Mig welders explained: what the specs mean and what to check
What a mig welder is, how to read its amps, duty cycle and input current, what gas and wire it needs, and where to go next to pick one.
“Mig welder” covers everything from a small box that plugs into a household outlet to a shop machine on its own 50 amp circuit. This page explains what the numbers on a listing mean, so you can compare them. When you are ready to choose, the ranked pages are linked at the end.
A mig welder feeds wire through a gun and shields the weld with gas. Three numbers tell you what a given machine can really do: rated output with its duty cycle, input current, and the input voltage it needs.
How it works
A motor pushes wire from a spool through a liner to the gun. The wire is both the electrode and the filler metal. When it touches the work the arc starts and melts wire and base metal together. Shielding gas flows out around the wire to keep air off the molten puddle. For a comparison with the other processes, see mig vs tig vs stick.
The numbers on a listing, decoded
| On the listing | What it means | What to look for |
|---|---|---|
| “140 amp”, “200 amp” | The top of the output range | The rated output, which is lower |
| Rated output, such as “90 A at 20 percent” | The output the duty cycle applies to | Minutes of arc per ten at the amperage you will use |
| Duty cycle | Share of ten minutes the machine can weld before cooling | The temperature or standard it was rated at |
| Input current, I1max, I1eff | What it draws from the wall | Whether your circuit can supply it |
| 120 V, 240 V, dual voltage | Which outlets it accepts | The rating at each voltage, since they differ |
| Wire sizes | Diameters the drive system accepts | That it takes the wire for your metal thickness |
| Spool gun ready | Can run an aluminum spool gun | Which gun, and whether it is included |
Each of these has its own page: duty cycle, amp draw, dual voltage and what size breaker a welder needs.
What else a mig setup needs
- A gas cylinder and regulator. The cylinder is often the biggest cost after the machine, and it is bought or leased locally. What to ask for is in mig welding gas.
- Wire. Solid wire for gas mig. Diameter follows metal thickness, and the contact tip and drive roll must match it.
- Consumables. Contact tips wear and are replaced often. Nozzles and liners less so.
- The same safety gear as any arc welding. See the list in welding for beginners.
Transformer or inverter
Older and some current machines from Hobart and Lincoln use a heavy transformer. They are simple and heavy, and their published duty cycles are modest. Inverter machines use electronics to do the same job in a much lighter case, and most budget imports are inverters. Neither type is automatically better. Compare the published numbers, model by model, in our welder comparison table.
Choosing a mig machine
- Ready to choose a gas-capable machine: best mig welders.
- First welder, household outlets: best welder for beginners.
- One machine for mig, stick and tig: best multiprocess welders.
- No gas bottle wanted: flux core welders explained.
- One machine for several processes: multiprocess welders.
- Cutting, not joining: plasma cutters explained.
- The other two processes: stick welders and tig welders.
Frequently asked questions
What is a mig welder?
A wire feed welder that shields the weld with gas from a cylinder. Pulling the trigger feeds wire from a spool, starts the arc and opens the gas. The formal name is gas metal arc welding, GMAW. Most machines sold as mig welders can also run gasless flux core wire.
What gas does a mig welder use?
For mild steel the common choice is a mix of argon and carbon dioxide, often 75 percent argon and 25 percent carbon dioxide. Pure carbon dioxide is cheaper and spatters more. Aluminum mig uses pure argon. Buy or lease the cylinder from a local gas supplier and follow their instructions for securing and storing it.
How many amps do I need in a mig welder?
Match the machine to the thickest steel you will regularly weld, using the thickness chart the manufacturer publishes for that model. Then check the rated output and duty cycle, not the top of the range, and check what input current that takes from your outlet.
Can a mig welder run without gas?
Most can, by switching to flux core wire and reversing the polarity as the manual describes. Some low cost machines are flux core only and can never use gas. If you may want gas mig later, confirm the machine has a gas valve and ships with or accepts a regulator.
Can a mig welder weld aluminum?
Some can, usually with a spool gun, which puts a small spool of soft aluminum wire at the gun so it does not tangle on the way. It needs pure argon. Check that the machine lists spool gun support and which gun fits.