LMTD Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026Why a simple average understates heat exchanger performance
In a heat exchanger, the temperature gap between hot and cold fluids typically changes along the length of the exchanger rather than staying constant - LMTD gives the mathematically correct average driving temperature difference for this changing gap.
Worked example
For temperature differences of 80 at one end and 20 at the other:
LMTD = (80 - 20) / ln(80/20) = 43.281 (temperature units)
A simple arithmetic average of the two end differences here would give (80+20)/2 = 50, noticeably higher than the correct LMTD of 43.28 - using the arithmetic average would overstate the true average driving force for heat transfer and lead to an undersized heat exchanger design.
LMTD is a foundational calculation in heat exchanger sizing, used alongside heat transfer coefficient and required duty to determine the necessary heat transfer surface area.