Computer-generated trochoids accurately describe profiles of both natural and laboratory antidunes. The ratio of current velocities at crest and trough can be calculated for flow over stationary trochoidal waves of any particular steepness, and must equal the inverse ratio of water depths at these positions. Antidune height H b can therefore be expressed as a function of water wave height H w , wave length L, and mean water depth d:H b = H w (1 - 2pi d/L). By assuming the steepest possible water waves (H w /L = 0.142), it is possible to derive a maximum estimate of water depth for a set of preserved antidunes. A technique for reconstructing the original bedform from a small amount of preserved cross-lamination allows these calculations to be applied even where the antidunes themselves have not been preserved.