The thermal expansion of uranium mononitride (UN) is calculated using in situ neutron powder diffraction data between 673 – 1273 K. The true, or instantaneous, thermal expansion describes the instantaneous rate of dimensional change at a given temperature. The true thermal expansion coefficient of UN calculated from this work and all available literature values are within error. The true linear thermal expansion coefficient (α_t) of UN between 250 K and 2523 K is described by: α_t=6.5(2)×10⁻⁶+3.0(2)×10⁻⁹T This can be converted to the mean linear thermal expansion coefficient (α_m), for any T₀ and T value in the temperature range, using the equation: α_m=exp[6.5×10⁻⁶(T−T₀)+3.0×10⁻⁹2(T²−T₀²)]−1T−T₀