Tantalum pentoxide films grown anodically on tantalum foil have been used as reference materials for a variety of purposes. For both the calibration of depth resolution in sputter-depth profiling instruments using Auger electron spectroscopy, and for calibrating the sputtering rate in these instruments, the certified reference material BCR 261 has been developed at NPL. Tantalum pentoxide reference materials have also been developed in Paris and at Chalk River with considerable characterization. Here we bring these separate standards to one common absolute calibration, show all to be of identical composition, and show that the sputtering rate is constant with depth. This establishes the NPL/BCR material for determining sputtering flux densities, and hence for calibrating Faraday cups, and also for calibrating nuclear scattering instruments for absolute measurements.