The correct method of residential air conditioning sizing uses the square footage of the area the unit is cooling. After calculating the area, compare it to a chart to convert to the British thermal units per hour it requires for cooling.
Under certain circumstances, adjustments to the required cooling capacity are necessary for proper sizing. Direct sunlight or heavy shade can increase or reduce the required cooling capacity by 10 percent. In rooms where there are normally more than two people, each additional person requires 600 Btu per hour. In kitchens, increase the cooling capacity by an additional 4,000 Btu per hour over the square footage requirement.