The electronic structure of calcium under pressure is re-examined by means of self-consistent energy band calculations based on the local density approximation and using the linear muffin-tin orbitals (LMTO) method with corrections to the atomic sphere approximation included. At zero pressure the topology of the Fermi surface as well as the calculated extremal areas are found to depend strongly on the positions of the p and d bands. The positions of the centres of the p and d bands are used as adjustable parameters in order to fit the measured Fermi surface. It is found that shifts of +0.037 and +0.149 Ryd, respectively, relative to the s band, give the best possible agreement. Under increasing pressure the s and p electrons are found to transfer into the d band, and Ca undergoes metal-semimetal-metal electronic transitions. Calculations of the bandstructure and the electronic pressure, including the adjustment to the zero-pressure Fermi surface, allow a simultaneous fit of the measured equation of state and the high-pressure resistivity data to be made.