K-指标以0-9的整数量化了地球磁场水平分量的变动，指出地磁风暴的程度，1是平静，5是强烈。它以三小时的间隔，以磁强计观察这段时期水平分量的最大扰动。字母'K'源自德文的'Kennziffer'，意思是'特殊的阿拉伯数字'。

从最大的扰动 (nT) 转换成K-指标，varies from observatory to observatory in such a way that the historical rate of occurrence of certain levels of K are about the same at all observatories.在实务上，这意味着在高地磁纬度的观测站，会观测到较高水准扰动的K-指标。即时的K-指标是在明定的3小时间隔(0000-0300, 0300-0600, ..., 2100-2400)确定结束之后测量的。在3小时内的最大正负偏差值将会与确定的最大扰动总值附在一起，但最大偏差值可以出现在这3小时内的任何时刻。

The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0–9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval. The label K comes from the German word Kennziffer meaning “characteristic digit”. The K-index was introduced by Julius Bartels in 1938.