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We present numerically extracted quasi-static parasitic coupling capacitance associated with geometric overlapping in hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) in large-area a-Si imaging systems. The capacitance is extracted using a newly developed computational technique based on exponential expansion of the Green's function. Values of the computed capacitance are compared with those obtained with the parallel-plate approximation. A large discrepancy in values is found when the overlap length is small, due to the dominance of the fringing field in such geometries. Furthermore, the capacitance is found to increase with increasing permittivity of the substrate.