17 CFR § 240.3a55-1
Market capitalization. For purposes of Section 3(a)(55)(C)(i)(III)(bb) of the Act (15 U.S.C. 78c(a)(55)(C)(i)(III)(bb)):
(1) On a particular day, a security shall be 1 of 750 securities with the largest market capitalization as of the preceding 6 full calendar months when it is included on a list of such securities designated by the Commission and the CFTC as applicable for that day.
(2) In the event that the Commission and the CFTC have not designated a list under paragraph (a)(1) of this section:
(i) The method to be used to determine market capitalization of a security as of the preceding 6 full calendar months is to sum the values of the market capitalization of such security for each U.S. trading day of the preceding 6 full calendar months, and to divide this sum by the total number of such trading days.
(ii) The 750 securities with the largest market capitalization shall be identified from the universe of all NMS securities as defined in § 242.600 of this chapter that are common stock or depositary shares.