High Quality Content by WIKIPEDIA articles! The Biot number (Bi) is a dimensionless number used in non-steady-state (or transient) heat transfer calculations. It is named after the French physicist Jean-Baptiste Biot (1774–1862), and gives a simple index of the ratio of the heat transfer resistances inside of and at the surface of a body. This ratio determines whether or not the temperatures inside a body will vary significantly in space, while the body heats or cools over time, from a thermal gradient applied to its surface.