# 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

$$\large{ Bi=\frac{h \; l_c}{k} }$$
Symbol 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 $$\large{\frac{Btu-ft}{hr-ft^2-F}}$$  $$\large{\frac{W}{m-K}}$$