Roymech engineering encyclopedia

Fillet Weld Strength and Size Calculator




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Use this calculator to check whether an existing fillet weld is strong enough for a direct shear load, or to estimate the minimum fillet weld size required for a given load.

What does this calculator do?

This calculator performs a simple direct shear check on a fillet weld. It uses the applied load, total weld length, fillet weld size, allowable weld shear stress and safety factor to calculate the stress in the weld.

The calculator gives two useful answers:

  1. Existing weld check: it checks the weld size you enter and reports the weld shear stress, utilisation percentage and pass/fail result.
  2. Required weld size: it calculates the minimum fillet weld size required for the same load, weld length, allowable stress and safety factor.

This is useful when you either already know the weld size and want to check it, or you know the load and want to estimate what weld size is required.

Engineering note: This calculator is intended for preliminary weld sizing and educational use. Final weld design should be checked against the relevant design standard, material properties, weld procedure, loading conditions, fatigue requirements, inspection requirements and safety factors.

How to Use the Calculator

  1. Enter the applied load, P. This is the force acting on the weld. Select either N or kN.
  2. Enter the total weld length, L. This is the total effective length of weld resisting the load. For two welds, add both weld lengths together.
  3. Enter the existing or proposed fillet weld size, h. Use this if you want to check an existing or proposed weld size.
  4. Enter the allowable weld shear stress. This is the allowable stress value you want to use for the weld material or design method.
  5. Enter the safety factor. The calculator divides the allowable weld shear stress by this safety factor to give the design allowable stress.
  6. Click calculate. The calculator will show whether the entered weld size passes, and will also calculate the minimum required weld size.

Fillet Weld Diagram

3D fillet weld strength calculator diagram showing weld length L, weld size h, applied load P and effective throat thickness a equals 0.707h
Fillet weld subject to direct shear load showing weld length L, weld size h and effective throat thickness.

Formulae Used

Effective throat thickness:

a = 0.707 h

Effective throat area:

A = 0.707 h L

Weld shear stress:

τs = P / A

Design allowable stress:

τdesign = τallow / SF

Utilisation ratio:

Utilisation = τs / τdesign

Required weld size:

h = (P × SF) / (0.707 L τallow)

Variables

Symbol Description Units
P Applied load N or kN
L Total weld length mm
h Fillet weld size mm
a Effective throat thickness mm
A Effective throat area mm²
τs Calculated weld shear stress N/mm²
τallow Allowable weld shear stress N/mm²
SF Safety factor -

Calculator Inputs

Results

Enter the weld details and click calculate.

Understanding the Results

Result Meaning
Effective throat thickness The theoretical throat of the fillet weld, calculated as 0.707 times the weld size.
Effective throat area The area of weld resisting the shear load.
Weld shear stress The calculated stress in the weld caused by the applied load.
Design allowable stress The allowable weld shear stress after applying the selected safety factor.
Utilisation The percentage of the available weld strength being used. Less than 100% means the entered weld size passes this simplified check.
Minimum required fillet weld size The estimated weld size required for the entered load, weld length, allowable stress and safety factor.

What This Calculator Tells You

  • Effective fillet weld throat thickness
  • Total effective weld throat area
  • Calculated weld shear stress
  • Design allowable stress after applying the safety factor
  • Weld utilisation percentage
  • Pass or fail result for the existing weld size
  • Minimum required fillet weld size for the applied load

Fillet Weld Strength Calculation Notes

This calculator is based on a simple direct shear check for a fillet weld. It is suitable for preliminary weld sizing where the load is assumed to be distributed over the effective weld throat area.

More complex weld groups, eccentric loading, bending moments, torsional loading, fatigue loading and code-specific design checks may require additional calculations.

For further weld strength formulae and weld group properties, see the RoyMech reference page: Weld Strength Formulae and Weld Group Properties.

Fillet Weld Strength Calculator FAQ

What is the effective throat thickness of a fillet weld?

For a standard 45 degree fillet weld, the effective throat thickness is commonly taken as:

a = 0.707 h

where h is the fillet weld size.

How do you calculate fillet weld throat area?

The effective throat area is calculated by multiplying the effective throat thickness by the total weld length:

A = 0.707 h L

How do you calculate weld shear stress?

The weld shear stress is calculated by dividing the applied load by the effective throat area:

τs = P / A

How is the required fillet weld size calculated?

The required weld size can be estimated by rearranging the direct shear stress equation:

h = (P × SF) / (0.707 L τallow)

Does this calculator replace a design code check?

No. This calculator is intended for preliminary weld sizing and educational use. Final weld design should be checked using the relevant design standard, material data, weld procedure, fatigue requirements and inspection requirements.

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