·m matches UK workshop torque wrench specifications.
Always consult the bolt manufacturer's torque specification chart for critical applications, especially in automotive, aerospace, and structural engineering. This calculator provides engineering estimates and should not replace manufacturer-recommended torque values for safety-critical joints.
Frequently Asked Questions
What is bolt torque?
Bolt torque is the rotational force applied to a bolt to generate tension in the bolt threads, which translates into clamping force between the fastened materials. Proper bolt torque ensures a secure joint that will not loosen under vibration or over-stress.
How is bolt torque calculated?
Bolt torque is calculated using the formula T = K × F × d × (1 - l/100), where T is the target torque in N·m, K is the nut factor depending on bolt material, F is the required clamping force in N, d is the bolt diameter in m, and l is the lubrication reduction in percent.
What is the K (nut) factor in bolt torque calculation?
The K factor, also called the nut factor or torque coefficient, accounts for friction between bolt threads and mating surfaces. Typical values are 0.20 for dry mild steel, 0.15 for lubricated or cadmium-plated steel, 0.22 for black finish, and 0.20 for zinc-plated steel. Lower K values mean less torque is required for the same clamping force.
How does lubrication affect bolt torque requirements?
Lubrication reduces friction between the bolt threads and nut, which lowers the torque needed to achieve the same clamping force. For example, SAE 30 oil reduces the required torque by approximately 40% compared to a dry bolt. Using lubricant allows more consistent clamping force and reduces the risk of thread galling.
What happens if a bolt is overtightened?
Overtightening a bolt can cause the bolt to stretch beyond its elastic limit, permanently reducing its clamping force. In severe cases, the bolt may break during assembly or fail under load. Overtightening can also damage the threads or deform the materials being fastened, leading to joint failure.
What happens if a bolt is undertightened?
Undertightening a bolt means insufficient clamping force is applied, and the joint may loosen over time due to vibration, thermal cycling, or service loads. This can lead to rattling, fluid leaks, or complete separation of components. Proper torque ensures the bolt stays tight throughout its service life.
What are typical torque values for common bolt sizes?
For a dry mild-steel bolt requiring 25,000 N clamping force: M8 (8 mm) needs about 40 N·m, M10 (10 mm) needs about 50 N·m, M12 (12 mm) needs about 60 N·m, and M16 (16 mm) needs about 80 N·m. Actual values depend on bolt grade, material, and lubrication, so always consult your bolt manufacturer torque chart for critical applications.
What is the difference between N·m and lbf·ft for torque measurement?
Both N·m (Newton-meter) and lbf·ft (pound-force-foot) measure torque. 1 N·m equals approximately 0.738 lbf·ft. Most automotive and engineering standards worldwide use N·m, while US imperial systems commonly use lbf·ft. This calculator outputs results in N·m for international use.