Method: Log mean temperature difference is calculated from: LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ / ΔT₂) For counterflow: ΔT₁ = T_h,in − T_c,out ΔT₂ = T_h,out − T_c,in For parallel flow: ΔT₁ = T_h,in − T_c,in ΔT₂ = T_h,out − T_c,out If both terminal temperature differences are effectively equal, the LMTD approaches that common temperature difference. LMTD is commonly used with: Q = U × A × LMTD where Q is heat-transfer duty, U is overall heat-transfer coefficient and A is heat-transfer area. This calculator gives the basic LMTD for ideal counterflow or parallel-flow arrangements. Multipass shell-and-tube exchangers and crossflow exchangers may require an LMTD correction factor. The calculation also assumes that no temperature cross or invalid terminal condition exists for the selected arrangement.