  This publication describes the methods used to measure the centrality of
inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding
nucleon pair with ALICE. The centrality is a key parameter in the study of the
properties of QCD matter at extreme temperature and energy density, because it
is directly related to the initial overlap region of the colliding nuclei.
Geometrical properties of the collision, such as the number of participating
nucleons and number of binary nucleon-nucleon collisions, are deduced from a
Glauber model with a sharp impact parameter selection, and shown to be
consistent with those extracted from the data. The centrality determination
provides a tool to compare ALICE measurements with those of other experiments
and with theoretical calculations.
