Bolted joint is a type of joint where minimum two parts are connected to each other with a bolt. Main functions of a bolted joint are keeping parts together during service life and transmitting torque, moment and force between parts. Bolt sizing is very critical issue and improper design of a joint can lead failures.

Metric bolt size calculator roughly determines bolt diameter in a bolted joint according to guideline VDI 2230 Part-1 , which was prepared by the Association of German Engineers (VDI), and treats the calculation of concentrically and eccentrically clamped bolted joints.

This calculator can be used for initial sizing of a bolt and preload in a joint according to metric unit system. The tool is applicable for:

  • Bolt dimension range from M3 to M39
  • Bolt strength grades of G8.8, G10.9 and G12.9
  • Axial and transverse loading
  • Concentric/eccentric and static/dynamic axial loading

A sample example about bolt sizing for pressure vessel cap is given in  "Examples" section of this page to show usage of the calculation tool. Reference information on friction coefficients, load introduction factor and loading types are given in "Supplements" section to assist the usage of the calculator.

This calculator can be used to roughly estimate bolt diameter for a joint but more detailed calculation shall be done according to VDI 2230 Part-1 guideline.

Metric Bolt Sizing Calculator:

Parameter Value
Axial Load [FA] N
Transverse Load [FQ]
Min. friction coef. between clamped parts [μ]+ -
Axial load type and introduction location *
Tightening technique
Bolt Strength Grade  

Note 1 : +Minimum values for static friction coefficients between materials which are used in general engineering application. Reference values are given in  "Supplements" section.

Note 2 : *Graphical representation of axial loading types which is given to assist selection of loading type among the alternatives is given in "Supplements" section.

Parameter Value
Required preload FMmax ---- N
Required bolt size for selected grade ---- ---

Note 1 : This calculator is applicable for bolt dimension range from M3 to M39 and for strength grades G8.8 , G10.9,  G12.9.

Note 2 : Use dot "." as decimal separator.


Axial load: Load parallel to bolt axis. Axial load can be concentric (along the bolt axis) or eccentric.

Bolt strength grade: Strength grade that shows tensile and yield strength of a bolt.

Preload: The tension load created in a fastener when it's tightened. This tension load presses clamped parts together. Preload must be large enough to prevent joint failure.

Transverse load: Load perpendicular to bolt axis.

General Information:

Calculation methodology of the calculator is summarized below. [Reference-1]

Load (N) Nominal Diameter Definition
Strength Grade Step-1:

If only transverse or longitudinal loads exist, then F= FAmax or FQmax

If there are both transverse and longitudinal loads exist, if FAmax< FQmax/ μTmin Then F= FQmax else F= FAmax
12.9 10.9 8.8
630 Step-2:

Select the closest larger load to F from the column 1 of the table. This load is new F. F= closest larger load
1000 M3 M3 M3
1600 M3 M3 M3
2500 M3 M3 M4
4000 M4 M4 M5 Step-3:

If F= FQmax in Step-1 then increase F found in Step-2  by 4 steps using column-1 of the table and find FMmin.

If F=FAmax in step-1 then increase F found in Step-2 by following steps using column-1 of the table and find FMmin.
- Increase by 2 steps for dynamically and eccentrically applied axial load
- Increase by 1 step for dynamically and concentrically applied axial load
- Increase by 1 step for statically and eccentrically applied axial load
- No increase for statically and concentrically applied axial load
6300 M4 M5 M6
10000 M5 M6 M8
16000 M6 M8 M10
25000 M8 M10 M12 Step-4:

Find FMmax by increasing FMin using column-1 of the table with following number of steps.
- 2 steps for tightening the bolt with a simple tightening spindle which has been set by the retightening torque
- 1 step for tightening using a torque wrench or precision spindle adjusted by means of dynamic torque measurement or elongation measurement of the bolt
- No steps for tightening by means of angle control within the plastic range or by means of computer-controlled yield-point monitoring
40000 M10 M12 M14
63000 M12 M14 M16
100000 M16 M18 M20
160000 M20 M22 M24 Step-5:

Next to the found value of FMmax, select a bolt size with desired strength (Column 2 to 4)  from the relevant row .
250000 M24 M27 M30
400000 M30 M33 M36
630000 M36 M39 --

Note: Increase 1 step means, getting the value which is next row under the current value. For example if current value is 1000 N, 1 step increase will result 1600 N.


Dynamic and Concentric Axial Loading Dynamic and concentric axial loading of a bolted joint
Dynamic and Eccentric Axial Loading Dynamic and eccentric axial loading of a bolted joint
Static and Concentric Axial Loading Static and concentric axial loading of a bolted joint
Static and Eccentric Axial Loading Static and eccentric axial loading of a bolted joint

(Source: VDI 2230 Part-1 page 114)
Material combination Static friction coefficient
Dry Lubricated
Steel - Steel 0.1 to 0.23 0.07 to 0.12
Steel - Gray cast iron 0.12 to 0.24 0.06 to 0.1
Gray cast iron - Gray cast iron 0.15 to 0.3 0.2
Bronze - Steel 0.12 to 0.28 0.18
Gray cast iron - Bronze 0.28 0.15 to 0.2
Steel - Copper alloy 0.07 ---
Steel - Aluminum alloy 0.1 to 0.28 0.05 to 0.18
Aluminum - Aluminum 0.21 ---


Link Usage
Bolt Sizing Example for Pressure Vessel Cap An example about calculation of bolts size which are going to fix pressure vessel cap to a vessel. Engineering unit converter is also used for unit conversion.


  • VDI 2230 Part-1 : Systematic calculation of high duty bolted joints - Joints with one cylindrical bolt