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Weld Strength Calculator

Check the capacity of a fillet weld to Eurocode 3 (EN 1993-1-8) - the effective throat, the normal and shear stresses on the throat, and the directional and simplified verifications with utilisation and PASS/FAIL. Butt welds, weld groups and the AISC 360 and TCVN codes are being added.

Free online calculator with step-by-step working and one-click Excel (.xlsx) and PDF export.

Using this weld calculator

What it checks

This calculator verifies a fillet weld, a butt (groove) weld or a weld group to Eurocode 3 (EN 1993-1-8), AISC 360 or TCVN 5575 - you pick the code, and the checks change with it, because each code defines weld resistance differently rather than simply restating the same rule.

For a fillet weld to EN 1993-1-8 the tool runs the directional method of clause 4.5.3.2 and the simplified method of clause 4.5.3.3 side by side, and reports the governing utilisation of the two. Both are permitted by the code, and the simplified method is the more conservative of the pair, so seeing them together tells you how much capacity a directional check would recover.

What you enter

Steel grade
S235, S275, S355 or S460 - sets f_u and the correlation factor beta_w
Leg size s
The fillet leg in mm; the effective throat a follows as s / sqrt(2)
Effective length
Length of one run, in mm, plus whether the weld is one-sided or two-sided
Design forces
Axial N_Ed (kN), shear V_Ed (kN) and in-plane moment M_Ed (kNm)

Design forces are at the ultimate limit state - the calculator does not apply load factors, so combine your loads first (the load-combination generator does this to EN 1990).

What you get back

The governing utilisation, a pass or fail against each individual check, and a step-by-step derivation showing every substitution: throat, throat area, weld section modulus, the throat stresses, and the design weld strength f_vw,d. Results export to Excel (.xlsx) and PDF.

Worked example - two-sided 6 mm fillet in S275

A bracket is welded with a 6 mm fillet on both sides of a 200 mm long connection in S275 steel, carrying an axial force of 30 kN, a shear of 50 kN and an in-plane moment of 4 kNm. These are the values the calculator opens with, so you can reproduce every figure below without entering anything.

Effective throat a = s / sqrt(2)
6 / 1.414 = 4.24 mm
Total effective length
200 mm x 2 sides = 400 mm
Throat area A_w
4.24 x 400 = 1697 mm2
Weld section modulus W_w
56 569 mm3
Normal stress from N_Ed
12.5 N/mm2
Normal stress from M_Ed
50.0 N/mm2
Perpendicular stress sigma_perp
62.5 N/mm2
Parallel shear tau_par
29.5 N/mm2
Design weld strength f_vw,d = f_u / (beta_w x gamma_M2)
430 / (0.85 x 1.25) = 404.7 N/mm2
Directional method, combined
135.0 <= 404.7 N/mm2, utilisation 0.33
Directional method, sigma_perp limit
62.5 <= 309.6 N/mm2, utilisation 0.20
Simplified method, per unit length
146.2 <= 1717.0 N/mm, utilisation 0.09
Governing utilisation
0.33 - the weld passes

The moment dominates here: 50 of the 62.5 N/mm2 perpendicular stress comes from M_Ed, not from the axial force. That is the usual pattern for a bracket, and it is why shortening the weld run is far more punishing than reducing the axial load - the section modulus goes with the square of the length.

Formula reference

Correlation factor beta_w
S235 0.80, S275 0.85, S355 0.90, S460 1.00
Effective throat, fillet
a=s2a = \dfrac{s}{\sqrt2}
Throat area
Aw=a LeffA_w = a\,L_{eff}
Weld section modulus, per side
Ww=a L26W_w = \dfrac{a\,L^{2}}{6}
Design weld strength
fvw,d=fuβw γM2f_{vw,d} = \dfrac{f_u}{\beta_w\,\gamma_{M2}}
Directional method, combined
σ⊥2+3(τ⊥2+τ∥2)≤fvw,d\sqrt{\sigma_\perp^{2} + 3(\tau_\perp^{2} + \tau_\parallel^{2})} \le f_{vw,d}
Directional method, normal limit
σ⊥≤0.9 fuγM2\sigma_\perp \le \dfrac{0.9\,f_u}{\gamma_{M2}}
Simplified method
Fw,Ed≤Fw,Rd=fvw,d aF_{w,Ed} \le F_{w,Rd} = f_{vw,d}\,a

gamma_M2 is 1.25 for the recommended EN 1993-1-8 value. The theory page derives where each expression comes from and why the directional method resolves the perpendicular force into equal sigma_perp and tau_perp components.

Assumptions and limits

The weld is assumed to be full-size along its stated effective length, with no allowance for start and stop craters; if your detail cannot guarantee that, reduce the length you enter. Throat stresses are taken as uniform over the throat, which is the basis of both EN 1993-1-8 methods.

The tool checks the weld only. It does not check the parent plate for bearing, block tearing or net-section failure, nor the member the bracket attaches to - the steel-connection tools cover those. It also assumes a static design situation: fatigue to EN 1993-1-9 is out of scope, which matters for welds in cranes, bridges and machine supports.

For the polar second moment used in weld groups, the "weld as a line" idealisation applies - see the theory page for where that idealisation stops being reasonable.

Frequently asked questions (FAQ)

You enter the leg size s, which is what appears on a drawing and what a fabricator measures. The throat a = s / sqrt(2) is derived internally and reported in the results, because the throat is the plane the resistance is checked on. A 6 mm leg is a 4.24 mm throat.

Both are permitted by EN 1993-1-8, but the simplified method ignores the direction of the stresses and so is conservative. In the worked example above the directional check governs at 0.33 while the simplified check reads 0.09 - on a different basis, force per unit length rather than stress, so the two are not directly comparable values. What matters is that each stays below its own limit. The tool reports both and takes the worst utilisation.

Enter the length of a single run, then set one-sided or two-sided. The tool multiplies to get the total effective length, so a 200 mm connection welded both sides gives 400 mm. Entering 400 as a one-sided weld would give the same throat area but the wrong section modulus, because the modulus depends on the length of each individual run.

No. The effective length you enter is taken as fully effective. Where the detail cannot guarantee a full-size weld at the extreme ends, common practice is to reduce the length by one leg size at each end before entering it.

Yes. The weld-group mode handles a group of runs resisting in-plane force and moment using the weld-as-a-line method, reporting the resultant force per unit length at the critical point. That is a separate mode from the single fillet run in the example above.

Use the code your project is designed to - the checks genuinely differ rather than being conversions of each other. EN 1993-1-8 uses the directional or simplified method on the throat with beta_w and gamma_M2; AISC 360 Chapter J2 works from a nominal weld stress with phi factors; TCVN 5575 checks two planes separately. Switching code changes the answer.

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