Root length density (RLD), a key factor for water and nutrient uptake, varies as a function of space and time, and is laborious to measure by root washing. In order to estimate RLD from root intersection density (RID), taking root orientation into account, RID was determined on three perpendicular soil planes of cubic samples and RLD was measured for the enclosed volume at different dates and positions in the soil. Results showed that the root system had a very slight preferential orientation whatever the depth, age, and tillage. This mainly isotropic root system led us to parameterise for rice a theoretical geometric model. The ratio between measured and theoretical RLD was stable and introduced as an empirical coefficient. The model was validated in two different countries (Madagascar and Ivory Coast) and accurately predicted RLD. Using this simple and generic model combined with field methods based on trench profiles, one can estimate upland rice RLD from RID.