Most of the closed top drums I've seen have an od at the top lip of 22 5/8". Open top drums have a slightly larger od right at the top lip.
Before anyone asks, they are usually about 35" tall, 34.5" from inside bottom to the top edge.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)