23 inches and most helmets fit 24, because I add padding AND a coif size medium helms dont fit over my coif. With this new arming cap they fit rather snug and it's much better, my only complaint is my kettle helm is so damn heavy. It was supposed to be 16 gauge but I think it's much heavier, the brim is really thick. Maybe I should sell it and get a better one from Royal Oak, but meh thats asking for too much.