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Calculator Description
Screw Thread Class Calculator v1
The screw thread class calculator is used to calculate the tolerances and strengths of metric and imperial screw threads according to ASME B1.13M-2005 (ISO 965-1) and ASME B1.1-2003

Screw Thread Class


Standard metric and Imperial screw thread strengths and machining dimensions, including tolerances, are defined in ASME B1.13M-2005 (ISO 965-1) and ASME B1.1-2003 respectively. The dimensions and tolerances defined in these codes are for the various types of fit required for the screw-nut assembly.

Not only do these standards include tables that include fit, size and tolerance range for each screw diameter, they also provide the formulas from which they have been derived. CalQlata has used the formulas and not the tables to calculate the dimensions as there is considerably less opportunity for error. However, this means that you may notice minor differences between some of the dimensions produced in Threads+ and those published in the associated standard.


The screw thread class calculator determines the thread dimensions, machining tolerances and thread strength of the screw (or bolt) and the nut according to the above design codes.

Whilst Threads+ has been designed to work in conjunction with CalQlata’s Threads calculator, in that you would normally select a standard screw thread size in Threads and transfer its dimensions into Threads+ for machining information, you may also enter any non-standard thread you wish in Threads+ and the calculation will be equally valid.

For strength considerations, the screw thread class calculator provides the minimum and maximum recommended engagement length for metric threads but allows you to enter your own engagement length for Imperial threads as the relevant specification does not include such recommendations.

For help using this calculator see Technical Help

The Screw Thread Class Calculator - Options

You enter the necessary input data and all maximum and minimum metal conditions will be calculated for the external thread (screw or bolt) and internal thread (nut). If you enter a value for material SMYS Threads+ will calculate the mated thread strengths (including the body of the screw).

Both calculation options (see Metric and Imperial below) have the same input and output data as follows:

You enter: and the screw thread class calculator will provide:
  • Diameter
  • Pitch
  • Length of engagement (optional for metric)
  • Screw SMYS
  • Nut SMYS
  • Screw outside diameter (max)
  • Screw outside diameter (min)
  • Screw pitch diameter (max)
  • Screw pitch diameter (min)
  • Screw root diameter (max)
  • Screw root diameter (min)
  • Nut outside diameter (max)
  • Nut outside diameter (min)
  • Nut pitch diameter (max)
  • Nut pitch diameter (min)
  • Nut root diameter (max)
  • Nut root diameter (min)
  • Length of engagement (max)
  • Length of engagement (min) (metric only)
  • Screw body strength
  • Screw thread strength
  • Nut thread strength


The formulas used in this calculation option are taken from ISO 965-1 and ASME B1.13M-2005 standards and apply to all metric thread configurations. If a value is entered for the length of engagement, the nut/screw strength will be calculated for the entered length otherwise it will calculate strength based upon maximum engaged length.


The formulas used in this calculation option are based upon ASME B1.1-2003 for the UN Series of threads but may also be applied to British imperial threads such as BA, BSW, BSF, etc. You must enter a length of engagement for Imperial threads as it is used to determine machining tolerances.

Check minimum system requirements

Price: £20.00

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