Enter the two different temperatures into the calculator to determine the logarithmic mean temperature difference (LMTD).

LMTD Formula

The following formula is used to calculate a logarithmic mean temperature difference. 

LMTD = (T2 - T1) / (ln(T2/T1))
  • Where LMTD is the logarithmic mean temperature difference
  • T2 is the hotter of the two temperatures
  • T1 is the cooler of the two temperatures

To calculate the logarithmic mean temperature difference, divide the difference in temperature by the natural log of the hot temperature divided by the cold temperature.

Fahrenheit, Celsius, or Kelvin can all be used in the formula as long as T2 and T1 are in the same units.

LMTD Definition

What is LMTD?

LMTD, short for logarithmic mean temperature, is a metric used in thermodynamics to determine the “driving” force for heat transfer in heat flow systems such as heat exchanges.

For heat flow systems with a constant area and heat transfer coefficients, a larger LMTD means more heat is transferred, and a small LMTD means less heat is transferred.

Example Problem

How to calculate LMTD?

First, determine the hotter of the two ends of heat flow. For this example, the heat exchanger has an inlet temperature of 50C.

Next, determine the cooler of the two ends of heat flow. In this case, the outlet has a temperature of 30C.

Finally, calculate the LMTD using the formula above:

LMTD = (T2 – T1) / (ln(T2/T1))

LMTD = (50-30) / (ln(50/30))

LMTD = 39.15