Take the design heat loss in BTUs (British Thermal Units) and multiply it by 24.
Multiply the sum of Step 1 by the total number of degree days in a season. Remember that a degree-day is not a representation of one 24-hour period, but the number of degrees below 65 degrees.
Divide the sum of Step 2 by the design temperature difference between the indoor and outdoor temperatures of the mill. Many buildings have an indoor temperature between 55 and 68 degrees, but you should use whatever the indoor temperature of your mill is.
If you prefer to use an online calculator or if you are unsure of the design outdoor temperature of your area, see "Resources" below for a link to a calculator.