Real gases are non-ideal gases whose molecules occupy space and have interactions; consequently, they do not adhere to the ideal gas law. To understand the behaviour of real gases, the following must be taken into account:

For most applications, such a detailed analysis is unnecessary, and the ideal gas approximation can be used with reasonable accuracy. On the other hand, real-gas models have to be used near the condensation point of gases, near critical points, at very high pressures, to explain the Joule–Thomson effect, and in other less usual cases. The deviation from ideality can be described by the compressibility factor Z.

Real gases are often modeled by taking into account their molar weight and molar volume $${\displaystyle RT=\left(p+{\frac {a}{V_{\text{m}}^{2}}}\right)\left(V_{\text{m}}-b\right)}$$

or alternatively: $${\displaystyle p={\frac {RT}{V_{m}-b}}-{\frac {a}{V_{m}^{2}}}}$$

Where p is the pressure, T is the temperature, R the ideal gas constant, and V_m the molar volume. a and b are parameters that are determined empirically for each gas, but are sometimes estimated from their critical temperature (T_c) and critical pressure (p_c) using these relations: $${\displaystyle {\begin{aligned}a&={\frac {27R^{2}T_{\text{c}}^{2}}{64p_{\text{c}}}},&b&={\frac {RT_{\text{c}}}{8p_{\text{c}}}}\end{aligned}}}$$

The constants at critical point can be expressed as functions of the parameters a, b: $${\displaystyle {\begin{aligned}p_{c}&={\frac {a}{27b^{2}}},&V_{m,c}&=3b,\\[2pt]T_{c}&={\frac {8a}{27bR}},&Z_{c}&={\frac {3}{8}}\end{aligned}}}$$

With the reduced properties ${\displaystyle p_{r}=p/p_{\text{c}}}$, ${\displaystyle V_{r}=V_{\text{m}}/V_{\text{m,c}}}$, ${\displaystyle T_{r}=T/T_{\text{c}}}$ the equation can be written in the reduced form: $${\displaystyle p_{r}={\frac {8}{3}}{\frac {T_{r}}{V_{r}-{\frac {1}{3}}}}-{\frac {3}{V_{r}^{2}}}}$$

The Redlich–Kwong equation is another two-parameter equation that is used to model real gases. It is almost always more accurate than the van der Waals equation, and often more accurate than some equations with more than two parameters. The equation is $${\displaystyle RT=\left(p+{\frac {a}{{\sqrt {T}}V_{\text{m}}\left(V_{\text{m}}+b\right)}}\right)\left(V_{\text{m}}-b\right)}$$