The golden rule to determine how deep your MIG welder can weld is by multiplying every 0.001 inches of your material’s thickness by 1 amp. For a 200A machine, this means welding up to 0.2-inch thick sheets. However, this is a rough guide and should be taken with a grain of salt. A more conclusive way to determine how deep your MIG welder can penetrate is by checking with the manufacturer.