We propose Sirius as an improved zero-point-defining star and calibrate its spectrum to an accuracy of ~0.6% in both the visible and infrared. This result is based on a newly derived independent calibration in the visible of similar accuracy to the previous standard one, with which it is combined. We use a large variety of approaches in the infrared to reach about three times smaller error than for previous absolute calibrations. The results in the two wavelength regimes are in agreement, providing a consistent link from the visible throughout the near- and mid-infrared. The Sirius-based zero-point at 5557.5{AA} (in vacuum) is 13.436{+/-}0.081x10^-12^W/cm^2^/{mu}m, based on the improved value for Vega of 3.473{+/-}0.018x10^-12^W/cm^2^/{mu}m and the measured magnitude difference between the two stars. At 2.1603{mu}m, the zero-point is 4.225{+/-}0.025x10^-14^W/cm^2^/{mu}m taking Sirius at a magnitude of -1.395. A jackknife analysis indicates that there are no serious systematic errors in these results. We consider selection of secondary standards that can extend the calibration over the sky. Despite more than a century in this role, normal A-stars are not suitable, although Am and Ap stars may be. G-stars older than ~1Gyr are good candidates if accurate temperatures can be measured. White dwarfs are suitable from the visible through the near-infrared, but their properties are unexplored at the necessary level at the longer infrared wavelengths, and for most facilities they are too faint there. Finally, as a further test of the calibration, we demonstrate an upgraded infrared flux method to determine accurate stellar diameters from K-band photometry.