Calibration of the CMS electromagnetic calorimeter at the LHC

The CMS electromagnetic calorimeter comprises 75848 lead tungstate scintillating crystals. The calibration of each channel is crucial to ensure excellent energy resolution. During data-taking in 2010 and 2011 a number of physics channels were used to compute the inter-calibration and absolute energy scale of the calorimeter. These included low mass di-photon resonances, electrons from Z and W decays and the azimuthal symmetry of low energy deposits from minimum bias events. The acquisition of the required data samples is described and results are presented for the precision of each method, together with the combined precision of the inter-calibration and absolute energy scale.