We compute the value of the thermal expansivity &agr; over a wide range of compression (&eegr;≡V/V0, 0.6≤&eegr;≤1.0) and temperature (300 K≤T≤2500 K). Three methods are combined to find &agr;. We utilize: 1) the high P,T data base from the PIB ab initio model for MgO, which specifies the Helmholtz energy and the volume and temperature derivatives; 2) the measured thermal expansivity and its various pressure and temperature derivatives; and 3) several thermodynamic identities relating T and V dependence of various physical properties. We present a simple equation relating &agr; to &eegr; along isochores and suggest values of &agr; for MgO over V,P,T conditions, including those of the earth's lower mantle. The parameters in the equation are evaluated by using the ab initio data base. We find that &agr; varies from about 1.40&agr;a to 0.40&agr;a along a geotherm through the upper and lower mantle, where &agr;a is &agr; at ambient conditions. ¿ American Geophysical Union 1992