To help assess whether adding or continuing an ex-situ program is the best way to conserve a species in the wild, we created PACES – the Planning and Assessment for Conservation through Ex-Situ management tool. This decision support tool is designed for use in expert workshops or by the individual, and guides users through determining the best strategy for conserving species in the long-term. A case study of the long-lived, Critically Endangered western swamp tortoise is included, to show how this tool can be applied. For the western swamp tortoise, the PACES tool showed that in the long term, only an ex-situ management approach yielded benefits for population growth and a downgrading of threat status. Following a facilitated PACES tool workshop, the participating conservation managers felt that the costs of captive breeding programs were reasonable for protecting this unique and iconic species.