These guidelines are intended to assist countries in understanding the agronomic parameters involved in the computation of the agricultural component of the Sustainable Development Goal (SDG) indicator 6.4.1 on the change in water use efficiency over time. They provide a detailed explanation of the calculation process for calculation by countries willing to generate a more accurate estimation using their national data. The guidelines provide the minimum standard method using an estimated or default value proposed by FAO, as well as the available methodologies to progressively improve the accuracy through a monitoring ladder for countries that have more comprehensive and accurate data on their main crops areas and productions.