The overall heat transfer coefficient for a multi-layered wall, pipe or heat exchanger - with fluid flow on each side of the wall - can be calculated as. 1 / U A = 1 / hci Ai + Σ (sn / kn An) + 1 / hco Ao (2) where. U = the overall heat transfer coefficient (W/ (m2 K), Btu/ (ft2 h oF))