# Rate of heat flow

${\displaystyle {\frac {\Delta Q}{\Delta t}}=-KA{\frac {\Delta T}{x}},}$
where ${\displaystyle {\frac {\Delta Q}{\Delta t}}}$ is the rate of heat flow, ${\displaystyle -K}$ is the thermal conductivity factor, ${\displaystyle A}$ is the surfaces area, ${\displaystyle \Delta T}$ is the change in temperature, and ${\displaystyle x}$ is the thickness of the material (${\displaystyle {\frac {\Delta T}{x}}}$ is called the temperature gradient and is always negative because the heat always flows from the higher temperature to the lower).