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Computer models are a rapid, inexpensive way to identify agricultural areas with a high potential for P loss, but most models poorly simulate dissolved P release from surface‐applied manures to runoff. We developed a simple approach to predict dissolved P release from manures based on observed trends in laboratory extraction of P in dairy, poultry, and swine manures with water over different water to manure ratios. The approach predicted well dissolved inorganic (R² = 0.70) and organic (R² = 0.73) P release from manures and composts for data from leaching experiments with simulated rainfall. However, it predicted poorly (R² = 0.18) dissolved inorganic P concentrations in runoff from soil boxes where dairy, poultry, and swine manures had been surface‐applied and subjected to simulated rainfall. Multiplying predicted runoff P concentrations by the ratio of runoff to rainfall improved the relationship between …