In a standard piston engine (an Otto or Diesel engine), displacement is calculated by multiplying the number of cylinders in the engine with the area of a piston and the length of the stroke. With circular pistons, displacement can be calculated from the bore diameter and stroke using the following formula:
displacement = pi /4 * bore^2 * stroke * number of cylinders
Displacement in a Wankel engine is much more complicated. For instance,Mazda's 13B is a two-rotor engine with combustion chambers of roughly 0.65 liters in displacement (difference in max vs. min combustion chamber volume). At 100% volumetric efficiency, 0.65 liter per rotor face * 3 faces per rotor * 2 rotors gives a total displacement of 3.9 liters. It takes 3 rotations of the eccentric shaft to complete one engine cycle, however. In 2 rotations of the eccentric shaft, comparable to 2 crankshaft rotations on a 4-stroke piston engine, the 13B would displace 2.6 liters. Mazda advertises the 13B as a 1.3 liter engine, which is the volume displaced during a single rotation of the eccentric shaft.