AbstractIt has been shown experimentally by Knudsen and others that gases flowing at decreasing total pressures through small capillaries pass from a region of Poiseuille flow to a so‐called slip flow region which contains a flow minimum. Frequently the practice has been followed of approximating this behavior with a linear relationship which, although it gives a fairly good approximation to the actual curve, is incomplete in theory.A new equation for the flow of gases in capillaries is presented, in which all flow constants can be calculated from the simple kinetic theory of gases. It is shown that this equation reproduces the experimental results of Knudsen and others over the entire range of laminar, slip, and Knudsen flow within the accuracy that might be expected from the simple kinetic model adopted.Some discussion of the use of this equation in the design of vacuum piping and its applicability to flow through porous solids is also given.