The Biot number (Bi) is a dimensionless parameter used in heat transfer analysis to characterize the relative importance of internal heat conduction within a solid body to external heat transfer at its surface. It’s named after the French physicist Jean-Baptiste Biot.

Biot number is proportional to { (thermal internal resistance) / (surface film resistance) } and is used in heat transfer in general and unsteady state calculations in particular. It is normally defined in the following form :




Mid-plane distance



Heat transfer coefficient



Thermal Conductivity


Depending on the value of the Biot number, different heat transfer regimes can be identified:

  • If Bi << 1, internal conduction dominates, and the temperature within the solid body is relatively uniform.
  • If Bi >> 1, external convection dominates, and the temperature distribution within the solid body varies significantly.
  • If Bi ≈ 1, both internal conduction and external convection are important, and the temperature distribution within the solid body is influenced by both mechanisms.

The Biot number is commonly used in various engineering applications, including thermal analysis of materials, heat exchanger design, and cooling of electronic devices.

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