What the index is
The Volcanic Explosivity Index was proposed in 1982 by Chris Newhall and Steve Self. USGS explains that eruption size is not read from a single instrument the way earthquake magnitude is. VEI combines the volume of erupted ash with other observations, because historical eruptions were recorded unevenly. Some have a measured deposit. Others are known from a written note.
The volume steps USGS states
A primary criterion is the volume of erupted ash. Each step is about ten times the ash volume of the step below, except VEI 1, which spans two orders of magnitude because those eruptions are very small. USGS gives these anchors: a VEI 5 eruption might have about 1 cubic kilometer of ash, and the 1980 eruption of Mount St. Helens is the example. A VEI 4 eruption is closer to 0.1 cubic kilometer, and the 2021 eruption of La Soufrière on St. Vincent is the example. A VEI 6 eruption is near 10 cubic kilometers, and the 1991 eruption of Pinatubo is the example.
Volume is not a rigid rule. Parícutin, which began erupting in a Mexican cornfield in 1943, ejected 1.3 cubic kilometers of ash and rock fragments over nine years. That volume looks like a VEI 5, but the eruption was not violently explosive at any one time, so it is classified as VEI 4. Lava is not counted in VEI. If the duration were unknown, the deposit alone could be misread.
Hawaiʻi, and the top of the scale
Eruptions that are only lava flows or domes, with no explosions, are VEI 0. A small explosive component, such as lava fountaining or minor ash, can be VEI 1. USGS says that description covers most eruptions in Hawaiʻi. See Kīlauea and Mauna Loa, which are outside this site’s Ring of Fire rectangles.
USGS calls the largest explosive eruptions VEI 8, with ash volumes in excess of 1,000 cubic kilometers. The eruption that formed Yellowstone Caldera 631,000 years ago is the example, because the deposits show at least that much explosive material. Yellowstone is not in our Pacific rectangles. The same article says the Toba eruption 74,000 years ago was possibly VEI 9. No VEI 8 or 9 eruption has been observed, so the duration of those events is not known from witnesses. USGS also says some Yellowstone deposits might have been several VEI 7 eruptions with gaps, rather than one VEI 8. That is an open geologic question, not a schedule.
Plume height is not a shortcut
USGS says there is no hard rule that turns plume height into a VEI. The January 2022 eruption of Hunga Tonga–Hunga Haʻapai sent an extraordinary plume up, but the ash volume was more like a smaller eruption. It was assigned VEI 5, and USGS said that value was still preliminary. The VEI explorer repeats these anchors. Picking a number there does not size a future eruption.