Measurement of the absolute absorption coefficient of various nanomaterials over a wide spectral range is important for a variety of photonic applications but is very challenging due to strong scatterings from the intrinsically granular features of nanomaterials. We report in this paper a two-step method to determine the absorption spectrum on the absolute scale for an ensemble of nanowires: first, the relative absorption spectrum over a wide spectral range is measured from the nanowire ensemble in a carefully designed experiment using integrating sphere. Second, the absorption coefficient at a single wavelength is measured on a single nanowire to serve as the calibration of the relative spectrum. The combination of the two measurements allows the determination of the absolute absorption spectrum over a wide range. We then apply this measurement strategy to a relatively new class of nanowire materials, the erbium compound (erbium chloride silicate (ECS)) nanowires to determine its absorption spectrum. The absorption coefficient in a wide spectral range from near infrared to visible was determined for the first time for this unique erbium compound material. The measurement strategy is generally applicable to other 1D nanomaterials. The precise determination of the ECS absorption spectrum in such a wide spectral range is of vital importance to various applications of ECS nanowires including nanolasers, solar cells, and nanoscale amplifiers.