I will consider supersymmetric gauge theories on a manifold with topology S^1 x S^3, allowing for a general class of metrics. I will present the exact computation of the partition function for these theories using localization of the path integral. The result reproduces the supersymmetric index, and includes a normalization factor that can be interpreted as a Casimir energy. The latter gives the leading order contribution in a large N expansion, and therefore provides a prediction for the on-shell action of dual gravity solutions.