The initial cost of a room air conditioning unit is significantly lower than the cost of central air. Because room air conditioners are designed for cooling small spaces, operating costs are reduced. And, room air conditioners can provide personalized temperature and humidity controls that central systems cannot. Calculating Cooling Capacity Cooling capacity is the critical factor in properly selecting a room air conditioner. Cooling capacity is measured in British Thermal Units (BTUs) and typical models will range in capacity from 5,000 BTUs to 28,500 BTUs. Choosing an undersized unit will overwork the unit and it will not cool properly. Choosing an oversized unit will cost more to buy and operate and it will not dehumidify properly. We can help you to calculate capacity. Be prepared to provide specific information on: Room Dimensions A simple floor plan to show the location of doors and north-facing windows The number of people it will serve Sources of heat such as lamps, TV and appliances An explanation of what's above the intended room Your insulation provisions Cooling Capacity by Room Size Measure the length and width of the area to be cooled. Multiply the length by the width to determine square footage or square meters. Lo.