Determine what kind of welding electrode you need. For instance, thoriated tungsten electrodes give good penetration and arc-starting characteristics over a variety of currents while electrodes alloyed with zirconium are used to make critical welds in the aircraft and missile industries. The type of welding electrode is critical to size since two different kinds of welding electrodes, of the same size, may provide markedly different currents.
Refer to the manufacturer's instructions for the particular material that you will be welding to determine how much welding current you require.
Use the electrode manufacturer's specifications to determine which electrode size, of the particular type and brand that you have chosen, will provide the welding current that you need. The higher current you need, the larger your electrode must be. For instance, if you require 275 amps of current, an electrode 5/16 inches in diameter will most likely provide it while an electrode 1/16-inch in diameter most likely will not.