The 39.8 ka Campanian Ignimbrite (CI) is the largest caldera-forming eruption of the Campi Flegrei during the Quaternary, which had a global-scale impact on the environment and human populations. The cooling following the eruption and the several effects of it strongly affected the paleoenvironment and the migration of hominids in Europe. The volume of the eruption is necessary to constrain the climate model of this area in the past. However, despite a large number of studies, the Dense Rock Equivalent (DRE) volume estimates range from 60 to 300 km3. Here we present a review of the previous volume evaluations and a new calculation of the volume of the ignimbrite. This estimate is constrained by the first total CI isopach map, developed through a method able to reconstruct the paleo-topography during the eruption, which is easily reproducible in all ignimbrites strongly topographically controlled and allows the calculation of well-defined uncertainties. The preserved total bulk extra-caldera volume of the ignimbrite is estimated at 61.5 km3 ± 5.5 km3. The total PDC deposit volume is then corrected for erosion, ash elutriation, the intracaldera deposit volume and the volume of tephra deposited in the sea. The total final volume estimate of the eruption ranges from 165 km^3 – 248 km^3 DRE. This value corresponds to a mass of 4.3 - 6.5 x 10^14 kg, a magnitude (M) of 7.7 and a VEI of 7. This M makes the CI the largest-magnitude Quaternary eruption in the Mediterranean area. The new detailed estimation of CI eruption physical parameters confirms this event has significantly affected human activity and the environment on a large scale at the time of the eruption and, in the future, an event of this size would be cataclysmic.