Fluids under stochastic dynamics in narrow cylindrical pores exhibit a dynamical transition from single-file diffusion (SFD) to Fickian bulk diffusion. For long time, the mean square displacement will change as the pore size increases, with a transition from SFD, similar tot(1/2), to Fickian, similar tot, while the diffusion coefficient (D-xx) increases from zero. We present a theory of this important process in terms of a hopping time, tau(hop), leading to D(xx)proportional to(tau(hop))(-1/2) which is verified with simulation. While the crossover is to be expected, the simple form is a priori unanticipated and is likely to be universal.