ESDU 00006 develops a method for predicting the drag where the cavity length is large compared to the depth and the shear flow enters the cavity and attaches to the floor before separating to pass over the rear wall with a stagnation point near the top of the wall (i.e. closed flow). A family of curves suggests an upper limit of cavity depth to length ratio for closed flow in terms of free-stream Mach number and cavity width to length or width to height ratio. In supersonic flow it is possible for vortices to form as the shear flow spills over the side edges of the cavity and their impingement on the rear wall gives rise to an increase in drag for which an estimation method is also provided. Tables give the ranges of parameters covered in the construction of the method. The prediction of the ratio of the drag coefficient@ based on floor area@ to the local skin friction coefficient at the cavity mid-length station (in the absence of the cavity) is assessed to be within 1 for low-speed flow (Mach number less than 0.1) and to within 2 for high-speed flow (Mach number between 0.5 and 3). Worked examples illustrate the use of the method. A companion ESDU document@ ESDU 00007@ deals with other types of cavity flow@ known as transitional and open. The third Item in the series@ ESDU 10016@ deals with the effect on cavity drag of a pair of doors open at 90??including the effects of three different treatments of the door leading and trailing edges.