Abstract The spin-echo attenuation corresponding to pulsed field-gradient NMR self-diffusion experiments has been simulated by Brownian dynamics. The accuracy of the Gaussian phase-distribution (GPD) approximation and the short-gradient-pulse (SGP) limit for spins confined to spheres and between planes has been investigated. The GPD approximation was found to be accurate enough for most experimental purposes, whereas the usefulness of the SGP limit is more restricted. Moreover, we present the expressions of the echo attenuation for the GPD approximation in the case of planes and for the SGP limit in the case of spheres.