Like the Richter Scale used to measure earthquakes and the Saffir-Simpson Wind Scale that rates hurricanes, volcanoes are measured using their own unique scale, called the Volcanic Explosivity Index (VEI).
The VEI ranks volcanoes on a scale of magnitude, ranging from one (minor eruptions) to eight (biggest volcano eruptions) based on the volume of erupted materials. With each number, the volume of ejected materials increases by a factor of 10, and only volcanoes of "VEI 8" magnitude are considered supervolcanoes.
Although a VEI 8 and VEI 5 are only three points away from each other on the scale, they depict quite different events. For example, the VEI 5 eruption of Mount St. Helen's in May of 1980 didn't even spew one-percent of the volume ejected by the VEI 8 Mount Toba eruption.