July 14, 2020

Structural Welds – Measuring Fillet Welds

Structural welds

My previous blog on welds got some interest as well as some queries, so today I’m following up on the issue of fillet weld size, leg length, and measurement.

My source is an article published in the Oct 2017 issue of the Canadian Welding Association Welding Journal. I apologise in advance for the imperial references!

So let’s recap. A fillet weld is specified by a size, for example 6 mm. But how do we define that? How do we measure that?

The size of a fillet weld is determined by its ‘leg length’.

Structural Welds - Weld length

The figure is pretty theoretical – it depicts an equilateral triangle – whereas a real weld is never flat, and will be either concave or convex, and not necessarily equal-length legs.

For strength calculations, we’re interested in the throat thickness. We get that by simply dividing the leg length by √2 because we’re used to specifying equal-leg welds. It’s therefore pretty important to ensure the specified weld is big enough to achieve the design throat thickness.

Measuring the weld size

The leg length refers to distance from the root to toe and should be measured in both directions.

The leg size is the length of the legs of an imaginary triangle that can be inscribed within the actual weld as shown with feature L1 and L2 in the image below for both a convex and concave fillet weld.

Structural welds - Convex & Concave

In the case of the convex fillet weld shown on the left, the measured leg (L) is equivalent to the size (S). In the case of the concave fillet shown on the right, the measured leg (L) provides an untrue measure of weld strength which is why these welds are instead assessed based on throat dimension to determine effective weld size (S).


In most cases, a standard fillet weld gauge set is used to verify that the leg length matches the specified size. These gauges may not measure exact size but instead verifies that the specified size has been attained. A typical gauge form BOC Gas is shown here:

Structural welds

When using the standard fillet weld set, the weld should first be visually examined and if it appears to be flat or convex, it should be assessed based on leg length only as shown below. The image below shows a fillet weld being inspected to verify the specified 3/8″ (9.5mm) leg size. If the fillet instead appears to be concave, the weld should instead be assessed using the side of the gauge with the centre tab which would need to touch the face of the weld.

Structural welds Gauge


In the next scenario, the measured leg length is again 3/8″ but the weld should be rejected as undersize due to its concave profile resulting in an effective weld size of only 1/4″ as shown on the right below. The black lines you see on the gauge points to the effective toe of the imaginary triangular fillet.

Structural welds - Convevity


While the above example illustrates how concave fillet profiles should be scrutinized, fillets that are convex may also be rejected due to profile or shape. At first glance, this fillet appears to be acceptable based on a specified leg length of 1/4″. In examining its profile however, noticeable convexity is evident.

Structural welds

To much convexity adds to the stress riser or notch effect at the toe of the fillet that can be a location for crack initiation. Such welds should be avoided in cyclic loading applications. Our welding code, AS 1554.1, places limits on the amount of convexity in welds.

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