Flexible heaters are devices that can be molded to the shape of the heated object. Most products can be bent around a small radius, and can be bent more than one without damaging the internal heating elements. Flexible heaters use two types of heating elements: etched foil and wire wound. Etched foil elements are etched in the heater's surface. They are low-profile and durable. Wire-wound heating elements are less common than etched foil or other low-profile elements. Length and width are important dimensions to consider when selecting flexible heaters. Specifications for power requirements and outputs include: maximum operating (sheath) temperature, AC voltage required, heating capacity, and watt density. Maximum operating (sheath) temperature is the highest temperature that the heater's sheath (or protective cover) may reach. This is not the maximum temperature a heated substance may reach. Heating capacity is the wattage which the heater can deliver. Watt density is a good measure of how quickly the heater can transfer heat to a heated surface. To calculate watt density, suppliers divide the available wattage by the heated surface area.