Solar Flares are classified according to the amount of energy emitted by them per unit area. The classification is based on the energy released in the 1-8 Angstrom band.

Typically we have 4 major classes designated by letters B,C,M and X, with X being the most powerful of flares. Each of these letter classes is further subdivided into 9 subdivisions. So a X class flare can exist from X1 to X9, with X9 being the strongest flare.