ESDU 87004 gives methods for estimating the factor by which the drag due to an isolated excrescence on a flat plate (for which ESDU 74036, 75031, 76008, 79015 and 84035 provide data) must be multiplied to obtain the drag increment when it is in a pressure gradient. The methods were developed theoretically by computing the change in drag of the aerofoil at constant lift when an increment of momentum thickness is introduced in the boundary layer calculation to represent the excrescence. The factor therefore includes the change in drag due to the incidence change made to keep the lift constant. Calculations were made for two aerofoils, designed for high subsonic speeds, over a range of Mach number and incidence, and graphs show the dependence of the factor on lift and excrescence chordwise position, together with the corresponding pressure distributions on upper and lower surfaces. Since the magnification factor depends primarily on pressure gradient, estimating the factor for any other aerofoil requires only that a similar pressure gradient to that at the excrescence location be located on these graphs, and the corresponding magnification factor read. A second method of calculating the magnification factor is also given that depends on the flow properties at the excrescence location and at the trailing- edge. The calculations are found to be largely independent of Reynolds number over a realistic range, although some variation was seen when shock waves were present. The accuracy of the results is discussed, with a comparison of both methods with one set of reliable data drawn from the literature. The full calculations, estimates using a matching pressure gradient, and results calculated from the flow properties all agree within 10 per cent.