Fits easily under the helmet. Hardly know I am wearing it but I feel reassured I am protecting my bald pate. Love that it saves me from applying sun block to my head and waiting for the eventual sting as perspiration and gets into my eyes.
Its a bit too big for my 21 12 inch head but I like that it still fits above not over my ears so it's more to my liking for a cap under my helmet. It absorbs sweat so it works well in hot or cooler weather.