Silicon undergoes a phase transition from the semiconducting diamond phase to the metallic β-Sn phase under pressure. We use quantum Monte Carlo calculations to predict the transformation pressure and compare the results to density-functional calculations employing the local-density approximation, the generalized-gradient approximations PBE, PW91, WC, AM05, PBEsol, and the hybrid functional HSE06 for the exchange-correlation functional. Diffusion Monte Carlo predicts a transition pressure of 14.0±1.0 GPa slightly above the experimentally observed transition pressure range of 11.3-12.6 GPa. The HSE06 hybrid functional predicts a transition pressure of 12.4 GPa in excellent agreement with experiments. Exchange-correlation functionals using the local-density approximation and generalized-gradient approximations result in transition pressures ranging from 3.5 to 10.0 GPa, well below the experimental values. The transition pressure is sensitive to stress anisotropy. Anisotropy in the stress along any of the cubic axes of the diamond phase of silicon lowers the equilibrium transition pressure and may explain the discrepancy between the various experimental values as well as the small overestimate of the quantum Monte Carlo transition pressure. © 2010 The American Physical Society.