We give an analytic description of the volume of a graded linear series, as the Monge-Ampère mass of a certain equilibrium metric associated to any smooth Hermitian metric on the line bundle. We also show the continuity of this equilibrium metric on some Zariski open subset, under a geometric assumption.2000 Mathematics Subject Classification. Primary 32J25, Secondary 32W20, 32A25, 14C20.