Fillet weld dimensions are given primarily by two measurements: leg length and throat thickness. Which one appears on a drawing depends on the standard being used. In the AWS (American) system, the number placed to the left of the weld symbol on the reference line represents the leg size. In the ISO/European system, either the leg length (prefixed with “z”) or the design throat thickness (prefixed with “a”) can be specified. Understanding these dimensions, where they appear on a welding symbol, and how they relate to each other is essential for reading or creating fabrication drawings.
Leg Length vs. Throat Thickness
A fillet weld sits in the corner where two pieces of metal meet, forming a roughly triangular cross-section. The two key dimensions of that triangle are the leg length and the throat thickness.
The leg length is the distance from the root of the joint (the inner corner where the two pieces touch) along each base metal surface to the toe of the weld. On an equal-leg fillet weld in a 90-degree joint, both legs are the same size. This is the dimension most commonly called out on American drawings.
The throat thickness is the shortest distance from the root of the joint to the face of the weld. For an equal-leg fillet in a 90-degree joint, the theoretical throat is 0.707 times the leg size. So a fillet weld with 1/2-inch legs has a theoretical throat of about 0.354 inches. This relationship only holds for joints at or very close to 90 degrees with equal legs and a flat or slightly convex face. If the weld surface is concave, the effective throat is reduced. Convex buildup, on the other hand, does not earn any bonus in calculations.
Engineers care about the throat because that is where a fillet weld is assumed to fail under load. The stress on a fillet weld loaded in shear is simply the applied load divided by the throat area (throat thickness multiplied by weld length). Leg size is what gets measured in the shop, and throat thickness is what gets used in design calculations.
How Dimensions Appear on Welding Symbols
In the AWS system, the weld size (leg length) is written to the left of the triangular fillet weld symbol on the reference line. If the weld is on the arrow side of the joint, it goes below the reference line; if on the other side, above it. A call for a 5/16-inch fillet weld means a 5/16-inch leg size unless otherwise noted.
The ISO system (covered by ISO 2553 and the older EN 22553) uses letter prefixes to distinguish between dimensions. A lowercase “z” before the number indicates leg length, while a lowercase “a” indicates design throat thickness. A third prefix, “s,” indicates a deep penetration throat thickness, used when the welding process is expected to melt beyond the root of the joint. So “z6” on an ISO drawing means a 6 mm leg length, and “a4” means a 4 mm throat requirement.
This distinction matters when working across international projects. A number on an AWS drawing is always the leg size. A number on an ISO drawing could be either the leg or the throat, and misreading the prefix can result in a weld that’s roughly 30% undersized.
Specifying Length and Spacing
When a fillet weld doesn’t run the full length of a joint, the drawing also specifies the weld length and, for intermittent welds, the pitch. These dimensions appear to the right of the weld symbol on the reference line.
For intermittent fillet welds, the format is typically the weld segment length followed by a dash and the center-to-center spacing (pitch). A callout like “3-12” means 3-inch weld segments spaced 12 inches apart, measured center to center. On staggered intermittent welds, the segments on opposite sides of the joint are offset from each other, and this is indicated by staggering the symbols above and below the reference line. The specified spacing values are generally treated as minimums, meaning you can weld segments closer together but not farther apart.
Minimum and Maximum Size Rules
AWS D1.1, the structural welding code for steel, sets minimum fillet weld 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 to 12 mm): minimum 3/16-inch (5 mm) fillet
- 1/2 to 3/4 inch (12 to 20 mm): minimum 1/4-inch (6 mm) fillet
- Over 3/4 inch (20 mm): minimum 5/16-inch (8 mm) fillet
These minimums exist to ensure enough heat input to avoid cracking, especially in thicker material that acts as a heat sink. The weld size never needs to exceed the thickness of the thinner part being joined.
For maximum sizes, D1.1 does not impose a hard cap on fillet welds in T-joints. Lap joints are the exception: the toe of the fillet weld should sit about 1/16 inch below the edge of the base metal, which effectively limits how large the weld can be relative to the plate thickness. As a practical rule of thumb, there’s little structural benefit to making a fillet weld larger than the thickness of the thinner member in the joint.
Deep Penetration Fillet Welds
Some welding processes (particularly submerged arc welding) consistently melt well beyond the root of the joint. When this extra penetration is reliable and verified by testing, the effective throat can be credited as larger than the standard 0.707 times the leg size. ISO drawings use the “s” prefix to call out this deep penetration throat thickness, distinguishing it from the nominal throat “a.”
Specifying a deep penetration fillet weld allows a designer to get more load capacity from a smaller visible weld, but it requires qualification testing to confirm the process consistently achieves the extra penetration. You won’t see this on routine shop drawings. It’s typically reserved for high-production structural work where the process parameters are tightly controlled.
How Fillet Welds Are Measured
In the shop and during inspection, fillet weld dimensions are checked with a fillet weld gauge, a small handheld tool with cutouts sized to standard weld dimensions. The correct way to use this gauge is to measure one leg at a time. You place the gauge flat against one base metal surface and check that the vertical tip of the gauge contacts the weld at the correct dimension. Then you flip the gauge to check the other leg from the opposite surface.
A common mistake is trying to fit the gauge so both edges contact the weld and the base metal simultaneously. Only the vertical distance from the flat base of the gauge to the tip of the cutout is accurate for checking leg size. The scribe lines sometimes marked on the concave end of the gauge are reference marks, not measurement tools for leg dimensions. The concave side of the gauge checks whether the throat of a concave weld meets the minimum throat requirement for an equivalent equal-leg triangle, which is a separate check from verifying leg size.
For unequal-leg fillet welds, the drawing will specify both leg dimensions, typically written with the two sizes separated by a multiplication sign or listed as the vertical and horizontal legs. The smaller of the two legs controls the effective throat, and both legs must be verified individually during inspection.