§2.4B · Under Welding calculations
Fillet weld size, leg and throat on the CWI
Fillet weld size and throat are two measurements of the same cross section. Size comes from a triangle drawn inside the weld. The throat runs from the root out to the face, and on an equal-leg fillet the theoretical one works out to about 0.707 × size. Whether the face bulges or dips decides which one your gauge reads.
- Exam
- CWI
- Clause
- §2.4B
- Tags here
- 6
- Parent
- Welding calculations
2.4B.1Same legs, two faces
2.4B.2Five words for one cross section
- Leg
- Distance from the joint root to the toe, measured along each member.
- Size
- Legs of the largest right triangle you can draw inside the weld. Equal-leg fillets use an isosceles triangle. On a convex face, size and leg match. On a concave face, size comes out smaller.
- Theoretical throat
- Perpendicular distance from the root to the hypotenuse of that triangle. About 0.707 × size on an equal-leg fillet.
- Actual throat
- Root to face at the closest point. Convex fillets carry more than the theoretical throat. On a symmetric concave fillet the two are equal.
- Effective throat
- The throat used for strength. Extra convexity earns no credit. Design rules sit in D1.1 Clause 4.
2.4B.3Convex vs concave at the gauge
| Check | Convex face | Concave face |
|---|---|---|
| Size comes from | The leg (the shorter leg if unequal) | The throat, since the legs overstate it |
| Gauge side to use | Leg side of the fillet gauge | Throat side of the fillet gauge |
| Throat vs theoretical | Actual throat is bigger, but strength stays at theoretical | About equal, so any dip is a real loss |
| What can reject it | Too much convexity, overlap at the toes | Throat under the required size |
Profile limits for convexity are in D1.1 Clause 7. Check the edition you're working to. Gauge handling is on weld inspection gauges.
2.4B.4The math you'll actually do
Everything here is in the welding calculations clause, and it's short:
- Theoretical throat = 0.707 × size. A 1/4 in fillet gives 0.177 in. A 1/2 in fillet gives 0.354 in.
- Cross-section area of an equal-leg fillet = ½ × leg². 1/4 in gives 0.03125 sq in. 1/2 in gives 0.125 sq in.
- Volume = area × length. A 1/4 in fillet 36 in long holds 0.03125 × 36 = 1.125 cu in of weld metal.
Look at those areas again. Doubling the size puts four times the metal in the joint. That's why an oversize fillet costs real time, wire and heat input even when it passes.
Reading the size off the drawing
On the welding symbol the fillet size sits to the left of the weld symbol, and length and pitch sit to the right. A size on both sides of the reference line means a fillet on both sides of the joint, and the two can differ. The fillet weld symbol page walks through the layout.
Unequal legs and metric drawings
An unequal-leg fillet is sized by both legs, such as 1/4 × 3/8 in. Its theoretical throat is the product of the legs over the hypotenuse: 0.09375 ÷ 0.451 ≈ 0.208 in. Its area is ½ × 1/4 × 3/8 ≈ 0.047 sq in.
On metric work a 6 mm leg gives a throat of about 4.2 mm. ISO drawings can call a fillet out by throat (a) instead of leg (z), so read the letter before you read the number.
2.4B.5Measure it on paper
Sketch the triangle before you touch the calculator. It's part arithmetic, part naming the dimension.
0 of 6 tagged · 0 accepted
Tag 01
What is the correct method for measuring fillet weld size?
Remarks on every option
- A That's the actual throat. Fillet size is a leg dimension.
- B The toe-to-toe line is the theoretical face, not the size.
- C That's convexity, measured from the theoretical face. It isn't weld size.
- D Correct: A convex face adds metal outside the triangle, so it doesn't add size, while a concave face measures smaller than its legs suggest.
Pick an option. The remarks on all 4 open here.
Tag 02
In a recent inspection of a fillet weld, the weld's throat is observed to be consistently lower than the original design specification, leading to insufficient weld strength. Which weld discontinuity is most likely causing this issue?
Remarks on every option
- A Extra penetration adds throat at the root. It doesn't make it too small.
- B Correct: An undersized weld, short legs or a thin throat, gives less throat than designed and less strength.
- C Porosity is voids inside the weld. It doesn't shrink the measured throat.
- D Incomplete fusion is missing bond, not a dimension that runs short all along the weld.
Pick an option. The remarks on all 4 open here.
Tag 03
What is the correct term for the perpendicular distance between the root of the weld and the face of the weld?
Remarks on every option
- A Correct: The actual throat is the shortest distance between the weld root and the face of a fillet weld.
- B Face height isn't a defined term.
- C Root distance isn't a defined term.
- D Weld depth isn't the term for this fillet weld dimension.
Pick an option. The remarks on all 4 open here.
Tag 04
Calculate the throat thickness of a 3/8 inch fillet weld
Remarks on every option
- A That's two-thirds of the leg, not 0.707 × leg.
- B Not 0.707 × 0.375.
- C Not 0.707 × 0.375 either.
- D Correct: Theoretical throat = 0.707 × 0.375 in. ≈ 0.265 in. for an equal-leg fillet.
Pick an option. The remarks on all 4 open here.
Tag 05
Calculate the cross-sectional area of a 5/16 inch fillet weld in square inches
Remarks on every option
- A Too low. The area of an equal-leg fillet is ½ × leg².
- B Rounded too far. ½ × 0.3125² is 0.0488.
- C Doesn't match ½ × leg².
- D Correct: A = ½ × 0.3125 × 0.3125 ≈ 0.0488 sq in for an equal-leg fillet.
Pick an option. The remarks on all 4 open here.
Tag 06
How should excessive convexity in a fillet weld be measured?
Remarks on every option
- A Toe-to-toe width is face width, not convexity.
- B Root to face is a throat measurement. Convexity is measured from the toe line.
- C Leg length is fillet size. Convexity is how far the face bulges.
- D Correct: Convexity is the maximum distance from the line joining the toes (the theoretical face) to the weld face.
Pick an option. The remarks on all 4 open here.
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