Selecting a proper-sized air conditioner for your room or space is crucial to getting optimal cooling and efficiency. An AC unit that is too small won’t lower temperatures adequately or run efficiently. One that’s oversized cools quickly but cycles on and off frequently, fails to remove humidity, and costs more to operate.
Follow this comprehensive guide on key considerations when determining the right BTU measurements and type of air conditioning unit for your unique needs.
Calculating the BTUs You Need
BTU, or British Thermal Unit, indicates the amount of energy needed to alter the temperature of one pound of water by one degree Fahrenheit. British Thermal Units measure the cooling capacity of air conditioners. Calculate the appropriate BTU rating you require by factoring in critical details about the room you want to cool:
Room Dimensions
Measure room length x width x height to determine total square footage. This is key for AC sizing. Total the connected areas if cooling multiple rooms. As a rule of thumb:
- 100-150 square feet = 5,000 BTUs
- 150-250 square feet = 6,000 BTUs
- 250-300 square feet = 7,000 BTUs
- 300-350 square feet = 8,000 BTUs
- 350-400 square feet = 9,000 BTUs
- 400-450 square feet = 10,000 BTUs
- 450-550 square feet = 12,000 BTUs
- 550-700 square feet = 14,000 BTUs
- 700-1,000 square feet = 18,000 BTUs
- 1,000-1,200 square feet = 21,000 BTUs
So, for example, a 12 x 20-foot bedroom that’s 9 feet high (240 square feet) would need around a 7,000 BTU unit.
Other Considerations
Factor in these other elements, too, for your BTU calculations:
- Windows: Add 100 BTUs for every square foot of window space. Large windows or numerous windows cause heat gain.
- Insulation: Poor insulation makes your AC work harder. Subtract 10% from your BTUs calculation if the insulation is low quality.
- Room Contents: More furnishings, electronics, and people increase the heat in a room. Add 100 – 200 BTUs per person.
- Climate: Hotter climates require higher BTU power to cool adequately. Add 10% for humid/hot climates.
- Sun Exposure: South-facing rooms see more sun exposure. Add 10% for ample sunlight.
- Ceiling Height: Air in taller rooms is harder to stabilize. Add 10% for rooms over 8 ft.
Use multiple online BTU calculators to verify your math, inputting room dimensions, windows, climate, and other factors. Overestimate BTUs if you aren’t sure.
SEER Rating
SEER stands for Seasonal Energy Efficiency Ratio. It is important to understand SEER and its relation to a proper AC unit size.
This metric measures the energy efficiency of an air conditioning unit over an entire cooling season. The higher the SEER rating, the more efficient the AC unit is. Older systems tend to have SEER ratings of 6 to 10, while today’s Energy Star-certified units are required to have a 14 SEER minimum rating. Split ductless systems can reach as high as 26 SEER, offering the greatest energy savings potential. The minimum SEER level has risen over time as technology improves and energy standards increase.
When properly matched to the cooling load using accurate room size and BTU calculations, higher SEER central, ductless, or window AC units use less electricity overall.
Air Conditioner Types
Once you determine the necessary BTU rating via room size and other specifications, the next step is selecting the kind of air conditioning unit.
Window Air Conditioners
As the name implies, these box-shaped units mount into a window opening. Affordable and simple to install, window ACs work for cooling down small spaces up to 350 square feet. Their capacities typically range from 5,000 to 12,000 BTU. Blend into your decor by opting for a narrower slimline type.
Portable Air Conditioners
Freestanding portable air conditioners have caster wheels and vents via adjustable window kits without permanent installation, making them versatile and great for rentals. Portable units cool spaces from 150 to 700 square feet, with BTU power spanning from 5,000 to 14,000. They’re costlier than window units.
Mini Split Systems
Mini split air conditioners are ductless systems that have an outdoor compressor linked to one, or multiple compact indoor wall or ceiling units installed high on walls. Operating quietly and out of sight, mini splits efficiently cool open-concept great rooms, kitchens, garages, and more up to 800 square feet through zoning control. Mini-splits typically range from 9,000 to 36,000 BTUs.
Central Air Conditioning Systems
For whole-home cooling, central air conditioning systems utilize an outdoor metal condensing unit containing a compressor, ventilation fan, and condenser coils. This links to an indoor evaporator coil and blower unit to circulate chilled air throughout your home via ductwork. Powerful central air conditioners carry BTU ratings from 16,000 to 65,000+, making them ideal for large homes over 2,000 square feet.
Matching Air Conditioner Size to Room Size
When looking at different kinds of air conditioning systems, ensure that you choose the appropriate system for your room or home size needs. Here are typical BTU recommendations by unit type per square footage:
Window Air Conditioners
- 5,000 to 6,500 BTUs = 150 to 300 sq ft
- 6,500 to 8,500 BTUs = 300 to 400 sq ft
- 8,500 to 12,500 BTUs = 400 to 700 sq ft
Portable Air Conditioners
- 8,000 to 13,500 BTUs = 300 to 600 sq ft
- 13,500+ BTUs = 700 to 1,500 sq ft
Mini Split Systems
- 9,000 to 12,000 BTUs = 400 to 800 sq ft
- 12,000 to 18,000 BTUs = 800 to 1,200 sq ft
- 21,000 to 30,000 BTUs = 1,200 to 2,000 sq ft
Central Air Conditioning
- 1.5 to 2.5 tons = 700 to 1,200 sq ft
- 2.5 to 3.5 tons = 1,200 to 1,800 sq ft
- 3.5 to 5.5 tons = 1,800 to 3,000 sq ft
- 5.5+ ton = More than 3,000 sq ft
So, for a living room with a connected dining area spanning 560 square feet, a portable AC with around 10,500 BTUs would suit this application. An undersized 6,000 BTU window unit wouldn’t cut it, while an oversized 28,000 BTU ductless split system breaks the bank and cycles on and off redundantly.
Other Important Considerations for Proper Sizing
Along with matching your air conditioner’s BTU specification to room size, proper sizing depends on these other cooling factors:
Energy Efficiency Ratio
Today’s air conditioning units must meet Energy Star efficiency standards of at least 10 EER for window units and 12 EER for mini splits. Select an inverter-driven model for even better efficiency and comfort.
Desired Comfort Level
Oversizing your air conditioner by 10% allows it to run less frequently to maintain more consistent temps and humidity on the hottest days. Size up too much, though, and it won’t effectively remove moisture.
Existing Insulation
Poor insulation levels, leaky windows, and high ceilings cause heat gain that your AC continually plays catch up on. Address insulation issues first before installing new units.
Sun’s Orientation
Rooms on the east and west sides of your home receive more intense solar heat in summer. South-facing rooms also warrant extra capacity.
By plugging all these secondary factors into BTU calculators, you can determine the “rule of thumb” size of the air conditioner plus any additions or deductions for your situation.
Problems Caused by Wrong Sizing
Selecting the incorrect central, ductless, or window AC size for your cooling needs leads to a variety of inefficiency issues:
Oversized AC Units
Oversized AC units can cause several issues that lead to an uncomfortable and inefficient cooling system. If an AC unit is too large for the space, it will short cycle and turn on and off frequently as it works to meet the temperature setpoint. Because it cools the air so quickly, it does not run as thoroughly through whole cycles and, therefore, does not effectively remove humidity from the space. This leads to rooms that feel cold and clammy despite running the AC.
Additionally, oversized units use more energy by turning on and off more often, raising your utility costs. All this excess cycling adds more wear and tear, which can lead to reduced lifespan and premature breakdowns that require costly repairs.
Undersized AC Units
Undersized AC units lead to continuous runtimes without ever actually reaching the thermostat’s set temperature. Because the unit capacity is too small for the space, it struggles to cool the interior and often runs continuously without shutting off. Even so, it fails ever to bring down the temperature to the desired level.
In addition to occupant discomfort, this leads to other problems like rooms taking excessively long to cool down despite running for prolonged periods. The lack of adequate moisture removal also causes excess humidity and possible mold or mildew growth. The compressor strains to keep up with the cooling demand and risks burnout from being overworked for extended runtimes.
Installing a correctly sized air conditioning unit to fit your interior space allows it to run just enough to keep even, comfortable temps without over or under-working.
Purchasing the correctly sized air conditioning unit ensures your system can cool your interior space efficiently without overworking. Consider room sizes, window space, climate conditions, and usage patterns when calculating the BTUs required. Match this capacity to the appropriate portable, window, mini split, or central air system type for adequate cooling and comfort all summer long while optimizing energy efficiency. Once you determine what size unit you need you can figure out how much your new AC unit should cost.
The Logan Difference
Logan A/C & Heat is a family-owned and operated business founded in 1969. For over 50 years, we have been committed to providing quality heating and cooling solutions with personalized customer service.
What truly sets Logan A/C & Heat apart is our dedication to the customer experience. All of our installers undergo extensive in-house training to ensure every job is completed properly, on time, and to the homeowner’s satisfaction. We make sure we have all the necessary parts on the truck before arriving so there are no frustrating delays. Throughout the process, we maintain clear communication every step of the way.
Our install managers are seasoned experts who review a comprehensive checklist completed by technicians after each and every installation or service call. We then do a thorough walkthrough with the homeowner, demonstrating how to operate the system and troubleshoot any potential issues. Our goal is for you to have the best possible experience that provides comfort and peace of mind for years to come.
We also care about our community. As a local business, we give back through donations, events, and by lending a helping hand to friends and neighbors in need. The “Logan Difference” is really our commitment to treating you like family. We stand behind our work, and your happiness is our top priority.
Frequently Asked Questions (FAQs)
How do I calculate the BTUs needed for an AC?
To find your air conditioner’s needed BTU amount, measure room length x width x height for total square footage—factor in windows, poor insulation, climate, sunlight, and height over 8 ft. Various online BTU calculators will do the math for your exact specifications.
What size AC is needed for a two-story house?
Figure the total combined square footage of rooms you’ll cool on both floors. Tailor your BTU amount and tonnage accordingly, sizing up a bit for a two-story home’s higher heat gain. A mini-split system with multiple indoor air handler units works great for multi-level cooling.
