What is the theoretical throat of a fillet weld?

For an equal-leg fillet weld, the theoretical throat is the perpendicular distance from the root to the hypotenuse, equal to 0.707 times the leg size. A 1/4 inch leg gives a 0.177 inch theoretical throat, which is the dimension used to compute weld strength.

How does throat differ for unequal legs?

For an unequal-leg fillet, the theoretical throat is the leg-times-leg product divided by the square root of the sum of their squares, which is the altitude of the right triangle formed by the two legs. This is smaller than either leg and governs strength.

What is the effective throat?

The effective throat is the throat actually counted in design. AWS treats flat and convex fillet welds as having an effective throat equal to the theoretical throat, ignoring convex reinforcement, while a concave weld's effective throat is measured to the actual weld face and is smaller.

Why is the weld area useful?

The cross-sectional area times the weld length gives weld volume for estimating filler metal and welding time, while throat times length gives the shear-resisting area for strength. The tool reports both so you can size the weld and estimate consumables.

Does convexity add strength?

No. Design strength is based on the throat to the original triangle face, so convex reinforcement is not counted and excessive convexity wastes filler metal and can raise stress concentration. Aim for a slightly flat to flat profile for the best strength-to-deposit ratio.

Fillet Weld Effective Throat Calculator

The effective throat is the dimension that actually carries load in a fillet weld, so getting it right is the basis of every weld-strength check. This calculator finds the theoretical throat from the leg size, adjusts for weld profile to give the effective throat, and reports the cross-sectional area for strength and filler-metal estimates.

How it works

For a right-triangle fillet, the throat is the altitude from the root to the face:

equal-leg theoretical throat   = 0.707 * leg
unequal-leg theoretical throat = (leg1 * leg2) / sqrt(leg1^2 + leg2^2)
effective throat (flat/convex) = theoretical throat
effective throat (concave)     = measured to the concave face (smaller)
cross-section area             = 0.5 * leg1 * leg2  (triangle)

Design per AWS D1.1 uses the effective throat times the weld length as the shear-resisting area, while the triangular cross-section area times length gives the deposited weld volume for consumable estimates.

Example and tips

A 5/16-inch equal-leg fillet has a theoretical throat of 0.707 x 0.3125 = 0.221 inches and a cross-section area of about 0.049 square inches. A concave profile on the same legs has a smaller effective throat, so it carries less load despite the same leg size. Specify and measure the leg, keep the profile flat to slightly convex, and never rely on convex reinforcement for strength, since design ignores it.