The heat transfer coefficient is a proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat. It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid. The heat transfer coefficient has SI units in watts per square meter per kelvin (W/m2/K) . The overall heat transfer coefficient, or U-value, refers to how well heat is conducted over a series of resistant mediums. The larger the coefficient, the easier heat is transferred from its source to the product being heated. The overall heat transfer coefficient is influenced by the thickness and thermal conductivity of the mediums through which heat is transferred.

In a heat exchanger, the relationship between the overall heat transfer coefficient (U) and the heat transfer rate (Q) can be demonstrated by the following equation:

Q = U x A x ΔTLM

where A is the heat transfer surface area, ΔTLM is the logarithmic mean temperature difference, and U is the overall heat transfer coefficient.

The convective heat transfer coefficient (h), sometimes referred to as the film coefficient, is included when calculating heat transfer between a fluid and a conductive wall. The heat transfer coefficient depends on both the thermal properties of a medium, the hydrodynamic characteristics of its flow, and the hydrodynamic and thermal boundary conditions.

The concept of heat transfer coefficient is also used in heat transfer with phase transformations in liquid (boiling, condensation) . When analyzing internal heat transfer in porous bodies, i.e., convective heat transfer between a rigid matrix and a fluid permeating through it, use is often made of the volumetric heat transfer coefficient.