Abstract Factors of importance in partial melting calculations are discussed. The thermal evolution of a geochemical and petrological model of the upwelling asthenosphere beneath a ridge crest is studied numerically. Partial melting, basalt eruption and differentiation of the upwelling asthenosphere is modelled. Melt distribution and density distribution in the top 100 km of the upper mantle are calculated. Partial melting takes place in a depth interval of 25—60 km below the ridge crest. The degree of partial melting is somewhat less than 20 %. About 2.5 times more liquid is produced by partial melting in the upwelling asthenosphere than is erupted at the ridge centre. This excess liquid solidifies in the lithosphere, off-ridge axis below the Moho. The calculated results are in agreement with the observations on the oceanic ridge basalt composition, its average eruption rate, and geochemical estimates of the degree of partial melting in the sub-ridge upper mantle.