# Biot number

The

**Biot number (Bi)** is a

dimensionless number used in unsteady-state and heat transfer calculations. It relates the heat transfer resistance inside and at the surface of a body.

It is defined as follows:

Where:

Values of the Biot number larger than 1 imply that the heat conduction inside the body is slower than at its surface, and temperature

gradients
are non-negligible inside it.

It is named after the French physicist Jean-Baptiste Biot (1774-1862)