What is the formula for working out the volts, amps and watts of a DC Motor?
The power of a motor is the product of its speed and torque. The power output is greatest at about half way between the unloaded speed (maximum speed, no torque) and the stalled state (maximum torque, no speed). 

The output power in watts is about (torque) x (rpm) / 9.57.

The efficiency of a motor η = Shaft Power Out/Electric Power In

Ref:http://www.me.ua.edu/me416/LECTURE%20MATERIALS/MotorEffic&amp;PF-CM5.pdf

(vector) Input Power = V·I and Loss = I²R 

If you can measure real power V*I but cannot measure HP, assume 87~90% for  η