An article deals with the derivation of machined layer cross section calculation formula for rotary ultrasonic machining with a hollow drill. The calculation of machined layer cross section is one of the basic characteristics of all machining technologies in technology of machining. The calculation is based on the kinematic characteristics of the Rotary Ultrasonic Machining process which is the combination of grinding and drilling. The kinematics is described as well. The cross section of machined layer is changing during the process in Rotary Ultrasonic Machining. The calculation of cross section consequently depends on the angular rotation of the tool. Because of that, for the calculation of cross section a simplification was used where the maximum deviation was only taken into account and the cross section is than considered as approximately constant. Final equation for approximated machined layer cross section is proposed as well as the equation for mean machined layer cross section in rotary ultrasonic machining with a hollow drill.