# 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:

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

\(\large{ l_c }\) = characteristic length

\(\large{ h }\) = heat transfer coefficient of film coefficient or convective heat transfer coefficient

\(\large{ k }\) = thermal conductivity of the body