A new method is proposed to correct shallow-water model resistance tests for the effect of the tank side walls, i.e. to correct the resistance curve towards shallow water of infinite width. From computed flow fields along ships in shallow water and channels, the tank or channel walls are found to increase the overspeed along the ship (return flow) by an amount that is roughly constant over the channel section. An algebraic equation is derived from which this amount can be solved, using a volume flux derived from a single potential-flow computation. The effect of the tank walls on the resistance curve is represented by a speed shift corresponding with this overspeed. The assumptions made are checked against computed flow fields and resistances. The overspeed increment is very well predicted; its effect on the resistance is fairly well estimated as long as it is not too strong. The method, in routine use for shallow-water tests at MARIN, is most practical; and in view of the large tank wall effects in shallow water, it is essential for good predictions.