brim, front to back: 15"
brim, side to side: 12 1/2"
height: about 7" (eyeballing with a vertical ruler, kinda tough to call)
brim: 3"
inner circumference: 26"

From my Samples, Singulars, and Somethings Completely Different collection -

A special order kettle helm made by GDFB (Get Dressed For Battle). These are not regularly commercially available in the U.S., but I have one that was made for me and two that are made for you. (I really like the looks of this particular helmet, and thought I'd share the joy). 14 gauge steel, leather suspension liner and chin strap.