In Uccle the total ozone content of the atmosphere is measured since 1971. With a Dobson spectrophotometer and since 1983 with a Brewer spectrophotometer. For observations on the direct sun, total ozone can be derived analytically. To handle the zenith observations an empirical method has to be implemented for each instrument and each location. In Uccle direct sun observations are often prevented by clouds. Therefore, special attention must be paid at the zenith sky observations. The zenith observations are separated in two groups, zenith blue and zenith cloudy. For the zenith blue values a polynomial is determined by a least squares method, using the data of quasi-simultaneous direct sun and zenith observations. For the observations with clouds a correction, linear dependent on the total ozone content, is introduced. In this way the mean difference between daily means based on direct sun and on zenith observations is-0.1±and -0.1±1.5% for the Dobson and Brewer instrument, respectively.