A fillet weld is a triangular-shaped weld that joins two metal surfaces meeting at an angle, typically 90 degrees. It’s the most common weld type in structural and industrial fabrication, used whenever two pieces of metal overlap or form an L or T shape. Unlike groove welds, which require the edges of metal to be beveled or prepared before welding, fillet welds are deposited on the outside corner where two surfaces meet, making them simpler to set up and execute.
Where Fillet Welds Are Used
Fillet welds show up in three main joint configurations. The most recognizable is the T-joint, where one plate stands upright on another like the letter T. This is common in structural steel, equipment frames, and anywhere a stiffener or bracket attaches to a flat surface. Pipes and tubes welded onto a base plate also form T-joints that call for fillet welds.
Lap joints are another frequent application. Two overlapping plates get welded along the edge where they overlap, and the fillet weld bridges the gap at the corner. You’ll see this in sheet metal work, tank fabrication, and automotive structures. Corner joints, where two plates meet at their edges to form an L shape, can also be completed with fillet welds, though they’re sometimes finished with groove welds depending on the load requirements.
Because fillet welds don’t require edge preparation like beveling or grinding, they save significant time in fabrication. That’s a big reason they account for roughly 80% of all welds made in structural work.
Parts of a Fillet Weld
Understanding fillet weld geometry comes down to a few key measurements. The legs are the two sides of the triangular cross-section that contact each base metal surface. In a standard equal-leg fillet weld, both legs are the same length. The weld size is defined as the leg length of the largest right triangle with two equal sides that can be inscribed within the weld’s cross-section. So when someone calls out a “1/4-inch fillet weld,” they mean each leg measures 1/4 inch.
The throat is the shortest distance from the root of the weld to the face, measured through the center of the triangle. For an equal-leg fillet weld, the throat is roughly 0.707 times the leg length. This dimension matters because the throat determines how much load the weld can carry. A weld with a thick face but a thin throat is weaker than it looks.
The root is the innermost point where the two base metals meet and the weld begins. Good fusion at the root is critical: if the weld metal doesn’t fully penetrate into this junction, the joint loses strength. The face is the exposed outer surface of the weld, and the toes are the points where the weld face meets the base metal on either side.
Fillet welds can also have unequal legs, where one leg is deliberately made longer than the other to distribute stress differently or accommodate an unusual joint geometry.
Minimum Size Requirements
Fillet welds need to be large enough to avoid cracking from the rapid cooling that happens when a small weld is deposited on thick metal. The AWS D1.1 structural welding code provides minimum leg sizes based on the thickness of the base metal being joined:
- Base metal under 1/4 inch (6 mm): minimum 1/8-inch (3 mm) fillet
- 1/4 to 1/2 inch (6–12 mm): minimum 3/16-inch (5 mm) fillet
- 1/2 to 3/4 inch (12–20 mm): minimum 1/4-inch (6 mm) fillet
- Over 3/4 inch (20 mm): minimum 5/16-inch (8 mm) fillet
The weld size never needs to exceed the thickness of the thinner piece being joined. For structures that experience repeated loading and unloading (cyclic loads), the minimum fillet size is 3/16 inch (5 mm) regardless of material thickness. Most international welding standards don’t specify minimums this explicitly, making AWS D1.1 the primary reference for this guidance.
Reading Fillet Weld Symbols on Drawings
On engineering drawings, a fillet weld is represented by a small right triangle placed on a horizontal reference line. The perpendicular leg of that triangle always sits on the left side of the symbol, no matter the weld’s orientation on the actual part.
The leg size appears as a number to the left of the triangle symbol. For unequal-leg fillet welds, both dimensions are listed, separated by a multiplication sign. The weld length, if it’s shorter than the full joint, goes to the right of the symbol. When no length is shown, the weld runs the full length of the joint.
Intermittent fillet welds, where short weld segments alternate with gaps, are noted by placing the segment length to the right of the symbol, followed by a hyphen and the pitch (center-to-center spacing). For example, “2-12” would mean 2-inch weld segments spaced 12 inches apart.
Common Defects and What Causes Them
Undercut is one of the most frequently encountered fillet weld defects. It appears as a groove melted into the base metal along the toe of the weld that doesn’t get filled by weld metal, effectively creating a thin notch. Undercut weakens the joint and always requires repair. The most common causes are excessive heat, moving the torch too fast, holding the arc too far from the workpiece, or using the wrong torch angle. Poor joint preparation, like failing to clean oil, rust, or mill scale from the base metal, also contributes by disrupting how the molten weld pool flows and fuses.
Overlap is essentially the opposite problem. It happens when too much filler material builds up as a convex bead that rolls over the toe without actually fusing to the base metal. The weld looks large but isn’t properly bonded at the edges. Using filler rod or wire that’s too thick for the joint is a typical cause.
Incomplete fusion at the root is harder to spot visually but equally serious. If the welder doesn’t direct enough heat into the root of the joint, the weld metal sits on top without bonding to the base metal where it matters most. This leaves a hidden weak point that can fail under load. Proper torch angle and adequate heat input are the main preventive measures.
Profile Matters: Flat, Convex, and Concave
The shape of a fillet weld’s face affects both its strength and how it handles stress. A flat or slightly convex profile is standard for most structural applications. Convex welds have extra material built up above the triangle’s hypotenuse, which adds some throat thickness but can create stress concentration points at the toes.
Concave fillet welds curve inward, creating a smooth transition between the weld face and base metal. This profile handles fatigue loading better because the gradual transition reduces stress risers at the toes. The tradeoff is a thinner throat, so the weld size may need to be increased to compensate. Inspectors measure concave fillet welds by the leg size of the inscribed right triangle, not the throat, which means the actual load-carrying throat is smaller than it would be on a flat or convex weld of the same leg size.
How Fillet Welds Are Measured and Inspected
Fillet weld gauges are the primary inspection tool. These are small, shaped templates that a welding inspector holds against the weld to check leg size, throat thickness, and convexity or concavity. The gauge has sets of blades or notches corresponding to standard weld sizes, and the inspector selects the one that matches the required dimension and checks whether the weld meets, exceeds, or falls short of the specification.
The effective length of a fillet weld is measured along the axis where the weld maintains its full, correctly proportioned cross-section. Any crater at the start or end of the weld, where the bead tapers off, is excluded from this measurement. For intermittent welds, only the actual welded segments count toward effective length. On curved joints, the length follows the curve rather than a straight-line distance.
Beyond visual inspection with gauges, fillet welds on critical structures may also undergo ultrasonic or magnetic particle testing to detect subsurface flaws like incomplete root fusion or porosity that aren’t visible from the outside.