Many biomass gasification models have been developed at reactor scale with the objectives of understanding the phenomena involved and optimizing the processes. Most of usual models consist in solving equations of heat, mass and momentum balance in a multiphase media. These equations call for source terms that provide production or consumption of heat and species by the reactions. The heat and mass source terms are directly derived from the particle conversion rate, dX/dt. This paper focuses on the determination of these source terms regarding the char gasification stage of the gasification process. We propose three simple functions to calculate wood particle conversion rate of steam gasification, Boudouard reaction and combustion. The determination of each function is based on a validated char gasification model at particle size and a numerical experimental design. The model of char particle gasification includes all the complexity of the transformation: intrinsic reaction kinetic and both internal and external heat/mass transfer limitations. The experimental design was used to define simulations to be carried out, and to establish a relevant correlation between the particle conversion rate and a limiting set of 5 influencing parameters. Two of them are representative of the operating conditions - temperature and partial pressure of the reactant gas, the three others corresponds to particle properties: porosity, thickness, and reactivity constant. The three functions were determined for the gasification of a wood chips char particle in a H2O/N2, CO2/ N2, and O2/N2 atmosphere respectively. And finally, considering wood pellet char, we showed that the functions could be adapted to this new char from only 3 to 5 experiments, showing a good correspondence between prediction functions and experimental results.