Biot Number Explained: Origins, Significance, Online Calculator & In Depth Guide

March 21, 2024

What Is The Biot Number?

The Biot number, a dimensionless quantity used in heat transfer calculations. It is named after the French physicist Jean-Baptiste Biot (1774–1862). The Biot number’s is used to determine how quickly heat transfers from the surface of the body to its interior, helping to predict temperature distribution within the body under certain thermal conditions. This number is important for engineers and scientists when designing experiments and interpreting their results.

When the Biot number is much less than 1, it indicates a minimal temperature difference between the surface and the interior of the object in question, thus a small/ insignificant temperature gradient.

On the other hand, if a Biot number is much greater than 1, it suggests a greater temperature difference between the surface and interior, thus a greater temperature gradient.

Biot Number Formula

Bi = \frac{h}{k} L_c


  • Bi – Biot Number (dimensionless)
  • h – Heat Transfer Coefficient at Surface (W/m2 K)
  • Lc – Characteristic Length of Material (m)
  • k – Thermal Conductivity of Material (W/m K)

We can compute the Lc if we have the Volume (m3) and Area (m2) of a surface through which the material is cooled down or heated up.

L_c = \frac{V}{A}

Biot Number Calculator

When using the calculator below, If the Characteristic Length is known, input the value into the Characteristic Length cell. If this is unknown, make sure the Characteristic Length cell is set to “0” and then input values into both the Volume and Surface Area in order for the calculation to compute the Characteristic Length for you.

Biot Number Uses In Industry

The Biot number plays a significant role in mutliple industrial applications, particularly in processes where heat transfer is a critical factor. Below is an overview of some of the key uses of the Biot number in industry:

  1. Material Processing:
    • In industries such as metallurgy, glass manufacturing, and polymer processing, the Biot number helps in designing and optimising processes like cooling and heating. It helps in determining whether the heat transfer within a material is controlled by its internal conduction or by the external convection/radiation, which is crucial for achieving desired material properties.
  2. Food Industry:
    • The Biot number is used in the food industry to make sure proper cooling and heating are applied during processes like pasteurisation, sterilisation, and freezing. It helps in making sure that the thermal processes are efficient and that products reach the desired temperature profiles for food safety standards and the correct quality.
  3. Energy Systems:
    • In the design and analysis of energy systems such as heat exchangers, boilers, and solar collectors, the Biot number is used to optimise heat transfer efficiency. It helps in selecting materials and design that enhance heat transfer while minimising energy losses.
  4. Electronics Cooling:
    • The Biot number is critical in the design of cooling systems for electronic components, where effective heat dissipation is required for reliable operation. It is used in the design of heat sinks and cooling strategies to manage the thermal load in electronic devices.
  5. Aerospace and Automotive Industries:
    • In aerospace and automotive applications, the Biot number is used to model and analyse the thermal management of components subjected to extreme temperature conditions, such as engine parts and spacecraft shielding. It helps in designing materials and systems that can withstand thermal stresses and maintain performance.
  6. Building and Construction:
    • The Biot number is applied in the analysis of thermal insulation materials and systems within buildings. It helps to optimise insulation thickness and materials to improve energy efficiency and comfort levels within buildings.
  7. Chemical Processing:
    • In reactors and other chemical processing equipment, the Biot number helps in the design and scaling up of processes by predicting how heat transfer limitations might affect reaction rates and product uniformity.

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