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ABSTRACT Arelationship was developed to predict flow resistance of granular materials as a function of the minimum length of the particle dimensions and surface roughness eliminating the need for costly field calibration. The resistance term was found to be a linear function of minimum particle dimension. The slope for the relationship was independent of surface roughness; however, the intercept varied with surface roughness. Without considering surface roughness effects on the intercept, the linear relationship predicted measured resistances with an R^ = 0.85. With the materials grouped into two types, smooth and rough, and individual intercept values determined for each group, the R2 values increased to 0.99 and 0.98 for Type I (smooth) and Type II (rough) materials, respectively. All results were significant, with an alpha level of 0.001. The relationship was developed for small orifices (12 cm diameter and smaller, assumed laminar flow); however, the relationship was extended to large orifices (12 cm diameter and larger, assumed turbulent flow) for three materials by dividing the small orifice resistance coefficient by a factor of three.