Whole mature chickens are marketed in the United States as fryers, broilers, and roasters. Fryers are the smallest size (2. 5-4 lbs dressed for sale), and the most common, as chicken reach this size quickly (about 7 weeks). Most dismembered packaged chicken would be sold whole as fryers. Broilers are larger than fryers. They are typically sold whole. Roasters, or roasting hens, are the largest chickens commonly sold (3–5 months and 6-8 lbs) and are typically more expensive. Even larger and older chickens are called stewing chickens but these are no longer usually found commercially. The names reflect the most appropriate cooking method for the surface area to volume ratio. As the size increases, the volume (which determines how much heat must enter the bird for it to be cooked) increases faster than the surface area (which determines how fast heat can enter the bird). For a fast method of cooking, such as frying, a small bird is appropriate: frying a large piece of chicken results in the inside being undercooked when the outside is ready.