We’ve discussed how to choose an AC that’s the right size for your home. Too big an AC will use too much energy and too small a unit won’t cool your space properly. But we know sizing is more complex than that. 

Cooling capacity is a determining factor in sizing an air conditioner. It is measured in British Thermal Units (BTUs); a BTU is a unit of heat needed to raise a pound of water by 1℉. The capacity needed for your home mainly depends on its square footage. However, there are other factors in determining the number of BTUs per sq. ft. your new AC should be.

A Look at AC Ratings

Every air conditioner has a BTUh rating. British Thermal Units per hour is a measurement, expressed in tons, of the unit’s cooling ability over one hour. For example, a one-ton air conditioner can remove 12,000 BTUs of heat per hour. In general, 12,000 BTUs of heat can be extracted from anywhere from 400 to 600 square feet of space in that time, depending on climate, shading, ceiling height, window size, etc.

U.S. Department of Energy (DOE) Recommendations

The DOE generally recommends 20 BTUs per square foot of living space. That is based on size alone. Therefore, a simple formula for calculating BTUs is to multiply the total square footage of your home by 20. But you may need more BTUs per sq. ft. if you have high walls, live in a hot climate like Florida or Arizona, a particular room usually faces the sun, and it has more/larger windows than average.

Calculating BTUs per Sq. Ft.

Using a BTU calculator, you can quickly determine the AC capacity your home needs. The general formula includes the room size in square feet, ceiling height (typically 8 or 9 feet for a home), and the type of room and how many people regularly occupy it. The calculator uses other variables as well.

To calculate how many BTUs per sq. ft. your AC needs to be:

Measure Your Entire Home’s Square Footage

The size of rectangular rooms is determined by measuring the length and width in feet and multiplying both numbers. The square footage of a triangular room is determined by multiplying the length by width and dividing by 2, while for circular rooms, measure the radius, then multiply 3.14 by the radius squared. Odd-shaped rooms can be measured by breaking them down into regular shapes and measuring them separately. Then add up the size of every room to get the total square footage of your home.

Identify Your Climate Zone

The U.S. is divided into climate zones. These zones are ranked from Zone 1 to Zone 5, generally going from south to north. Climate zone BTU calculations differ slightly from the DOE’s. Per zone, the BTU per sq. ft. requirements look as follows:

Zone 1: 30 to 35 BTUs per sq. ft.

Zone 2: 35 to 40 BTUs per sq. ft.

Zone 3: 40 to 45 BTUs per sq. ft.

Zone 4: 45 to 50 BTUs per sq. ft.

Zone 5: 50 to 60 BTUs per sq. ft.

Why You Need a Manual J Calculation

There are many factors in determining HVAC load. The actual number of BTUs per sq. ft. can vary, so it’s more important to look at the overall AC size requirements for your home. You’ll need to multiply the height of the ceiling by room square footage, add 100 BTU per occupant, and 1,000 BTU each per exterior door and window. Contractors use sophisticated software in addition to manual calculations to determine the AC that’s the best size, provides the most comfort, and is most efficient.

Contact Sky Heating & Air Conditioning

Failing to properly size a unit is one of the biggest mistakes an AC contractor can make. At Sky, we use precise calculations to determine not only BTUs per sq. ft. but also the total capacity needed to keep your home comfortable. Our technicians focus on precise attention to detail so will find the perfect make, model, and size of AC. To get started with AC replacement, schedule an appointment online or call (503) 673-9083.