ESDU Aero B.S.02.03 12 presents empirically derived curves of base pressure coefficient (based on conditions outside the boundary layer at the base of the body) for a boat-tailed body with a plain base (it is assumed there is no base bleed or jet flow but there can be a jet orifice). For a fully turbulent boundary layer the curves are plotted against the Mach number outside the boundary layer at the base (up to a value of 4). There is some effect of Reynolds number but it is small and for the ratio of base diameter to boundary layer momentum thickness above 60 the curves predict the coefficient to within 10 per cent. When the boundary layer at the base is laminar it is found that the base pressure is dependent on the position of transition in the wake, being a maximum when transition occurs downstream of the confluence of the separated layers and a minimum when it occurs in the separated layers. Curves of both maximum and minimum base pressure coefficient are plotted against the Mach number outside the boundary layer at the base, again up to a value of 4, the minimum values being approximately the same as those that occur with a fully turbulent boundary layer. As it is not possible to predict where transition will occur downstream of the base values of base pressure coefficient cannot be precisely predicted between those bounds.