# Biot Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

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