Listed below are some useful stage lighting formulas which will allow you to convert units, work out beam diameters, etc.

Light output calculations
Lux = candela / (distance in metres x distance in metres) ie distance in m squared.
Foot-candles = candela / (distance in feet x distance in feet) ie distance in ft squared

Power
Watts = Volts * Amps
Amps = Watts / Volts

Light Beam calculations
Beam diameter = distance * (2 * tan(beam angle / 2))
Throw distance = Square root ((horizontal dist. * horizontal dist.) + (vertical dist. * vertical dist.))

Unit conversions
inches = millimeters / 25.4
millimeters = inches * 25.4

feet = meters / 0.3048
meters = feet * 0.3048
footcandles = lux / 10.8
lux = footcandles * 10.8
Centigrade = 0.556 * (Fahrenheit - 32)
Fahrenheit = 1.8 * (Centigrade + 32)
pounds = kilograms * 2.205
kilograms = pounds * 0.454
degrees = 180 * radians / PI
radians = PI * degrees /180

Sine Wave (VAC)
RMS Volts = 0.707 x Peak Volts
RMS Volts = 1.11 x Average Volts

Peak Volts = 1.57 x Average Volts
Peak Volts = 1.414 x RMS Volts

Average Volts = 0.637 x Peak Volts
Average Volts = 0.9 x RMS Volts