In simple terms, the formula for horsepower is 1hp = 33,000 pound-feet per minute. For a conveyor, you will need to determine the distance to be moved in a period of time and the effects of friction. The effect of friction is that the hp required will increase by 1/(1- CF). For example if the coefficient of friction is 0.2, the hp required will need to be increased by 1/(1 - 0.2) or 1.25. Another factor that is not simple to deal with is the starting torque required if the conveyor is loaded. For that, you should consult with the motor manufacturer.