Abstract The dentistry literature shows consensus to use the Garvie and Nicholson equation modified by Toraya to quantify the Y-TZP phase transformation. However, this method does not include the possibility of cubic phase transformation and crystallographic texture after artificial ageing, and in this case, it is possible to observe errors of quantification. Objectives The aim of this study was to evaluate a dental Y-TZP ageing kinetic of phase transformation under pressure and hydrothermal conditions (130° C, 2 bar) and to compare the methods of quantification by the equation of Garvie and Nicholson modified by Toraya and the Rietveld refinement method. Methods Discs of Y-TZP (12 mm Ø × 1.2 mm in height) were divided into groups ( n = 4) according to the ageing times (in the range of 6 and 138 h). The superficial characterisation was made using SEM and the XDR for crystallographic analysis. Results An aggressive superficial degradation process at the beginning of phase transformation in 6–10 h of ageing was observed by SEM. The phase transformation quantification showed differences between the methods. It was observed the increase and stabilisation of monoclinic phase until 80% at 40 h of ageing by the Garvie and Nicholson modified by Toraya equation, compared to 60% of monoclinic phase and approximately 30% of cubic phase observed by the Rietveld method. Conclusion The Toraya equation showed an overestimated result of monoclinic quantification compared to the Rietveld method. Clinical significance The overestimated result of monoclinic phase could lead to different interpretation about the dental Y-TZP ageing process.