Weld Strength Calculator
Enter a weld you already have — size, length and electrode — and find out how much load it can actually carry, with a clear pass / fail against your applied load and factor of safety.
How Strong Is My Weld?
A weld’s strength is set by the metal that actually fuses across the joint — not by the size of the plates it connects. Capacity comes from three things: the effective throat (the thinnest slice of weld metal that load must pass through), the effective length of weld, and the strength of the filler (your electrode). Multiply throat area by the weld metal’s allowable stress and you have its capacity. The connection is then checked against the lesser of the weld-metal strength and the surrounding base-metal strength, and divided by a factor of safety to give the load you can actually rely on.
This calculator does exactly that for fillet, groove, plug, slot and grouped welds, then tells you plainly whether your weld passes or fails for the load you enter, and by how much margin.
Fillet Weld Strength Formula
For an equal-leg fillet weld of leg size w and effective length Le, using an electrode of classification strength FEXX:
The 0.60 factor is the nominal shear strength of weld metal in AWS D1.1 / AISC. An applied combined load is resolved to a resultant, R = √(V² + T²), before comparison.
Worked example
A 1/4 in fillet weld, 6 in long on both sides, laid with an E70 electrode:
te = 0.707 × 0.25 = 0.177 in · Le = 12 in · Aw = 2.12 in²
Rn = 0.60 × 70 × 2.12 = 89 kips ≈ 89,000 lb
At a factor of safety of 2.0, Ra = 44,500 lb.
Against a 10,000 lb shear load that is only ~22% utilization — a comfortable PASS.
Weld Throat Explained
The throat is the shortest path through a weld, and it is where failure happens. For an equal-leg fillet it sits at 45° across the triangle, so the throat is 0.707 (that is, 1/√2) times the leg size. Double the leg and you double the throat and the strength; but a weld twice as long also carries twice the load, which is why length and number of sides matter just as much as size.
For a full-penetration groove weld the throat equals the plate thickness, so a properly made CJP weld develops the full strength of the base metal. For a partial-penetration groove the throat is only the depth that was actually fused.
Frequently Asked Questions
Which electrode strength should I pick?
E70 (70 ksi) is the most common choice for mild steel and is the default here. E60 suits lighter work; E80 and above pair with higher-strength steels. Always match or slightly exceed the base metal.
Does the direction of load matter?
Yes — transverse fillets are roughly 1.5× stronger than ones loaded along their length. This tool uses the conservative shear-on-throat capacity for all directions, so real transverse welds have extra margin.
Why does the base metal sometimes govern?
On thin plate the connected steel can shear before the weld does. The calculator checks base-metal shear on the connected thickness and reports whichever limit is lower.
What factor of safety is reasonable?
A factor of safety of 2.0 on the nominal weld strength matches the familiar AISC allowable value (0.60 FEXX ÷ 2.0 = 0.30 FEXX). Raise it for fatigue, impact or uncertain loads.
Related calculators
- Weld Size Calculator — work the other way: size a weld for a known load.
- Factor of Safety Calculator — choose the right safety factor for your load case.
- Stress Calculator — check the base-metal stress around the joint.
- Steel Weight Calculator & Material Weight Calculator — weigh the parts you are joining.
- Material Strength & Welding Cost calculators — coming soon to the Fabrication hub.