Аннотация:Solar flare activity is characterized by classification systems, both in optical and X-ray ranges. A new classification of solar flares is proposed - the X-ray index of flare XI, based on measurements of radiation in the range 0.1-0.8 nm on the GOES satellites. The XI index has a clear physical interpretation associated with the total flare energy in the range 0.1-0.8 nm. XI is easily calculated for each flare with use of available GOES data. The XI index can be used to assess flares along with other important geoeffective parameters.