Spacecraft employing low-thrust propulsion must be able to recover from an event in which the spacecraft stops thrusting due to an unforeseen problem. Extra propellant is carried to allow the spacecraft to reach its target in the form of propellant margin. An automated method is presented for estimating the propellant margin a spacecraft should carry. The method is applied to three cases: a transfer to Mars, a gravity-assist trajectory to the asteroid Psyche, and a solar sail transfer to the near-Earth asteroid 1991 VG. Examining different power levels for the Mars transfer, it is shown that a 5% margin is sufficient to recover from a 20 days missed thrust for a 10 kW spacecraft, whereas a 15% margin is required for a 20 kW spacecraft. The gravity-assist case exhibits greater sensitivity to missed thrust, but this problem is alleviated by adding a coast arc before the gravity assist in the nominal case. For the solar sail mission, a spacecraft outage of more than 10 days late in the mission may delay arri...