How it works
Weld metal is a prism: its cross-section times its length. A fillet weld with equal legs is a right triangle, so its area is half the leg squared. A V-groove weld fills a wedge whose width grows with depth, plus the root opening across the full thickness.
The weight of that metal is its volume times the density of the filler. Not every pound of wire ends up in the joint, though, because some is lost as spatter, slag, stubs, and fume. Deposition efficiency is the share that reaches the weld, so you divide the weld metal weight by it to know how much to buy.
Worked example
10 feet of 1/4 inch equal-leg fillet weld in steel, welded with solid MIG wire, with a 10% allowance:
- Fillet area = 0.25² ÷ 2 = 0.0313 in²
- Volume = 0.0313 in² × 10 ft × 12 × (1 + 10%) = 4.13 in³
- Weight = 4.13 in³ × 0.2836 lb/in³ = 1.17 lb
- Filler to buy = 1.17 lb ÷ 95% = 1.23 lb
| Input | Value |
|---|---|
| Joint | Fillet weld, equal legs |
| Fillet leg size | 0.25 in |
| Plate thickness | 0.5 in |
| Groove included angle | 60 ° |
| Root opening | 0.125 in |
| Root face | 0 in |
| Total weld length | 10 ft |
| Filler metal | Carbon steel (7.85 g/cm³) |
| Deposition efficiency | Solid wire MIG/GMAW (about 95%) |
| Allowance for reinforcement and waste | 10 % |
| Result | Value |
|---|---|
| Weld metal cross-section | 0.0313 in² |
| Weld metal volume | 4.13 in³ |
| Weld metal weight | 1.17 lb |
| Weld metal per foot of weld | 0.117 lb/ft |
| Filler metal to buy | 1.23 lb |
Assumptions and limits
- The fillet is an ideal triangle with equal legs and a flat face. Convex or concave profiles change the area, which the allowance can cover.
- The groove is a single V with complete penetration and no backing bar. Bevels, U-grooves, and double-V joints have different areas.
- Deposition efficiencies are midpoints of typical ranges: 60–65% for stick, 92–99% for solid MIG, about 82–88% for gas-shielded flux-cored, 75–85% for self-shielded flux-cored, and 94–97% for TIG.
- This estimates weld metal only. It does not size the weld for strength; use the engineering drawing or code for that.
Common questions
How much does a 1/4 inch fillet weld weigh per foot?
About 0.106 lb of steel per foot with no allowance: 0.03125 in² × 12 in × 0.2836 lb/in³. Add your waste allowance on top.
What is deposition efficiency?
The weight of weld metal deposited divided by the weight of filler consumed. Stick electrodes are lowest because of the stub and flux coating; solid MIG wire is highest.
How many spools do I need?
Divide the filler-to-buy weight by the weight of the spool you use, and round up. Keep some margin for stub ends and restarts.
Why does root opening matter so much?
The gap runs the full plate thickness, so widening it adds weld metal quickly. Holding a tighter gap saves filler and heat input.
Sources
- Geometry of weld cross-sections; steel density 7.85 g/cm³ (0.2836 lb/in³), stainless 8.0 g/cm³, aluminum 2.70 g/cm³ (standard handbook densities).
- Typical deposition efficiency ranges by process, as published by welding trade sources: stick 60–65%, gas-shielded flux-cored 82–88%, self-shielded flux-cored 75–85%, solid MIG 92–99%, TIG 94–97%.
Updated 2026-09-30