A fillet size needs the geometric quantity it reports.
Define the ideal cross-section
TWI distinguishes fillet-weld leg and throat dimensions. For an ideal right-angle, equal-leg triangular section with flat face, geometric throat is leg length divided by √2. Primary reference: TWI technical explanation.
The stated shape assumption matters. Unequal legs, convexity, concavity or penetration need their relevant section interpretation; the simple geometric relation should not silently describe every actual weld profile.
Calculate a fictional section
Assume an invented ideal leg length of 8 mm. Its geometric throat is 8/√2, approximately 5.657 mm, while the triangular cross-sectional area is 8²/2 = 32 mm². A bare value eight cannot describe both leg and throat.
If another record specifies throat 8 mm for the same ideal shape, the corresponding leg would be approximately 11.314 mm. These different sections do not share the same dimensions merely because their callouts contain the same number.
Keep geometry separate from capacity
Make a cross-section worksheet with leg convention, throat convention, face shape and the assumed root point. Retain the actual drawing requirement alongside the interpretation.
Request the appropriate joint and design evidence for capacity or acceptance. This article calculates geometry only; it supplies no permitted load, stress limit or approved weld size. Its useful outcome is a correctly identified section before a quote or inspection description is compared with the intended requirement.
Customer Questions
What is the invented eight-millimeter leg throat?
Approximately 5.657 mm under the ideal shape.
Does throat eight mean leg eight?
They are different geometric quantities.
Does the calculation establish load capacity?
The actual joint/design assessment is separate.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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