When pressure approaches zero, all real gas will behave like ideal gas, that is,  of a mole of gas relying only on temperature. Therefore, we can design a scale with  as its argument. Of course any bijective function will do, but for convenience's sake a linear function is the best. Therefore, we define it as
The ideal gas scale is in some sense a "mixed" scale. It relies on the universal properties of gas, a big advance from just a particular substance. But still it is empirical since it puts gas at a special position and thus has limited applicability—at some point no gas can exist. One distinguishing characteristic of ideal gas scale, however, is that it precisely equals thermodynamical scale when it is well defined (see ).