Welding Calculator

Welding Strength Calculator

The Welding Strength Calculator computes the load capacity of welded joints for common weld configurations used in structural and mechanical engineering. Select from fillet welds (transverse, parallel, or combined) and butt welds (single or double-sided), enter joint dimensions and material strength, and get the weld strength in Newtons, kilonewtons, or pound-force.

Fillet welds vs. butt welds

Fillet welds are triangular cross-section welds used for lap joints, T-joints, and corner joints. The weld size $s$ (leg length) defines the triangle leg, and the effective throat thickness is $0.707 \times s$ (derived from $\cos 45^\circ$). Fillet welds are the most common type in structural fabrication because they require minimal edge preparation.

Butt welds join two plates edge-to-edge with the weld metal deposited in the gap between them. The throat thickness $t$ equals the plate thickness for full-penetration welds. Butt welds develop the full base-metal strength and are preferred for primary load paths.

Transverse fillet welds (tension)

A single transverse fillet weld resists a load perpendicular to the weld axis. Its strength is:

$$P = 0.707 \times s \times \sigma_t \times l$$

where $s$ is the weld leg size, $\sigma_t$ is the allowable tensile stress, and $l$ is the weld length. A double transverse fillet doubles the capacity:

$$P = 2 \times 0.707 \times s \times \sigma_t \times l$$

Parallel fillet welds (shear)

When the load is parallel to the weld axis, the weld is in shear. The strength of a double parallel fillet weld is:

$$P = 2 \times 0.707 \times s \times \tau \times l$$

where $\tau$ is the allowable shear stress. For structural steel, shear strength is typically $0.3$ to $0.4$ times the tensile strength.

Combined welds

A combined weld has one transverse fillet (length $l_1$) and two parallel fillets (each of length $l_2$). The total capacity is the sum:

$$P = 0.707 \times s \times \sigma_t \times l_1 + 2 \times 0.707 \times s \times \tau \times l_2$$

Combined welds are common in bracket and gusset-plate connections where the weld runs along three sides of a plate.

Butt weld strength

The strength of a single butt weld depends on the throat area and tensile stress:

$$P = t \times l \times \sigma_t$$

For a double butt weld (welded on both sides), the total throat area is the sum of both sides:

$$P = (t_1 + t_2) \times l \times \sigma_t$$

When both sides have equal throat thickness $t$, this simplifies to $P = 2 \times t \times l \times \sigma_t$.

The 0.707 factor explained

In a fillet weld, the leg size $s$ defines the visible triangle leg. The throat thickness is the shortest distance from the root to the face of the weld, measured at $45^\circ$ to the legs. This gives $\text{throat} = s \times \cos(45^\circ) = s \times \sin(45^\circ) = s \times 0.707$. The throat is the critical dimension because it is the minimum cross-section that resists the applied load.

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