Weld size and weld metal
Fillet weld cross-section and weld metal mass per metre
- where
- weld cross-section area (mm²)
- equal leg length (mm)
- convexity, cap height above the flat face (mm)
- weld metal mass per metre (kg/m)
- weld metal density (kg/m³)
- total weld metal mass (kg)
- total weld length (m)
Explain this equation
Works out the cross-section of a fillet weld and how much weld metal it takes per metre and in total. Use it for estimating consumables and welding time.
The basic fillet is a right triangle of legs z, area z²/2. Any convexity adds a curved cap across the face, whose width is √2 z; a parabolic cap has two-thirds of the area of the rectangle around it. Area in mm² times density in kg/m³, divided by a million, gives kg per metre (1 mm² × 1 m is 10⁻⁶ m³).
Triangle plus parabolic segment, then volume times density.
Sense check: an 8 mm flat fillet is 32 mm², about 0.25 kg per metre of steel weld metal. Weld metal goes up with the square of leg length, so a 10 mm fillet needs about 56% more than an 8 mm.
Watch out: this is deposited metal. Divide by deposition efficiency to get wire or electrodes to buy.
Works out how much weld metal goes into an equal-leg fillet: the triangle plus any convex cap. Use it to estimate wire or rods and arc time.
Enter your values to see the result.
Good to know
The triangle is z²/2. The convex cap is treated as a parabolic segment over the face width b = √2 × z, area 2/3 × b × h. Steel weld metal is taken as 7850 kg/m³ (use about 8000 for austenitic stainless). An 8 mm leg flat fillet is about 0.25 kg/m.
Common mistake
Ignoring oversize. A fillet that is 2 mm over on the leg carries over 50 % more metal on an 8 mm weld, which is where much consumable cost goes.
Limits
Deposited metal only: excludes spatter, stubs and slag (see the filler consumption calculator). Penetration and fused parent metal are not counted.
Sources
- BS EN ISO 2553:2019 Welding and allied processes – Symbolic representation on drawings – Welded joints (a = throat thickness, z = leg length)
- Kiwa: ISO 5817 quick guide (excess weld metal 502 and excessive convexity 503: h ≤ 1 + 0.25b / 0.15b / 0.1b for levels D / C / B; fillet maxima 5 / 4 / 3 mm)
- BS EN ISO 5817:2023 Welding – Fusion-welded joints in steel, nickel, titanium and their alloys – Quality levels for imperfections
- BS EN 1991-1-1:2002 Eurocode 1, Annex A: nominal density of steel 77.0–78.5 kN/m³ (7850 kg/m³ is the usual design value)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.