How do I convert watts to amps?

Watts divided by voltage equal amps.

A 400 watt ballast plugged into ordinary house current (120v) would draw 3.33 amps.

Using a little math we can see then:

Amps = Watts / Voltage
Watts = Voltage x Amps
Voltage = Watts / Amps

Don't forget to include ballast losses!

400W ballast burns up 65 watts in core.

That's why they actually draw closer to 3.95 Amps.