One of the largest expected ionospheric effects of the increasing concentrations of greenhouse gases is a decrease in the height of the F2 region peak density, hmF2, by ∼20 km. Analyses of long-term trends of hmF2 in historical data to confirm this change must rely on models of hmF2 based on a limited number of ionogram characteristics, because a full analysis of historical analog (pre–digital age) ionograms to derive the plasma frequency profiles and hmF2 was a tedious and usually rare procedure. This paper discusses the accuracy of three models/formulas of hmF2 that have been used for long-term trend studies, namely (1) the simple parabolic model, which sets hmF2 equal to the ionogram virtual height at 0.834 foF2, (2) the Dudeney (1974, 1983) model, and (3) the Bilitza et al. (1979) model. The models have been validated using ionograms calculated from plasma frequency profiles given by the International Reference Ionosphere (IRI). The best model would be the one with the smallest seasonal and solar cycle variations in the errors, even if it has a systematic error. Analysis of the errors (relative to the IRI) in the three models shows that all three are useful, but at different times of day.