``Crosshead''‐slit dies are widely used for the extrusion of polymer sheeting, flat film, and blown film. In this paper we analyze the laminar flow of liquids in such dies, deriving an equation relating pressure to position along the slit for materials that obey the ``power law.'' The equation yields an index of uniformity (UI), which is the ratio of the rate of extrusion at the far end to that at the feed end, given byUI=1−[(1+0.05n)n&agr;L](n+1)/n(1+0.05n)(n+1)n,wheren=the exponent of the power law,L=the length of the die from feed end to far end, and &agr;=a parameter depending on the other die dimensions and the exponentn, having the unit (in)−1. An illustrated procedure is given for designing dies that will make film whose caliper is satisfactorily uniform over its whole width. We show that theUIfor center‐fed dies is always greater than for end‐fed dies of the same length.