A correlation is proposed to estimate the frictional drag for a surface covered with irregular three-dimensional roughness in the fully rough regime. The correlation relies solely on a measurement of the surface roughness profile and builds on previous work utilizing moments of the surface statistics. A relationship is given for the equivalent sandgrain roughness height (k s ) as a function of the root-mean-square roughness height (k rms ) and the skewness of the roughness probability density function (sk). Boundary layer similarity scaling then allows the overall frictional drag coefficient to be determined as a function of the ratio of the equivalent sandgrain roughness height to length of the surface.