Obtain the total power or wattage of the heating source. The power is identified by having watt as the unit.
Determine the heated surface area. The heated surface area is the total contact surface of the heater less any insulated areas or areas where there is no heating. Unheated areas are typically found around the terminals and mounting holes and at the edges of heating strips. Use the following formula to calculate the heated surface area for a flat, rectangular heater: Heated Area = (Length of heated area -- length of unheated area) x Width of heated area. Use the following formula to calculate the heated surface area for a cylindrical or tubular heater: Heated Area = (Length of heated area -- length of unheated area) x (inner diameter of tube x 3.14)
Divide the total power of the heating source by the heated surface area to obtain the watt density of the device.