Watts divided by volts equals amps.

If you have the fixture the exact rating is right on the label inside on the ballast.

You need to know what you of HID lamp you have. Stan is correct, the total amps will be on the nameplate.

A400 W HPS will draw about 464 watts, a metal halide will draw about 454 to 464.

Calif, while your formula is correct, calculating amp draw of an HID lamp and ballast is not that simple. The lamp is 400 watt, the ballast also uses power, which will will show up in the total amps listed on the nameplate.

Or find the datasheet at the manufacturers website for the exact unit you have.