A mathematical model of an unheated biogas plant described in this paper can be used to predict biogas generation at any given geographical location by using laboratory experimental data of methane gas generation at different values of temperature and retention time. So, this model obviates the need for repeating field experiments at different geographical locations. The introduction of new concepts of equivalent temperature and biogas coefficient help to convert laboratory data into field data and minimize the necessity of conducting field experiments. Further, retention time and size of the biogas plant for a given daily requirment of biogas are optimized through life cycle economic analysis. The model has been elaborated by calculating results for biogas generation with cattle dung in a fixed dome type biogas plant located at Ludhiana. Results are presented for various values of daily biogas requirement and cost of feedstock.