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What Size of an Air Conditioner Do I Need?2021-08-25T13:13:39+00:00
air conditioner repair

Do you know how to pick the right kind of air conditioner for your home – one that is just the right size for your home and your needs? You may think you do, but if you’re like most homeowners, you could be considering an AC that is too big (or too small).

Choosing the right Size of an Air Conditioner is not as simple as a one-size-fits-all solution. And because your comfort is what matters most, it pays to learn how to find the ideal AC size for your home.

This guide by experts at Conejo Valley Home Services – your preferred Southern California HVAC company – reveals everything you should know concerning AC sizing. Let us dive right in.

Importance of a Proper Size of an Air Conditioner. Why Size Matters?

When it comes to air conditioner sizing, most homeowners think bigger is better. But it is often the wrong approach.

An oversized AC will likely run frequent short-cycles, where it turns on and off constantly, leading to a dramatic rise in bills – the exact costs you intended to minimize when buying a massive AC unit. Additionally, short cycles cause wear and tear, leading to constant air conditioner repair and shortening your equipment’s lifespan.

An oversized air conditioner will also cool your home rapidly, leading to humidity issues. The excess moisture can cause duct leakage, compromised indoor air quality, and mold growth. Water damage can be expensive to repair.

In comparison, an undersized air conditioner will have to work harder to maintain the desired indoor temperature. As a result, it consumes a great deal of energy and wears out faster. You may be in for a hectic summer because the unit will likely distribute cooling unevenly, leading to hot spots in more distant rooms.

The right AC size for your home frees you from either of the above scenarios. Take advantage of the expertise of an HVAC contractor to choose a cooling system that keeps your home comfortable and saves energy as you desire.

Factors Affecting the Size of an AC Unit?

Room Size

An air conditioner will provide comfort and save you money on energy bills only when it is appropriately sized for the room it is intended to cool. When we talk about room size, we often refer to the floor surface area. The table below compares room sizes and AC size estimates.

Room Size (m2) Example AC size (kilowatts)
Up to 20 Bedroom, small kitchen 2-2.5
20-40 Small lounge, bedroom with ensuite 2.5-5
40-60 Large bedroom, large kitchen, mid-size lounge 4-6
60-80 Large lounges 5-7
80+ Large open areas 6-9

Available are formulas that give specific AC sizes for particular room sizes. The rule of thumb is to pick an air conditioner with equal or slightly greater capacity for a specific room. For example, if you find that your space requires a 4kW model, you will certainly not go wrong with a 4-4.5kW air conditioner.

Room volume comes into the picture when we consider the ceiling height. A room with a higher ceiling has a greater volume, translating to a larger volume of air that needs cooling. Such spaces require high-capacity ACs.


How much heat or cold a room can keep will influence the size of an air conditioner you need. Insulation comes in here. Generally, the better the insulation, the easier it is to heat or cool a room, and the smaller the AC needed.

Several factors determine the overall insulation. They include construction materials used, quality of insulation material installation, flooring type, gaps between the floors and doors, and window coverings. Most cities have building codes stipulating the minimum amount of insulation for walls and ceilings. However, there is a likelihood homes built before the regulations came into effect have inadequate insulation, demanding higher capacity AC systems.


The orientation of your room will impact your AC size requirements. A shaded, southern-facing window will feel cooler, while a west- or north-facing window will be much in summer. You will also need to consider the size and number of windows. Bigger, multiple windows will make a room warmer in summer and force an appropriately sized air conditioner to work harder to cool it down.


Dominant climatic conditions factor in the right size of an AC. Generally, homes in colder, drier regions need smaller air conditioners than homes located in hot, humid climates. Based on this argument, a house in Florida or Texas will have different cooling requirements than a similar-sized house in Colorado.

The term “location” also encompasses properties unique to your home. For example, if your yard has many large trees offering commendable midday shade, you will save money by investing in a lower-capacity AC.

What Does BTU Mean?

BTU is an acronym for British Thermal Units. It is a measure of heat defined as the amount of heat required to increase the temperature of one pound of water by one degree Fahrenheit. The unit is typically used as a rating for HVAC units.

A single BTU is nearly the amount of energy generated by a burning match stick, according to the U.S Energy Information Administration(EIA). BTU rating tells you how powerful an air conditioner is. The more the BTUs, the more cooling effect the unit provides.

How do I Calculate the Right Size of an Air Conditioner?

Picking the right size of an air conditioner means choosing a model with the right BTU rating for your room. The principal factor when calculating the size of your AC is your home’s square footage. As a rule of thumb, each square footage of living space requires 20 BTU.

To obtain your room’s measurements, you only need to multiply the length by the width (in feet). A tape measure comes in handy here. Combine the sizes of rooms not separated by doors as the AC will need to cool both areas.

To obtain the specific BTU rating needed, multiply the total square footage by any figure between 20 and 25. Below is a list of BTU ratings for different ranges of room sizes:

Square feet BTUs Square feet BTUs
150-250 6,000 450-550 12,000
250-300 7,000 550-700 14,000
300-350 8,000 700-1,000 18,000
350-400 9,000 1,000-1,200 24,000
400-450 10,000    

If your room, for example, measures 470 square feet, the above chart tells you that you need an air conditioner with a capacity of 12,000 BTUs. Note that other considerations, including environmental factors, may call for less or more AC power.

If your room receives direct sunlight, consider increasing the necessary capacity by 10%. Using our previous example, you will require an AC with a BTU rating of 13,200. For heavily shaded rooms, cut down the capacity by 10%.

The human body generates heat, with the amount varying depending on the activity. If more than two people occupy your living space, add 600 BTU for each additional individual. A gym requires an AC with a higher capacity due to higher levels of activity in such spaces.

An AC will have to work harder in rooms with heat-generating appliances, such as refrigerators. Ovens and stovetops release a great deal of heat, which is why you should add 4000 BTUs for any cooling unit you intend to install in a kitchen.

DIY Calculations for Ballpark AC Size

An AC can be rated in tonnage. In air conditioning, a ton indicates how much heat removed by a cooling unit would be required to melt one ton of ice in 24 hours. AC tonnage is expressed in BTUs per hour.

Typically, it takes 288,000 BTUs to melt a ton of ice in 24 hours, equating to 12,000 BTUs per hour. Simply put, one ton is the ability of an AC to cool 12,000 BTUs in an hour.

So, you want to calculate the tonnage of your new conditioner. Take the steps below to obtain a ballpark estimate:

  1. Calculate square footage: Your home’s blueprint may contain the square footage of the area you want to cool. Else, use a tape measure to measure each room’s length and widget, calculate the square footage of each room, and add the surface areas together. Alternatively, you can take your home’s exterior measurements and subtract the area of any space that does not require cooling, e.g., an attached garage.
  2. Calculate the BTU Value: To get the base BTU value, multiply the square footage value obtained from the above step by 25. Remember the extra considerations we discussed earlier.
  3. Obtain the tonnage: Divide the total BTUs by 12,000 to determine the estimated tonnage.

We can summarize the above steps in a simple equation:

{(Total square footage x 25/12,000)-1}

For illustration, let us calculate the required tonnage for a 3,000-square foot house:

{(3,000 x 25/12,000)-1} = 5.25 tons

This means you need a 5.25-ton AC for a 3,000 sq. ft. house.

Your location affects the AC tonnage formula. If you live in a hot, arid region, use the formula below:

(Total square footage x 25/12,000)

Based on this equation, you will need a 6.25-ton air conditioner for a 3,000 sq. ft. home in a desert.

What is Manual J Load Calculation?

The AC sizes of two similarly-sized homes can vary incredibly based on various factors. That is why the Air Conditioning Contractors of America (ACCA) developed the Manual J Load calculation.

By considering various aspects of your home’s design, Manual J load calculation offers you and your HVAC contractor a clearer picture of your cooling requirements.

An HVAC professional performs Manual J load calculation by entering your home’s specifications in software designed to calculate the proper size of an air conditioner. These aspects include:

  • Total square footage
  • Shape and orientation of your house
  • Amount of insulation
  • Number, size, types, and direction of windows
  • The amount and efficiency of home appliances
  • Airtightness of the building

The primary types of Manual J load calculation are:

  • Block load calculation: It determines the cooling requirements for the entire home
  • Room-by-room load calculation: It provides load requirements for individual rooms.

HVAC contractors pair Manual J Calculation with other HVAC design guidelines, including Manual D duct design and Manual S equipment selection.

BTUs are Heat, Right? BTUs and Central Air Conditioners

No doubt, BTU rating makes more sense for heaters (referring back to the definition), but what does it mean for an AC?

You see, an air conditioner extracts heat from the air inside your home and circulates cool air. Inside your home (the “cold side” of the system), warm air cools off as it blows across your AC’s indoor coil full of refrigerant. As the refrigerant turns from liquid to gas, it absorbs heat from the warm indoor air. The refrigerant transfers the heat to the outdoor unit (condensing unit), which dissipates heat outside your home. The cooled air is circulated throughout your house.

With that explanation in mind, it becomes easier to understand why ACs are rated by BTUs. The rating simply denotes the amount of heat an AC can remove from your home in one hour.

SEER also Matters on Your Tonnage

When sizing an air conditioner, do not just focus on the tonnage. Compare the SEERs of different models, too!

SEER stands for Seasonal Energy Efficiency Ratio. It denotes the energy efficiency of your air conditioner over a typical cooling season. Mathematically, SEER is the ratio of an AC’s cooling output to the total electric energy input (kW). The higher the SEER rating, the higher the equipment’s energy efficiency and the lower the energy costs.

The minimum standard SEER rating for ACs is 13, but available are models that fall in the range of 20+. Most older systems have a maximum SEER rating of 10. The ideal SEER for your AC will depend on several variables including, total square footage and ductwork.

Installing a higher SEER air conditioner often means you will enjoy more comfort in the summer months. However, some of these models have smaller compressors, losing cooling power quickly at higher outdoor temperatures and providing less than expected comfort.

Combining a 2-stage/variable compressor and a variable blower, the latest high SEER models are more effective and energy-efficient. Lower SEER ACs are typically single-stage and only operate at one speed. Such a model is more likely to cause hot and cold spots and humidity issues in your home.

With so many variables to consider for AC SEER and tonnage, it helps to consult with a reputable HVAC company like Conejo Valley Home Services. Our technicians will size your home and recommend (and install) a cooling system that ensures maximum home comfort while saving energy.

The Best ACs for the Room or Zone Size

Ready to invest in a high-quality air conditioner? If so, you may need to explore different air conditioner brands. In this section, we will focus on two popular brands: Trane and Lennox.

In a survey conducted by Consumer Reports, of the 21 AC brands considered, only Trane earned an excellent mark for reliability and owner satisfaction. Trane ACs have distinctive, sleek designs. SEER ratings range between 14.5 and 22, while the sound rating averages 70 decibels. The units also incorporate advanced features, such as TruComfort, which minimizes humidity by running at lower speeds rather than shutting off. Trane air conditioners come with a standard 12-year compressor warranty and a 10-year warranty for parts and labor.

Founded in 1895 and headquartered in Richardson, TX, Lennox is famous for its high-efficiency AC units- some record as high as 26 SEER. The brand offers 11 models with variable speed/ two-stage compressors for more precise cooling. Some models have noise levels as low as 41 dB.

Talking of warranties, Lennox covers the unit and parts for five years. A few models come with a 10-year warranty.

There’s More to Picking an AC than Size

Now that you know the entails of air conditioner size and efficiency, you may be wondering what else constitutes an educated buying decision. Additional things to consider when choosing an AC include:

  • Add-ons
  • Your home/office’s electrical system
  • Your budget
  • Existence or non-existence of ductwork

HVAC pros at Conejo Valley Home Services have the experience and knowledge to help you pick a cooling system that matches your cooling requirements and budget. We are an HVAC company offering air purification installation and HVAC services in Conejo Valley and nearby regions, including Los Angeles County, Ventura County, and Santa Barbara County. Call us today at 805-499-0448 for immediate help choosing or installing an appropriate AC for your home or office.

Conejo Valley Home Services?
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  • We Charge by the Job – Not by the Hour
  • We Offer Only the Most Dependable Customer Satisfaction With Each Job Completed
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