Your welder's manual will include a chart for setting the heat (amperage) range for the thickness of the metal you are welding. If you are using a wire feed welder, this chart will also suggest a wire speed setting. You may need to make minor adjustments for the best possible weld bead. Some new welders automatically make the proper heat and wire speed settings once the operator dials in the thickness of the metal. If you are using a stick welder, the size of the rod in thousandths of an inch is about equal to a workable welding current for that rod. All of these usually have a range of current that extends from below to above these numbers. So:
3/16 inch rod = 0.187 inch = 190 amps. 5/32 inch rod = 0.155 inch = 150 amps. 1/8 inch rod = 0.125 inch = 125 amps. 3/32 inch rod = 0.094 inch = 90 amps. 5/64 inch rod = 0.078 inch = 75 amps. 1/16 inch rod = 0.062 inch = 60 amps.
My experience indicates that your actual settings would be a little lower than these suggestions. But, it is still a neat correspondence that the rod diameter in thousandths of an inch is very near to the correct amperage setting.