This paper describes the development of a new multiple week reach model for Dutch radio data. The new reach formula has been developed to solve the inconsistency between available radio data that describe average week results and radio campaigns consisting of three to eight weeks of advertising. The result of the conducted research is a formula that enables an accurate evaluation of the reach and frequency of multiple week radio campaigns based on multiple observations in radio research. Most of todays radio research can deliver only limited data on the listening behaviour of radio audiences. Because of the constraints of methods, data covering more than one week of the radio life of respondents are rare. But radio-advertising campaigns tend to stretch out between three to eight weeks. To solve this inconsistency a new multiple week reach model for the Dutch market has been developed. This paper describes the process and results of developing this new reach calculation. An actual live case from a user perspective is included in this paper.