Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings. The law is frequently qualified to include the condition that the temperature difference is small and the nature of heat transfer mechanism remains the same. As such, it is equivalent to a statement that the heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. This condition is generally met in thermal conduction (where it is guaranteed by Fourier's law) as the thermal conductivity of most materials is only weakly dependent on temperature, but it is only very approximately true in conditions of convective heat transfer. Newton's Law is more closely followed for forced air or pumped fluid cooling, where the velocity of the fluid does not vary with temperature. Finally, in the case of heat transfer by thermal radiation, Newton's law of cooling holds only for very small temperature changes, and a more accurate description is given by Planck's Law.

Sir Isaac Newton did not originally state his law in the above form in 1701, when it was originally formulated. Rather, using today's terms, Newton noted after some mathematical manipulation that the rate of temperature change of a body is proportional to the difference in temperatures between the body and its surroundings. This final simplest version of the law given by Newton himself, was partly due to confusion in Newton's time between the concepts of heat and temperature, which would not be fully disentangled until much later.

*Newton’s law of cooling explains the rate at which a body changes its temperature when it is exposed through radiation. This is nearly proportional to the difference between the temperature of the object and its environment. However, don’t forget to keep in mind that the difference over here is quite small.

*When we use newton’s law of cooling formula, we can calculate how fast a substance at a particular temperature would cool in any particular environment. Moreover, it also tells us how the rate of cooling of an object depends on the temperature difference between the substance and the surroundings, but, also on the cooling constant of the substance.