← Back to Bead BoardHow much filler metal do I need?
Pick your joint type, weld size, and welding process. Get the total pounds of wire or rod, number of spools or boxes to order, and the estimated cost. Accounts for deposition efficiency and waste.
One-pager with your shop name and numbers. Never leaves your browser.
How to calculate filler metal for a weld joint
The formula is straightforward: figure the cross-section area of the weld, multiply by length and density to get deposited weight, then divide by deposition efficiency and add your waste factor to get the amount you actually need to purchase.
For a fillet weld, the cross-section is a triangle: half the leg size squared, times 1.2 for reinforcement (typical convexity adds about 20% over a flat-faced triangle). For a butt weld, it is the plate thickness times the root gap plus a reinforcement allowance.
The density of mild steel is 0.2836 lb/in³. Stainless is slightly higher (0.289), aluminum much lower (0.098). This calculator uses mild steel density. For stainless or aluminum fillers, the deposited weight is close enough for ordering purposes — the cost difference matters more than the weight difference.
Deposition efficiency by welding process
Values from ESAB Quick Weld Productivity Analyzer and Welding Answers deposition efficiency research. E7018 at 62% assumes 14" electrode length. Shorter electrodes (12") have higher stub-to-rod ratio and drop to 59%.
Fillet welds vs butt welds: filler metal differences
A fillet weld fills a triangular gap between two perpendicular plates. The cross-section grows with the square of the leg size — doubling the leg from 1/4" to 1/2" quadruples the metal required. This is why oversizing fillet welds costs so much: a 3/8" fillet uses 2.25× the material of a 1/4" fillet for proportionally less added strength.
A butt weld fills a groove between two plates. The cross-section depends on plate thickness and root gap. Thicker plates with large gaps can require significantly more filler than an equivalent-strength fillet on the same joint. V-grooves with backing require even more because of the bevel angle — this calculator covers the simpler square-groove case that most small fab shops encounter.
Questions welders keep asking about filler metal
How much MIG wire for a 1/4 inch fillet weld?
A 1/4" equal-leg fillet weld in mild steel deposits approximately 0.128 lb per foot of weld (0.0107 lb/inch). At 95% deposition efficiency for GMAW solid wire plus 10% waste, you need about 0.148 lb per foot purchased. A 4-foot weld needs about 0.6 lb of wire. A 10 lb spool covers about 68 feet of 1/4" fillet.
How many pounds of 7018 per foot of weld?
For a 1/4" fillet weld with E7018 stick at 62% deposition efficiency and 10% waste, you need about 0.227 lb of electrodes per foot of weld. That accounts for the stub loss and coating weight. A 50 lb box of 1/8" E7018 covers roughly 220 feet of 1/4" fillet weld, assuming typical multi-pass technique.
What is deposition efficiency in welding?
Deposition efficiency is the ratio of weld metal deposited to filler metal purchased. MIG solid wire is about 95% (5% lost to spatter). Flux-core is 78–86% (flux and spatter). Stick is 55–62% (stub ends, coating, and spatter). TIG filler rod is 97–98% (almost no loss). A lower efficiency means you buy more material per pound of weld deposited.
How do you calculate weld metal weight?
Cross-section area (in²) × weld length (inches) × density of steel (0.2836 lb/in³) × number of passes = deposited weight. For a fillet weld, cross-section = 0.5 × leg² × 1.2 (reinforcement factor). For a butt weld, cross-section = thickness × (gap + 0.1 × thickness). The 1.2 factor accounts for typical weld reinforcement/convexity.
What size MIG wire spool should I buy?
Hobby and light-duty: 2 lb spools. Small fab shop on a 200A machine: 10 lb spools (most common). Production MIG on a larger machine: 33 lb or 44 lb spools/drums. Larger spools have a lower per-pound cost and fewer changeovers. If you weld more than 10 hours per week, step up from 10 lb to 33 lb.
How much waste should I add for welding filler metal?
Standard practice is 10% for shop work (spatter cleanup, test beads, stub ends on stick, wire trim). For field work or less experienced welders, use 15–20%. For production MIG with good technique, you can drop to 5%. The waste factor is separate from deposition efficiency — efficiency covers the inherent process loss, waste covers human and operational factors.
Does multi-pass welding use more filler metal?
Yes, proportionally. A 3/8" fillet typically requires 2 passes, each depositing roughly half the total cross-section. The calculator multiplies deposited weight by number of passes. In practice, multi-pass welds use slightly more total metal than the math suggests because of inter-pass grinding and cleanup, but the 10% waste factor covers this.
Other tools in the Tack Box
Deposition efficiency data from ESAB QWPA and Welding Answers. Fillet weld formula from TWI Job Knowledge 95. Reinforcement factor 1.2× from Lincoln Procedure Handbook of Arc Welding. Other Tack Box tools.