Denitrification bioreactors for treatment of nitrate in agricultural drainage have recently gained popularity as a water quality improvement strategy in several agricultural regions. Due to increased interest in this new technology, many bioreactors have been installed in the field though design methodology is still under development. In order to improve design techniques, it is valuable to communicate these potential methodologies. This paper presents one such design approach to generate dimensions (length and width) for denitrification bioreactors treating agricultural drainage. Though this is just one approach, it is an engineering-based method developed to address the wide fluctuations in agricultural drainage flow rates. By coupling Darcy's equation and the retention time model, bioreactor dimensions that allow treatment of a portion of peak estimated flow rate at an adequate retention time can be developed. The use of Darcy's flow through porous media equation allows calculation of an allowable bioreactor flow rate given an iteratively chosen length and width. By then using these dimensions in the retention time model, an appropriate length and width can be refined. In addition to explaining the design method, this paper presents a range of values for each model parameter to allow site specific designs. Also, several denitrification bioreactor designs generated using this technique are described. Though this design approach is only a preliminary method, it provides a starting place for design optimization and further bioreactor design enhancement.