Simulating leaf area development is of paramount importance in crop models. The objective of this study is to develop a generic function for partitioning biomass to leaves when simulating grain crops growth. Information on total aboveground and green leaves biomass during crop development was obtained for spring barley, spring wheat, winter wheat, and corn. Leaf area index in these crops peaked near flowering. However, plant area index peaked well after flowering due to the increased contribution of reproductive structures to the plant area. In most cases, the fractional allocation of biomass to leaves decreased linearly from emergence to flowering. The initial fractional allocation to leaves ranged from 0.62 to 0.75. A bilinear relationship between leaves and aboveground biomass also accommodated some situations, with the fraction of the biomass partitioned to leaves ranging from 0.51 to 0.75 from emergence to jointing (beginning of stem elongation), and from 0.12 to 0.35 between jointing and flowering. Coupled with functions to estimate specific leaf area, these functions allow simulating leaf area index. The impact on simulating solar radiation interception of using a linear decrease in the partition to leaves as phenology progresses or using two fixed pre- and post-stem elongation partition coefficients is analyzed.