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Weld Throat Thickness

Preamble: this article has been published in the Oct 2017 issue of the Canadian Welding Association's Welding Journal.

Some cynicism is intended in the title of this article as I have often seen mistakes made or shortcuts taken in assessing the humble, everyday fillet weld. I also intend to clarify with examples how to accurately measure fillet welds with respect to their size and shape.

A fillet weld joins two pieces at a right angle (for simplicity’s sake, let’s stick with that). Code dictates the size of weld required for a joint. The code for structural welding in Canada is W59 - Welding Steel Construction, it states: 4.3.2 Fi. Welding gauges 2. At B, throat thickness of the fillet weld is 5-6 mm which is a point on the gauge touching the centre of the face of the weld. Types of gauges for welding. On each gauge numerals are punched indicating the thickness of the work in millimeters for butt welds, and the leg in millimeters for fillet welds.

Throat thickness. TABLE 7.1: Fillet weld comparison Fillet size (mm) Weld strength relative to 4mm size Weld area relative to 4mm size Increase in weld strength for next size (%) Increase in weld area for next size (%) 4 1.00 1.00 25 56 5 1.25 1.56 20 44 6 1.50 2.25 33 78 8 2.00 4.00 25 56 10 2.50 6.25 20 44 12 3.00 9.00 33 78 16 4.00 16.00. Weld Length: Select Process: 99 Submerged Arc Welding 92 MIG (GMAW) Short Arc 95 MIG (GMAW) Spray Arc 98 MIG (GMAW) Pulsed Arc 82 Flux Cored (FCAW) E70T-4 85 Flux Cored (FCAW) E71T-1 88 Flux Cored (FCAW) E70T-5 92 Flux Cored (FCAW) E70C-6M 80 Hard Facing Gas Covered 56 Stick (MMAW) GP's 59 Stick (MMAW) Low Hydrogen 60 Stick (MMAW) Iron Powder. Throat thickness. TABLE 7.1: Fillet weld comparison Fillet size (mm) Weld strength relative to 4mm size Weld area relative to 4mm size Increase in weld strength for next size (%) Increase in weld area for next size (%) 4 1.00 1.00 25 56 5 1.25 1.56 20 44 6 1.50 2.25 33 78 8 2.00 4.00 25 56 10 2.50 6.25 20 44 12 3.00 9.00 33 78 16 4.00 16.00.

Weld Throat Thickness

Weld Features to Measure

I do need to emphasize that this article will deal with fillet welds measured by 'Leg Length' which is the standard dimensional feature specified on engineering drawings, at least in North America. This refers to distance from the root to toe and should be measured in both directions. In a future article, I will review how measuring fillet weld size based on throat dimension instead of leg size can have both engineering and productivity benefits.

To clarify the features specified in the symbol below that can be inspected for, on the arrow side of the joint a fillet weld is required with a 1/4' leg size and for a finite length of 8' (location may be noted on dwg). On the other side of the joint an unequal leg length fillet weld is required measuring 1/4 x 3/8'.

Weld Throat ThicknessWeld Throat Thickness

Assuming the length feature should be a straight forward measurement, let's look at how to properly measure leg length. 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. 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). More discussion on the various throat dimensions will follow in a future article.


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. The standard set is shown below along with some other gauges that may be used. In cases where larger or non-standard fillet sizes are specified beyond gauge capabilities, special gauges may be required.

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' 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.


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.


Weld Throat Thickness Fillet Weld Symbol

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. Listed below the image is convexity criteria from one structural welding code which should be used to assess this weld.

Convexity Criteria: Convexity, C, of a weld or individual surface bead shall not exceed 0.07 times the actual face width of the weld or individual bead, respectively, plus 1.6 mm (1/16 in).


The criteria above is particularly important for cyclic loading applications as increased convexity adds to the stress riser or notch effect at the toe of the fillet that can be a location for crack initiation. While many inspectors may make a judgement call based on experience or best guess, the image below shows how a 1/4' convex fillet should be assessed to evaluate if convexity is excessive. This will require the proper gauges to measure actual throat and yes, some mathematical calculations. As shown, the maximum measure throat dimension would be 0.264' based on this calculation. A proper gauge measuring the actual throat dimension should be used to assess if this criteria has been met.


Throat of fillet weld

One last thing … that in fact, should have been the first thing the inspector did prior to welding. If there is a gap between members being joined, the measurement methods already mentioned may not state the effective weld size. Listed below is criteria from one structural welding code that places limitation on gaps between parts being joined via fillet welds. In some cases the measured gap would be subtracted from the measured fillet leg while gaps in excess of 3/16 may cause outright rejection.

Throat Of A Weld

Fillet Weld Assembly Criteria: The parts to be joined by fillet welds shall be brought into as close a contact as practicable. Nobita ki nayi duniya. The separation between parts shall normally not exceed 5 mm (3/16 in) except in cases involving shapes and plates 75 mm (3 in) thick or greater when, after straightening and in assembly, the gap cannot be closed sufficiently to meet this tolerance. In such cases, there may be a maximum gap of 8 mm (5/16 in), provided that a sealing weld or suitable backing material* is used to prevent melt-through. If the separation is 2 mm (1/16 in) or greater, the leg of the fillet weld shall be increased by the amount of the separation.

In performing fitup and preparation inspection, inspectors noticing gaps between parts to be joined should use a proper gauge to measure the magnitude of gap.

Weld Throat Thickness Calculation

In the case of the fillet weld measured with a 1/4' leg length as shown on the left below, the weld would be rejected as undersize based on the criteria listed above. Based on the measured gap of 5/64' a measured leg of 21/64' would be result in an effective fillet size of 1/4'.

In my next article, I will look more closely at how fillet welds can be assessed based on throat dimension and emphasize important benefits this can provide. In the meantime, make sure you have fresh batteries in your flashlight, a proper suite of gauges in your inspection kit and a diligent mindset as you tackle your inspection chores.