# Biot Number

Biot Number, abbreviated as Bi, a dimensionless number, is the ratio of internal thermal resistance of solid to fluid thermal resistance. This is used for heat transfer between fluids and solids.

## Biot Number formula

\(\large{ Bi=\frac{h \; l_c}{k} }\) |

### Where:

Units |
English |
Metric |

\(\large{ Bi }\) = biot number | \(\large{dimensionless}\) | |

\(\large{ l_c }\) = characteristic length | \(\large{ft}\) | \(\large{m}\) |

\(\large{ h }\) = heat transfer coefficient of film coefficient or convective heat transfer coefficient | \(\large{\frac{Btu}{hr-ft^2-F}}\) | \(\large{\frac{W}{m^2-K}}\) |

\(\large{ k }\) = thermal conductivity of the body | \(\large{\frac{Btu-ft}{hr-ft^2-F}}\) | \(\large{\frac{W}{m-K}}\) |

Tags: Heat Transfer Equations