Canopy architecture has a prominent role in fundamental processes of crop growth including light interception, evapotranspiration and photosynthesis. A widely used bio-physical parameter to quantify vegetative canopy architecture is leaf area index (LAI). The objective of this study was to develop a quantitative model to predict LAI for sorghum (Sorghum bicolor (L.) Moench) from emergence to flag leaf stage. Measurements for two cropping seasons included LAI, leaf number, leaf length, maximum leaf width and leaf area for eight sorghum genotypes under water-and nutrient-sufficient field conditions. LAI was calculated from an algorithm developed to consider the area of mature leaves (leaves with a ligule/collar), area of expanding leaves (leaves without a ligule/collar), total leaf area per plant and plant population.  Linear functions were used, for all mature leaves, to quantify a) leaf length from leaf sequence number and b) maximum leaf width from leaf length. The leaf shape factor (slope of the regression line between leaf area and product of leaf length and maximum width) was constant (0.73 in 2009 and 0.81 in 2010) for all mature leaves across all genotypes. Phyllochron (thermal time between ligule development of successive leaves on the culm) varied among genotypes. Area of mature leaves was calculated as the product of length, maximum width and shape factor. Area of expanding leaves was linearly related to length, relative to expected mature length and was modeled assuming linear leaf expansion rates. Slope of regression of modeled versus observed LAI varied for photoperiod sensitive (PPS) and insensitive (non-PPS) genotypes for the year 2010. A good correlation was found between the modeled and observed LAI with coefficient of determination (R²) 0.96 in 2009 and 0.94 (non-PPS) and 0.88 (PPS) in 2010. The proposed model has applications in canopy light distribution and interception studies and identification of drought stress on crops.