How much welding wire a job really needs
The wire you buy is always more than the weld metal you keep. Spatter, slag, stub ends, and burn-off all mean some filler never makes it into the joint. This calculator starts from the weld size and length, works out the deposited metal, then divides by your process deposition efficiency to give the wire or electrode you actually have to order — and how many spools that is.
The calculation
Deposited weld metal = (leg squared / 2) x length x density, for an equal-leg fillet. Filler required = deposited weight / deposition efficiency. Efficiency is the fraction of purchased filler that ends up in the weld.
Deposition efficiency by process
| Process | Typical efficiency |
|---|---|
| Stick / SMAW | 60 – 70% |
| Flux-cored / FCAW | 78 – 85% |
| MIG / GMAW (solid wire) | 90 – 98% |
| Submerged arc / SAW | 95 – 99% |
Stick loses the most to stub ends and slag, which is why a stick job needs noticeably more electrode than the same weld in MIG.
Where this fits
Size the weld first with the fillet weld calculator, then bring the length here. To turn wire, gas, and labor into a price, use the welding cost calculator.
Worked example
A 1/4 in fillet, 120 in total, in steel deposits about 1.06 lb of weld metal. At 95 percent MIG efficiency that is roughly 1.12 lb of wire — a small dent in a 33 lb spool, but it adds up fast across a production run.
FAQ
Does wire diameter change how much I need?
Not the weight. A given weld needs a given mass of filler regardless of wire diameter; diameter affects deposition rate and amperage, not total consumption.
Should I add a waste margin?
Buying about 10 percent extra is sensible for spatter, lead-ins, and the odd restart, on top of the efficiency factor already applied here.