divided into high-, medium-, and low-volatile categories based on fixed carbon content, and high-volatile bituminous coal is further subdivided by energy content.

ISO classification of bituminous coal is based on vitrinite reflectance. This classification divides medium rank coal (approximately equivalent to bituminous coal) into four subranks. In order of increasing rank, these are:
* Medium D: Vitrinite reflectance of 0.5 to 0.6. Corresponds approximately to ASTM high volatile C bituminous or sub-bituminous A.
* Medium C: