毕奥数（Biot number，Bi）是热传学中的无因次量，以法国物理学家让-巴蒂斯特·毕奥的名字命名。

热量传递中，毕奥数指传热阻力与对流阻力之比，决定固体温度的一致性，计算式为：

其中

质量传递中，毕奥数指扩散阻力与反应阻力之比，决定固体浓度的一致性，计算式为：

其中

The Biot number (Bi) is a dimensionless quantity used in 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.