Fillet Weld — What It Is and Why It Matters
A fillet weld is a triangular cross-section weld deposited in the internal corner formed by two surfaces meeting at an angle — most commonly at a tee joint or lap joint. Fillet welds are the most frequently used weld type in structural steel fabrication, accounting for roughly 80% of all welds made in the structural steel industry.
The key dimensions of a fillet weld are the leg length (measured along each surface) and the throat (the shortest distance from the root to the face). The throat determines the weld's effective cross-sectional area and therefore its load-carrying capacity. A concave fillet has a thinner effective throat than a convex fillet of the same leg length.
Fillet weld sizes are specified on engineering drawings using standard welding symbols. Typical structural fillet welds range from 3/16" to 5/16" leg size. Achieving consistent fillet welds in all positions — especially vertical and overhead — is a core skill tested in most welder qualification tests.
Frequently Asked Questions
How is fillet weld size measured?
Fillet weld size is measured by the leg length — the distance from the root of the joint to the toe of the weld along each surface. A 1/4" fillet weld has 1/4" legs on both sides. The throat (shortest distance from root to face) determines the weld's effective strength and is typically about 0.707 times the leg length for an equal-leg fillet.
What is a common fillet weld defect?
Undercut (a groove melted into the base metal along the weld toe that reduces the base metal's cross-section) and insufficient throat (the weld is too small to meet the specified size) are the most common fillet weld defects found during inspection.