The commercialization of microfluidic fuel cells remains difficult because of their low-power density. In this study, microfluidic fuel cells with a planar single-stack structure are proposed to improve the power density. The proposed stacks connect multiple cells in series, parallel, series-parallel, and parallel-series configurations. The electrolyte flow patterns of the stacks were numerically analyzed, and cell performances were experimentally measured with a platinum electrode using formic acid as the fuel. With a minimum size, these planar cell single stacks provide better power density than a single cell. The cell stack connected in parallel and then in series, where the velocity and pressure distributions of the electrolytes were simulated as almost uniform and few inner electrical connections existed, produced the best scaling-up efficiency of 1.93. Additionally, a common feed inlet configuration was developed to further reduce the size of the cell stack further. The results show that well-balanced fluid flow between inlets is necessary to obtain high scalability.