Fillet Weld Effective Throat

The cross-section of an equal-leg fillet weld is a right-angled triangle with its two legs lying along the two plates. The throat is the altitude from the right angle at the root to the hypotenuse at the face — the shortest path across the weld, and therefore the plane the weld actually fails on. For a triangle with equal legs of length $z$, that altitude is $z/\sqrt{2} = 0.7071\,z$. Codes print 0.707, and this page uses the printed figure so that hand arithmetic and the calculator agree to the last digit; the difference is fifteen parts in a hundred thousand and nothing whatever depends on it.

Using the leg where the throat belongs overstates the weld by 41 per cent, because $1/0.707 = 1.414$. It is the most common arithmetic error in weld design, and it is dangerous specifically because both numbers are properties of the same weld and both are called "the weld size" in ordinary speech. The drawing gives you a leg. The calculation wants a throat. Convert once, deliberately, and write down which is which.

Four assumptions sit under the 0.707, and every one of them is routinely forgotten.

The legs are equal. An unequal-leg fillet has a throat that is neither leg times 0.707 — it is the altitude of a scalene triangle, and it has to be worked out from the actual legs or measured on a macro-etch. Unequal legs are not rare: they are specified deliberately where one plate is much thicker, and they happen accidentally whenever the gun angle drifts.

The face is flat or slightly convex. A concave fillet has a throat SMALLER than its legs suggest, because the face has been drawn in toward the root. This is the real and common overestimate of capacity, and it is insidious because a concave fillet looks tidier and more professional than a flat one — it is what a welder produces when trying to make a neat weld, particularly in the horizontal position with a fluid puddle. A fillet gauge that measures legs will pass it. A gauge that measures the throat will not. If capacity matters, measure the throat.

There is no penetration credit in this number. Some codes permit a qualified deep-penetration process — submerged arc in particular — to count root penetration as part of the effective throat, which can be a substantial gain. That credit has to be earned through procedure qualification and it belongs to a specific process, current and joint. It is not available by assumption, and it is not in the 0.707.

And the leg on the drawing is a minimum, not a target — but oversizing is not free either. Weld metal grows as the square of the leg, so a fillet welded 25 % oversize costs about 56 % more metal, more arc time, more heat into the plate and more distortion. Meanwhile the arithmetic is bracketed from both ends by code requirements this page will not guess at: a minimum fillet size on thick material, because a small weld on a heavy plate is chilled by the mass around it and cools too fast, keyed to the thicker part joined; and a maximum along the edge of a plate, so the plate edge stays visible and the weld cannot be built out past it. Both are table entries in the governing code.