Maintaining or increasing soil organic carbon (SOC) stocks is crucial to meet the 2050 carbon neutrality target. The contribution of crop and grassland to SOC stocks was shown to be variable in France according to the soil, climate, and management situations. To assist farmers assessing how their land use and management modifies SOC stocks, a user-friendly interactive tool was developed. This tool is based on random decision forests built on fine gridded model simulations of SOC changes over France, carried out within the framework of the ‘4p1000’ INRAE study. The tool estimates a 30-year SOC evolution (0-30cm layer) for three land use types: permanent grasslands, and crop rotations with and without temporary grassland. The tool inputs are 26 easily available information sources related to soil, climate, and length of the rotation (including the share of grassland and cover crops), fertilization and irrigation practices and grass management. The tool is a good approximation of complex process-based crop and grassland models. It is therefore a promising tool for estimating in situ SOC evolution of livestock farms.