Variational perturbation theory is used to determine the decay rates of metastable states across a cubic barrier of arbitrary height. For high barriers, a variational resummation procedure is applied to the complex energy eigenvalues obtained from a WKB expansion. For low barriers, the variational resummation procedure converts the non-Borel-summable Rayleigh-Schrödinger expansion into an exponentially fast convergent approximation. The results in the two regimes match each other well and yield very accurate imaginary parts for the energy eigenvalues. This is demonstrated by comparison with the complex eigenvalues obtained from solutions of the Schrödinger equation via the complex-coordinate rotation method.