The efficiency of the waterwheel is determined by the hydraulic and mechanical losses. Under normal operating conditions, there are losses during water entry and exit. At the entry the losses are dependent on the shaping of the water supply (open channel or sluice gate) and the speed at which the water exits the channel. Furthermore, the distance between the exit of the channel and the cell bottom should be minimized to reduce the losses at the entrance. For exit losses, dynamic and static losses have to be distinguished. The discharge of water begins in the lower third of the waterwheel and the water falls down a head difference, which is a static loss. Furthermore, the water has a certain kinetic energy when it leaves the cells. This momentum flow is a dynamic loss. Two methods how to determine the exit losses are presented. These losses will be calculated in more detail. The losses due to wading and overloading are only briefly discussed, as they can be avoided by proper wheel design.