Precision on primordial abundances, deduced from observations, have now reached the percent level for 4 He and deuterium. Precision on big bang nucleosynthesis (BBN) predictions should, hence, reach the same level. The uncertainty on the 4 He mass fraction is strongly affected by theoretical uncertainties on the weak reaction rates that interconvert neutrons with protons. All these corrections have been calculated in a self-consistent manner and implemented in a new, and public, Mathematica code PRIMAT [1], together with an extensive data base of reaction rates. Both can be obtained at