A simplified model of an unconfined dry snow slab is used to derive expressions for the rate at which elastic strain energy is released and for the fracture resistance during the propagation of the rapid brittle fractures that result in slab avalanches. A relatively constant energy release rate is derived based on arguments that associate most of the released energy with the crown fracture. In contrast, advancing basal and crown fractures result in increasing fracture resistance. By assuming fracture propagation will stop when the strain energy release rate is just greater than the fracture resistance, the width of a slab avalanche is found to be proportional to an arrest parameter. This parameter includes the slab thickness, slab tensile strength, and basal shear strength, each of which can be measured prior to avalanching. It is argued that, compared to the three variables which can be measured, the remaining variables have limited influence on the width of unconfined slab avalanches. These less influential variables are grouped into an empirical constant that is evaluated using field data from 13 unconfined slab avalanches. The width of these slab avalanches appears to be related linearly to the arrest parameter and to be independent of whether the avalanches result from natural, skier, or explosive triggering.Key words: avalanche, avalanche width, energy release rate, fracture arrest, fracture resistance, slab tensile strength, basal shear strength.