Weld Size Calculator

Size a fillet weld from a joint load, or go the other way and check how much load a chosen weld size can actually carry. Pick electrode class, enter the weld length and either the load or the leg size, and read back the required (or achievable) leg size, the effective throat thickness, and the load capacity. Everything runs in your browser — no uploads, no sign-up.

How it works

This models the standard AISC/AWS allowable-stress-design shear check for a fillet weld loaded along its length:

Allowable shear stress: τ = 0.30 × F_EXX

Effective throat: t = leg × 0.707 (a 45° equal-leg fillet)

Load capacity: P = τ × t × L

where F_EXX is the electrode's classified tensile strength (E70XX = 483 MPa / 70 ksi, the most common structural electrode) and L is the total effective weld length carrying the load.

How to use it

1. What do you want to find? — Required weld size solves for the leg from a load; Load capacity solves for the load from a leg you've already picked.
2. Joint load / leg size — whichever the mode above needs.
3. Total effective weld length — add up every run of weld sharing the load. A double fillet on both sides of a joint counts each side separately (two runs, not one).
4. Electrode classification — E70XX covers most mild-steel fabrication; switch to a higher class for higher-strength base metal, or Custom to type a tensile strength directly.
5. Thickness of the thicker part joined (optional) — fill this in to also check the result against the AWS D1.1 Table 5.7 code-minimum fillet size, which can require a bigger weld than the strength calculation alone.

In Required weld size mode, the calculated size is rounded up to the nearest fillet a shop will actually run (3, 4, 5, 6, 8, 10mm, …) — the tool then reports that practical size's real capacity, not the raw fractional-millimeter answer.

Why does the code-minimum sometimes require a bigger weld than the load calls for?

Thin welds on thick material cool too fast and are prone to cracking, regardless of how little load they're carrying — AWS D1.1 Table 5.7 sets a minimum fillet size purely from the thickness of the thicker part being joined. A weld sized only for strength can be technically adequate for the load and still fail a code check on a thick base plate.

Does this cover a transversely loaded weld, or fatigue?

No — this is the longitudinal-shear case, the one that governs sizing for most general fabrication. Transverse (end) loading, cyclic fatigue loading, and other AWS D1.1/Eurocode 3 provisions can shift the governing size and stress limit; a qualified welding engineer should verify anything load-bearing or safety-critical against the applicable code before fabrication.

What's the difference between the leg size and the throat?

The leg is the size you specify to the welder and measure with a fillet gauge — the length of each side of the weld's triangular cross-section. The throat is the shortest distance across that triangle to the joint root, which is what actually resists the shear load. For a standard 45° equal-leg fillet, throat = leg × 0.707.

Related tools

Sizing a bolted joint instead of a welded one? The Bolt Torque Calculator covers preload and tightening torque from thread size and grade. Need the weight of the plate you're welding to? The Plate Weight Calculator looks it up from dimensions and material.