In situ crystallography based on diamond anvil cells have been extended to the multimegabar regime. Temperatures in these experiments have crossed the 2500 K mark. Yet, current high pressure-temperature (PT) standards of calibration produce uncertainties that inhibit clear conclusions about phenomena of importance to planetary processes, e.g., the postperovskite transition in Earth's mantle. We introduce a new thermal equation of state (EOS) of MgO which appears to be predictive up to the multimegabar and thousands of kelvin range. It is obtained by combining first principles local density approximation quasi-harmonic (QHA) calculations with experimental low-pressure data. This EOS agrees exceptionally well with shock compression data. The postspinel and postperovskite phase boundaries recalculated using our EOS match seismic observations. The latter, in particular, supports the idea that postperovskite transforms back to perovskite before the core-mantle boundary. The recalculated experimental Clapeyron slope of the postperovskite transition is also more consistent with those obtained by first principles calculations.