The breakdown probability in a single-photon avalanche diode is calculated using a technique formulated by McIntyre that allows for dead space in impact ionization. Breakdown probability increases faster with overbias ratio when the dead space of impact ionizing carriers occupies a larger fraction of the avalanche region width and when the enabled ionization coefficients are more dissimilar. As the avalanche region width, and hence the operating electric field, changes, these effects may compete and partially cancel.