Formulas for the calculation of the length and height of the sand-waves are suggested. These formulas are valid for steady and uniform turbulent flow and for cohesionless bed material. The judgement as to whether sand-waves are ripples or dunes is made according to the value of the grain-size Reynolds number. The suggested formulas have an extremely simple mathematical form and can, therefore, be easily used in engineering practice for the prediction of the size of irregularities of a mobile bed. The formulas were determined from a wide range of experimental data obtained from both laboratory flumes and rivers. The experimental data used covers the range of grain-sizes 0.085 mm to 2.45 mm, and the range of water depths 1.32 cm to 92.5 ft.