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Scientists classify solar flares according to their brightness at X‐ray wavelengths, i.e. their X‐ray flux. There are five classes: A (the weakest), B, C, M, and X (the strongest). Each category has nine subdivisions, e.g., C1 to C9, M1 to M9, and X1 to X9. Very occasionally, extremely powerful flares are given a much higher designation. For example, one storm of November 4, 2003, has been variously classified as X28 or X45.

These scales are logarithmic, much like the seismic Richter scale, so an M flare is 10 times as strong as a C flare. The most important categories are:

 X‐class flares – the most powerful. These major events can trigger radio blackouts around the world and long‐lasting radiation storms in Earth's upper atmosphere which can damage or destroy satellites.

 M‐class flares – medium‐sized, releasing 10% of the energy of X‐class flares. They generally cause brief radio blackouts that affect Earth's polar regions. Minor radiation storms sometimes follow an M‐class flare.

 C‐class flares – small, releasing 10% of the energy of M‐class flares, with few noticeable consequences on Earth.