In physics, a **perfect gas** is a theoretical gas that differs from real gases in a way that makes certain calculations easier to handle. Its behavior is more simplified compared to an ideal gas (also a theoretical gas). In particular, intermolecular forces are neglected, which means that one can use the ideal gas law without restriction and neglect many complications that may arise from the Van der Waals forces.

The terms *perfect gas* and *ideal gas* are sometimes used interchangeably, depending on the particular field of physics and engineering. Sometimes, other distinctions are made, such as between *thermally perfect gas* and *calorically perfect gas*, or between imperfect, semi-perfect, perfect, and ideal gases. Two of the common sets of nomenclatures are summarized in the following table.

Along with the definition of a perfect gas, there are also two more simplifications that can be made although various textbooks either omit or combine the following simplifications into a general "perfect gas" definition.

A **thermally perfect gas**

is in thermodynamic equilibrium
is not chemically reacting
has internal energy *e*, enthalpy *h*, and heat capacities *C*_{V},*C*_{P} that are functions of temperature *only* and not of pressure, i.e.,
e
=
e
(
T
)
,
h
=
h
(
T
)
,
d
e
=
C
v
(
T
)
d
T
,
d
h
=
C
p
(
T
)
d
T
.
This type of approximation is useful for modeling, for example, an axial compressor where temperature fluctuations are usually not large enough to cause any significant deviations from the *thermally perfect* gas model. Heat capacity is still allowed to vary, though only with temperature, and molecules are not permitted to dissociate. The latter implies temperature limited to 2500 K.

Even more restricted is the **calorically perfect gas** for which, in addition, the heat capacity is assumed to be constant:
e
=
C
v
T
and
h
=
C
p
T
.

Although this may be the most restrictive model from a temperature perspective, it is accurate enough to make reasonable predictions within the limits specified. A comparison of calculations for one compression stage of an axial compressor (one with variable *C*_{p}, and one with constant *C*_{p}) produces a deviation small enough to support this approach. As it turns out, other factors come into play and dominate during this compression cycle. These other effects would have a greater impact on the final calculated result than whether or not *C*_{p} was held constant. (examples of these real gas effects include compressor tip-clearance, separation, and boundary layer/frictional losses, etc.)