How to calculate required torque for a stud bolt?

The nut factor K is multiplied by the bolt diameter and the tightening force. The formula is: T = K × d × F. Select the appropriate value for K based on surface condition and lubrication, determine the required tightening force for the connection, and the result is the required torque. If the "Strength Class" option is enabled, the tool will automatically calculate the tightening force based on the strength class of the bolt.

What friction coefficient k should I use?

For dry, clean steel material it is about 0.20. For lubricated threads it can be between 0.10 and 0.15 while with the use of anti-seize or PTFE values of between 0.08 and 0.12 can be achieved. As K has a significant effect on torque (more than a factor of two) select the value for K based upon the actual condition of the bolt. For critical connections, use the manufacturer's specified values.

What is the torque for an m10 bolt?

It depends on the grade and lubrication level. For an M10 bolt of grade 8.8 tightened to about 75% of its yield strength, it would take around 40-50 Nm in dry conditions, while less torque is required for lubricated connections. Higher grades like 10.9 or 12.9 require higher torques. Use the "grade" option in this tool to get the appropriate value for your bolt.

Does the lubricant change the tightening torque?

Yes, there is a significant difference. Lubricants reduce friction, which allows more of the torque to be converted into bolt tension. For the same amount of tightening force, less torque is required when the bolt is lubricated than when it is dry. This is why a lower tightening torque is recommended in low-friction conditions.

What are the applicable ISO standards for bolt tightening torque?

The ISO 898-1 standard specifies the mechanical properties and strength classes of steel bolts and screws including yield strength and guaranteed load. The torque values are then determined based on desired preload level, contact area, and friction conditions and are not specified by this standard itself.

Bolt Torque Calculator

Bolt and load

Bolt diameter

Surface and lubrication

Apply an extra lubrication reduction

Lower the required torque by a lubrication percentage.

Clamping force

Tightening torque

Target preload from the bolt grade

Derive the recommended clamp force and torque from the bolt's strength class.

Enter any two of bolt diameter, clamping force, and torque. The calculator solves for the third.

Please input values into the fields

Charts and reference

The Screw Torque Calculator tool converts the tightening force required for a connection into the necessary torque and also does the reverse conversion. Enter the diameter of the screw, select surface condition and lubrication conditions then enter either the tightening force or the torque. Leave the other field blank to let the tool do the calculation.

In addition, the strength class of the screw directly determines the target preload so you can check the recommended torques without having to consult tables.

What is torque?

Torque is a rotational force acting at a distance from an axis of rotation. When you turn a screw, this force slightly stretches the screw and thereby presses parts together. This stretching manifests as tensile stress inside the screw, and the compressive force holding parts together is the clamping force of the connection.

The importance of the clamping force is greater than one might think. If the clamping force is too low, vibrations can cause the screw to gradually loosen over time. If it is too high, the screw may flow, causing the tightening force not only to fail to increase but to actually decrease, and there is a risk of breakage during assembly or use. Each connection has a specific target torque that should be set.

Formula for torque when tightening screws.

The simplified torque equation establishes a relationship between the torque and the tightening force taking into account the friction coefficient.

T = K \cdot d \cdot F

In this case T is the tightening torque, K is a nut coefficient that takes into account total friction on the threads and bolt head, d is the nominal diameter of the bolt and F is the tightening force. By transforming the equation, the tightening force can be calculated based on the measured torque.

F = \frac{T}{K \cdot d}

Some reference materials include in addition to surface selection an explicit lubricant factor and adjust the torque by a percentage, l.

T = K \cdot d \cdot F \left(1 - \frac{l}{100}\right)

This is a practical approximation. As the thread pitch is not directly modelled, for important connections it is recommended to use values published by the manufacturer of the fasteners.

Nut coefficient k:

K represents the condition of the thread surface. Dry, rough or corroded threads have a higher K because more torque is lost to friction. Coatings, engine oil, grease and lubricants reduce friction, lowering K and allowing more torque to be converted into tensile force. As a rule of thumb: for dry, general purpose steel it's about 0.20, for slightly viscous oil about 0.15, and for low-friction coatings about 0.10.

Condition	Typical K
Dry, rusty or as-rolled	0.25
Dry, plain steel	0.20
Zinc-plated	0.18
Machine oil or SAE 30	0.16
Graphite grease	0.14
Moly (MoS2) grease	0.11
PTFE or wax	0.10
Anti-seize	0.09

The influence of K on the torque is more than twice as high, therefore it is most important and easiest to adapt the value of K to the actual condition of the screw.

Pre-tensioning force, strength class, tension range

Instead of estimating the clamping force, engineers usually target a certain percentage of bolt strength. The preload is part of the yield load, where the yield load is the product of the yield stress and the tensile area.

F = \frac{y}{100} \cdot \sigma_y \cdot A_s

For metric coarse threads, the tensile area is calculated from the diameter and pitch p.

A_s = \frac{\pi}{4}\left(d - 0.9382\,P\right)^2

If you enable the grade option and select a grade, then the tool will calculate the tensile range, target pretension force, and recommended torque. A common target is between 60 percent and 75 percent of yield strength with 90 percent used for controlled one-time connections.

Grade	Yield strength
Class 8.8	640 MPa
Class 10.9	900 MPa
Class 12.9	1080 MPa
SAE Grade 5	621 MPa
SAE Grade 8	830 MPa

How to use this calculator.

Specify the bolt diameter and surface condition or lubrication condition. Then enter the required tightening force for your design and read off the torque. Alternatively, you can enter the torque set on the wrench and find out what the resulting tightening force is. If you leave one of the values blank and enter the other two values, the tool will calculate the missing value.

The units can be freely switched between each other. For the diameter there are millimeters or inches available, for the force Newtons, kiloNewtons or pounds of force and for the torque newton meters or pound feet. This allows using the tool in both metric and anglo american systems. If you want to determine the recommended torque directly from the bolt's strength class, activate the "strength class" option.

Example calculation:

Let's take a 3/4 inch diameter low carbon steel bolt with zinc plating. The coefficient of friction for the lubricated surface is about .12, and the tightening force is 25,000 pounds-force. The torque required is equal to the product of K, the diameter, and the force, which results in a value of approximately 187 pound-feet. If you set your wrench to this value, then the bolt will be tightened to the desired preload. If you change the surface condition to "dry steel", much more torque is required for the same tightening force. This is reflected by the slider bar for lubrication conditions.

This calculation tool is for general guidance and learning purposes only, it does not replace the torque values specified by the manufacturer of the fasteners. For safety critical connections you should follow published values and if necessary measure the preload directly.

Frequently asked questions

How to calculate required torque for a stud bolt?: The nut factor K is multiplied by the bolt diameter and the tightening force. The formula is: T = K × d × F. Select the appropriate value for K based on surface condition and lubrication, determine the required tightening force for the connection, and the result is the required torque. If the "Strength Class" option is enabled, the tool will automatically calculate the tightening force based on the strength class of the bolt.
What friction coefficient k should I use?: For dry, clean steel material it is about 0.20. For lubricated threads it can be between 0.10 and 0.15 while with the use of anti-seize or PTFE values of between 0.08 and 0.12 can be achieved. As K has a significant effect on torque (more than a factor of two) select the value for K based upon the actual condition of the bolt. For critical connections, use the manufacturer's specified values.
What is the torque for an m10 bolt?: It depends on the grade and lubrication level. For an M10 bolt of grade 8.8 tightened to about 75% of its yield strength, it would take around 40-50 Nm in dry conditions, while less torque is required for lubricated connections. Higher grades like 10.9 or 12.9 require higher torques. Use the "grade" option in this tool to get the appropriate value for your bolt.
Does the lubricant change the tightening torque?: Yes, there is a significant difference. Lubricants reduce friction, which allows more of the torque to be converted into bolt tension. For the same amount of tightening force, less torque is required when the bolt is lubricated than when it is dry. This is why a lower tightening torque is recommended in low-friction conditions.
What are the applicable ISO standards for bolt tightening torque?: The ISO 898-1 standard specifies the mechanical properties and strength classes of steel bolts and screws including yield strength and guaranteed load. The torque values are then determined based on desired preload level, contact area, and friction conditions and are not specified by this standard itself.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

ISO 898-1 mechanical properties of fasteners
Strength grades, yield and proof loads for steel bolts and screws.
NIST Special Publication 811: Guide for the Use of the International System of Units
The US national standard for SI units, unit names and symbols, and conversion factors.