Heat Transfer Formula

Heat Flux Formula

Heat is one of the significant components of phase change that is associated with work and energy. Heat transfer is defined as the process of flow of heat from an object at a higher temperature to an object at a lower temperature. The heat flow equation covers the heat transfer mechanism, such as the conduction equation, convection formula, thermal radiation, and evaporate cooling.

On this page, we will understand the heat transfer formula, the rate of heat transfer formula, and the type of heat flow equation in detail.

Heat Flow Equation

The formula heat energy describes the amount of heat transferred from one object to another.

So, the amount of heat transferred from one object to another is determined by the following heat transfer formula:

                 Q  =  mcΔT


Q is the amount of heat added to the system

c  =  Specific heat capacity of the system

At constant Volume, c becomes cV

Similarly, at constant pressure, c becomes cP

Besides this, 

The mass of the system is “m,” and ΔT is the temperature difference, measured in K.

The transfer of heat occurs through the following three different processes:

  •  Conduction

  •  Convection

Now, let’s understand the formula for the types of heat transfer:

Conduction Formula

Heat conduction is the transmission of internal thermal energy as a result of the collisions of microscopic particles and the motion of electrons within a body.

The conduction equation is given by:

                           q   =  - k ▽T


q  =   Local heat flux density 

- k   = material’s conductivity, and

▽T = temperature gradient

Now, talking about the heat flux formula, it is given by:

Фq  =  - k \[\frac{dT(x)}{dx}\]


Фq  =  heat flux = the heat divided by the area = \[\frac{Q}{A}\] 

Thermal conductivity is k, and T is the temperature

The SI unit of heat flux is W/m\[^{2}\] or Watt per meter square.      

Convection Formula

The convection formula is:

                    Q  =  h A ΔT 


Q = the rate of heat transfer      

h = convection heat transfer coefficient

A = the exposed surface area, and            

ΔT = the difference in temperature 

The temperature difference is between a solid surface and surrounding fluid

For the convection equation unit, we have the following heat transfer coefficient formula: 

                      h = \[\frac{Q}{\Delta T}\]

Therefore, the SI unit of convection coefficient is W/(m\[^{2}\]K).

Rate of Heat Transfer Formula

The rate of heat transfer formula is:

\[\frac{Q}{t}\] = \[\frac{kA(T_{2} - T_{1})}{d}\]


\[\frac{Q}{t}\] = rate of heat transfer in watts per second (W/s) or kilocalories per second (Kg/s)

k = a thermal conductivity of the material

(T\[_{2}\] - T\[_{1}\]) = a temperature difference across the slab

d = thickness of the slab, and

A = surface area of the slab

Thermal Conductivity Equation

The thermal conductivity equation is:

                          Q = \[\frac{Qd}{A \Delta T}\]


k = thermal conductivity, measured in W/m.K

Q = amount of heat transfer, measured in Joules/second or Watts

d = distance between the two isothermal planes

A = surface area in square meters    

ΔT = the temperature difference


The heat generated by the movement of particles in the system. Heat transfer is a process of the exchange of heat from a high-temperature body to a low-temperature body.

FAQs (Frequently Asked Questions)

Q1: What is Heat?

Ans: Heat is the form of energy that transfers between systems/objects with varying temperatures, also, referred to as heat energy/thermal energy. Heat is measured in units: Btu, calories, or joules.

Heat is a kinetic energy parameter, as a result of the motion of the particles in the system. As a system temperature rises, the kinetic energy of the particle in the system also increases.

Q2: How is the Heat Transfer Coefficient Calculated?

Ans: Heat transfer coefficient is used in calculating the heat transfer, usually by convection or phase transition between a fluid and a solid. 

The most common method of calculating the heat transfer coefficient is by dividing the thermal conductivity (k) of the convection fluid by a length scale. 

Q3: What is Meant by the Term Heat Transfer?

Ans: Heat transfer, or a few sorts of marvels, considered as mechanics, that pass on the energy and entropy from one location then onto the next. The particular mechanisms are normally alluded to as convection, warm radiation, and conduction.