A formed sheet heat exchanger is provided for exchanging heat between fluids is provided. The apparatus includes flow divider sheets that are positioned in a stacked configuration and extend in a longitudinal direction so that adjacent pairs of the sheets define flow passages therebetween for receiving first and second fluids. Each of the sheets is nonuniform in the longitudinal direction, having a manifold portion and a corrugated portion. The corrugated portions of each adjacent pair of sheets define a plurality of fluid channels therebetween that are connected to the portion of the flow passage defined between the manifold portions. The fluid channels are configured to receive the first or second fluids and transfer thermal energy therebetween through the flow divider sheets.