Thermodynamic temperature

When pressure of real gas is decreased, it act as an ideal gas. Ideal gas has relationship between pressure p, volume V, temperature T is

pV/T = constant

T is thermodynamic temperature of the body, and measured in Kelvin (ºK)

One kelvin is the fraction of 1/273.16 of the

thermodynamic temperature of the triple point of water. 

To avoid complications and confusion between empirical centigrade scale based on experiments with real gases and constant-volume gas thermometer scale, a new scale, the Celsius scale was defined by international agreement which is 

Unit of Celsius scale, which is degree Celsius (℃)
has same scale as thermodynamic scale, which is Kelvin(ºK).

Thus, Temperature θ ℃= T/K - 273.15 or T/K = θ ℃ + 273.15

To avoid confusion of absolute zero, which is 273.16, and number used in this equation, 273.15, and for various purposes, it is sufficient to work with the number 273