We compute the full disc irradiance at 2800 MHz ('F10.7') on the basis of individual active regions (ARs) on the sun as observed in the light of the Ca II K line at 393 nm. The solar radio emission is used as a stand-in for UV measurements from above the atmosphere, since it does not have significant calibration problems, and since day by day records are available for many years. We illustrate in detail the techniques for parametrization of the model calculations which, with few changes, are applicable to UV measurements. In addition to the excellent overall agreement and the impressively close correspondence of day to day variations, we find no indication of a systematic discrepancy between model and observations over the entire 10-year period. Nor do we find a contribution to the 2800 MHz irradiance that is not directly related to individual ARs on the sun at any given day. We believe that our analysis, by comparison with previous work, is much more detailed without being much more complex, and that it offers a serious chance for an independent evaluation of short-term as well as long-term fluctuations of the solar observed irradiance.