Precipitation scavenging coefficients, such as the HTO washout rate, are often derived from raindrop intensity and average velocity of drop. In this study, several Drop Size Distribution (DSD) functions based on experimental data are compared to calculate HTO concentration, by using Chamberlain equation, then the washout rate. Variability of Gamma, Lognormal and Marshall-Palmer distributions on the rain concentrations and washout rates are estimated. The fall velocities of drops are computed for every DSD with Kessler, Andronache, Seinfield and Loosmore and empirical formula used in the CEA code called CERES which is the CEA reference computational tool for impact assessment. Results show that the HTO concentration in drops and the scavenging coefficient depends very closely on rain characteristics. More than the choice of velocity drop calculations, which represent an uncertainty less than 20%, the choice of the distribution is important and depends on the rain observed. Certain assumptions can lead to uncertainty about a factor 2 to 3 on HTO concentration in rain and on the washout rate.