The average number of prompt neutrons p emitted per fission event has been evaluated for 239Pu with a special emphasis on the fluctuations experimentally observed in the low-energy range. These fluctuations have a significant impact on applications, especially the reactivity coefficient of advanced water reactors. Consequently, the p curve has to be defined in the same fine energy mesh as the fission cross section for accurate neutron source calculations. In this range, formalisms are proposed to calculate p from the resonance parameters, resolved or averaged. Using the JEF-1 library as a data base, an analysis of several thermal, low-moderated, or fast systems shows a good convergence of the selected microscopic and integral information.