VEI (Volcanic Explosivity Index) is the measurement of how explosive a Volcanic eruption is. The scale is logarithmic – each interval on the scale is a tenfold increase in observed ejecta criteria. Scientists are still determining where the recent Hunga Tonga eruption ranks, but it could be as high as 6. This would give it the same rank as Krakatoa in 1883. A supereruption would rank 8 on the VEI, ~100x more explosive than Krakatoa in 1883, and most recently erupted in New Zealand almost 30,000 years ago.

Source: one, two, three, four