Bolt Torque Calculator

Find the tightening torque needed for a bolt to reach a target clamp (preload) force, from bolt diameter and the torque coefficient.

🔩 Bolt Torque Calculator
mm
kN
Required torque (T)
—
In lb-ft
—
Step-by-step working

🔩 What is Bolt Torque?

Bolt torque is the tightening torque applied to a bolt or nut to develop a target clamp (preload) force in the joint. The relationship, T = K × D × F, connects the torque a wrench applies to the actual clamping force stretching the bolt and squeezing the joint together, through a torque coefficient K that captures how much of that applied torque is lost to friction rather than converted into clamp force.

Mechanical, automotive, and structural engineers use this relationship constantly when specifying assembly torque values for bolted joints, from engine head bolts to structural steel connections to simple equipment mounting hardware. Since a torque wrench measures torque directly but the joint actually needs a specific clamp force to function correctly (resist shear, prevent gasket leaks, maintain preload under vibration), this formula is the practical bridge between the two.

A common point of confusion is assuming torque and clamp force are simply proportional with no other factors, or that K is a fixed universal constant. In reality, roughly 90 percent of applied torque is consumed by friction at the threads and under the bolt head, only about 10 percent actually stretches the bolt, so K, which captures that friction, has an outsized effect and varies significantly with lubrication, surface finish, and plating.

This calculator computes the tightening torque needed to reach a target clamp force from your bolt diameter and chosen torque coefficient, showing the result in both newton-meters and pound-feet with full step-by-step working.

📐 Formula

T = K × D × F
T = tightening torque (N·m)
K = torque coefficient (nut factor), typically about 0.15-0.25 depending on friction/lubrication
D = nominal bolt diameter (mm)
F = target clamp (preload) force (kN)
Example: K = 0.2, D = 10 mm (M10 bolt), F = 20 kN → T = 40 N·m.

📖 How to Use This Calculator

Steps

1
Enter the torque coefficient. Type K, the torque coefficient (nut factor), typically about 0.20 for standard steel fasteners.
2
Enter the bolt diameter. Type D, the nominal bolt diameter, in millimeters.
3
Enter the target clamp force. Type F, the desired clamp (preload) force, in kilonewtons, then read the required tightening torque.

💡 Example Calculations

Example 1 — M10 Bolt, Standard Steel

K = 0.2, D = 10 mm, F = 20 kN

1
T = K × D × F = 0.2 × 10 × 20
2
T = 40.00 N·m (29.50 lb-ft)
T = 40.00 N·m (29.50 lb-ft)
Try this example →

Example 2 — M6 Bolt, Standard Steel

K = 0.2, D = 6 mm, F = 8 kN

1
T = K × D × F = 0.2 × 6 × 8
2
T = 9.60 N·m (7.08 lb-ft)
T = 9.60 N·m (7.08 lb-ft)
Try this example →

Example 3 — M16 Bolt, Lubricated Threads

K = 0.15, D = 16 mm, F = 50 kN

1
T = K × D × F = 0.15 × 16 × 50
2
T = 120.00 N·m (88.51 lb-ft)
T = 120.00 N·m (88.51 lb-ft)
Try this example →

❓ Frequently Asked Questions

What is the formula for bolt torque?+
T = K x D x F, where T is the tightening torque, K is the torque coefficient (nut factor), D is the nominal bolt diameter, and F is the desired clamp (preload) force. With D in millimeters and F in kilonewtons, T comes out directly in newton-meters.
What is the torque coefficient K?+
K, also called the nut factor, represents the combined effect of thread friction, under-head friction, and thread geometry. Typical values are about 0.20 for plain as-received steel fasteners, 0.15 to 0.17 for lubricated fasteners, and higher for dry, rough, or galvanized surfaces.
Why does most of the tightening torque not go into clamp force?+
Roughly 90 percent of applied tightening torque is consumed overcoming friction, about 40-50 percent at the threads and 35-45 percent under the bolt head or nut face, leaving only about 10 percent to actually stretch the bolt and generate clamp force. This is why friction (captured in K) has such a large effect on the torque-to-clamp-force relationship.
How accurate is torque-based bolt tightening?+
Torque-based tightening typically has plus or minus 25 to 30 percent scatter in actual achieved clamp force, due to friction variability between individual fasteners, even with identical torque and nominal K. For critical joints, angle-controlled tightening or direct tension indicators give tighter control.
What units does this calculator use?+
Bolt diameter is entered in millimeters (mm) and desired clamp force in kilonewtons (kN). The resulting torque is shown in both newton-meters (N·m) and pound-feet (lb-ft).
Does bolt grade affect the torque coefficient K?+
Not directly, K primarily reflects friction at the threads and bearing surface, which depends on surface finish, plating, and lubrication rather than the bolt's material grade. However, higher-grade bolts can safely be tightened to a higher clamp force (limited by their proof load), which changes the target F used in the formula, not K itself.
How do I choose a target clamp force?+
A common engineering target is 65 to 75 percent of the bolt's proof load (a conservative preload that maximizes joint fatigue resistance without risking yield), always check your specific application's design standard or the fastener manufacturer's recommendation rather than guessing.
What happens if I apply too much torque to a bolt?+
Excessive torque can stretch the bolt past its yield point (permanent deformation, losing clamp force and fatigue resistance) or strip the threads. Always stay within the fastener's rated proof load, and use a calibrated torque wrench rather than tightening by feel.
Can this formula be used for wheel lug nuts or engine bolts?+
The same T = K x D x F relationship underlies torque specifications for lug nuts, cylinder head bolts, and structural connections, but always use the exact torque value specified by the manufacturer for safety-critical applications rather than calculating your own from an assumed K, since the true K for a specific fastener/lubricant combination can vary.
Why does a larger bolt diameter need more torque for the same clamp force?+
Torque is directly proportional to diameter in T = K x D x F, because a larger diameter thread has a larger effective radius, so the same clamp force requires proportionally more torque (force times lever arm) to achieve through the threads.
Tags: Bolt Torque Calculator Torque to Clamp Force Fastener Preload Calculator Torque Coefficient K Calculator Formula Free Online

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What is the formula for bolt torque?

T = K x D x F, where T is the tightening torque, K is the torque coefficient (nut factor), D is the nominal bolt diameter, and F is the desired clamp (preload) force. With D in millimeters and F in kilonewtons, T comes out directly in newton-meters.

What is the torque coefficient K?

K, also called the nut factor, represents the combined effect of thread friction, under-head friction, and thread geometry. Typical values are about 0.20 for plain as-received steel fasteners, 0.15 to 0.17 for lubricated fasteners, and higher for dry, rough, or galvanized surfaces.

Why does most of the tightening torque not go into clamp force?

Roughly 90 percent of applied tightening torque is consumed overcoming friction, about 40-50 percent at the threads and 35-45 percent under the bolt head or nut face, leaving only about 10 percent to actually stretch the bolt and generate clamp force. This is why friction (captured in K) has such a large effect on the torque-to-clamp-force relationship.

How accurate is torque-based bolt tightening?

Torque-based tightening typically has plus or minus 25 to 30 percent scatter in actual achieved clamp force, due to friction variability between individual fasteners, even with identical torque and nominal K. For critical joints, angle-controlled tightening or direct tension indicators give tighter control.

What units does this calculator use?

Bolt diameter is entered in millimeters (mm) and desired clamp force in kilonewtons (kN). The resulting torque is shown in both newton-meters (N·m) and pound-feet (lb-ft).

Does bolt grade affect the torque coefficient K?

Not directly, K primarily reflects friction at the threads and bearing surface, which depends on surface finish, plating, and lubrication rather than the bolt's material grade. However, higher-grade bolts can safely be tightened to a higher clamp force (limited by their proof load), which changes the target F used in the formula, not K itself.

How do I choose a target clamp force?

A common engineering target is 65 to 75 percent of the bolt's proof load (a conservative preload that maximizes joint fatigue resistance without risking yield), always check your specific application's design standard or the fastener manufacturer's recommendation rather than guessing.

What happens if I apply too much torque to a bolt?

Excessive torque can stretch the bolt past its yield point (permanent deformation, losing clamp force and fatigue resistance) or strip the threads. Always stay within the fastener's rated proof load, and use a calibrated torque wrench rather than tightening by feel.

Can this formula be used for wheel lug nuts or engine bolts?

The same T = K x D x F relationship underlies torque specifications for lug nuts, cylinder head bolts, and structural connections, but always use the exact torque value specified by the manufacturer for safety-critical applications rather than calculating your own from an assumed K, since the true K for a specific fastener/lubricant combination can vary.

Why does a larger bolt diameter need more torque for the same clamp force?

Torque is directly proportional to diameter in T = K x D x F, because a larger diameter thread has a larger effective radius, so the same clamp force requires proportionally more torque (force times lever arm) to achieve through the threads.

📌 Quick Tips

💡The torque coefficient K (also called the nut factor) is not a fixed physical constant, it depends heavily on friction: K is around 0.20 for standard as-received steel fasteners, drops to about 0.15-0.17 with lubrication, and can rise above 0.25 for dry, corroded, or galvanized threads.
💡This formula assumes most of the applied torque overcomes thread and under-head friction, only about 10 percent of the tightening torque actually goes into stretching the bolt to create clamp force, which is exactly why K matters so much.
💡Always use the manufacturer's or engineering drawing's specified torque value when one is given, this calculator is for estimating torque when only a target clamp force and a reasonable K value are known.
💡Torque wrench accuracy and friction variability mean torque-based tightening has real scatter (commonly plus or minus 25 percent) in actual achieved clamp force, angle-based or direct tension methods are more precise where critical.
💡Never reuse this formula to exceed a bolt's proof load or yield strength, always check the fastener's rated tensile and proof load before targeting a high clamp force.