Due to the commercial transfer of billions of cubic meters of natural gases, the knowledge of the gross calorific value (GCV) of the main natural gas components and, in particular, of methane, is of outstanding interest. On the basis of previous work carried out by a Groupe Européen de Recherches Gazières (GERG)–Physikalisch-Technische Bundesanstalt collaboration, the so-called GERG calorimeter was further developed on the hardware as well as on the software side. With the renewed GERG calorimeter, the GCV of CH4 could be determined with unprecedented precision and accuracy. Important elements for improving the measuring methodology of flame calorimetry included the in situ calibration of the mass of the burned gas, the determination of the actual exhaust gas temperatures, and the detection of the water input by countercurrent water absorption from ambient air. For the first time, it was possible to determine the GCV not only via direct online weighing of the mass of burned gas but also via the stoichiometric water balance with a consistency of about 3.5 ppm. Based on 27 weighings of the mass of burned gas, the real-gas GCV of methane is determined as Hs(CH4) = 890 202.1 J mol−1 with a confidence interval of ±52.6 J mol−1 (t95% = 2.056). This value is by ΔHs/Hs = (−0.0436 ± 0.0059)% below the real-gas GCV of Hs(CH4) = (890 590 ± 380) J mol−1 (k = 2) converted according to ISO 6976:2016. The difference can be explained by systematic influences as well as by failures in the stoichiometric water balance in all other measurements.