How do you calculate power from torque?

For metric conversion:-

P=Power, kW

T=Torque, Nm

N=Engine rotational speed, rpm

P=T*pi*N/30000 or approximately T*N/9550

1 PS = 0.73549875 kW = 0.986320 hp

for imperial conversion:-

P=Power, horsepower

T=Torque, foot-pounds

N=Engine rotational speed, rpm

P=T*N/5252

The "magic numbers" are a result of the conversion steps from rotational to linear dynamics.