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Weld Throat

  1. The throat is the shortest distance from the root to the face of the weld. To measure this dimension in a regular mitre or flat faced fillet weld is relatively simple. The shape is that of an isosceles triangle, the throat being 0.7 of the leg length.
  2. To get the throat size for an equal-leg fillet weld simply multiple the leg size (w) by the cosine of 45˚, which is 0.707. So a 0.330 leg would yield a 0.330 x 0.707 = 0.233” throat. This is why leg sizes are called out for fillet welds.
  3. ⇒ A butt weld is specified by effective throat thickness plate thickness size of weld penetration thickness ⇒ The maximum slenderness ratio of a steel column, the design of which is governed by wind or seismic forces is 150 180 250 350 ⇒ The shape factor of an isosceles triangle for bending about the axis parallel to the base is: 1.5 1.7.
  4. Given: Weld throat thickness = 8.2 mm. Formula: Weld size (h) = t √2 - ( It is the relation between weld size and weld throat thickness) Solution: weld size (h) = t √2. Weld size (h) = 11.59 mm.

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

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.

Throat of fillet weld

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.

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'.

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 depth


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).

Actual Throat Of Weld

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.


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.

Weld Throat

Fillet Weld Assembly Criteria: The parts to be joined by fillet welds shall be brought into as close a contact as practicable. 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.

Actual Throat And Theoretical Throat

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'. Cool muster android assistant free alternative version.

Weld Throat Calculator

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.