What Size HVAC Do I Need? The Ultimate Sizing Guide

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October 24, 2019

Is this this the answer to my question, what size HVAC do I need?

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Utility bills are a sizeable part of any household budget. When you live in a cold or warm climate, and especially one with severe summers and winters, your HVAC heating bills can leave you feeling like there’s a hole in your wallet. Some households must even go on level payment plans to afford their bills during the hottest and coldest months. To keep their homes affordable, households need to slash energy costs wherever they can. Did you know that the size of your HVAC unit is one of the biggest factors in heating/cooling costs? That’s why we found the answer to the question, What size HVAC do I need?

What Size HVAC Do I Need?

Determining HVAC size needs relies on calculating the British thermal units (BTU) required to heat/cool your home. Once you have the BTU number, it’s a matter of selecting an HVAC unit with the right BTU rating.

Classic America Ductless Wall Mount Mini Split Inverter Air Conditioner

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Accurately determining BTUs requires an extensive calculation, which can be performed by an HVAC professional. Though you can guesstimate BTU needs based on square footage, there are too many other factors for the estimate to help you fine tune your HVAC setup and save on your monthly bills. Here are the two steps you must follow to answer the question: What size HVAC do I need?

Step 1

Daikin 24,000 BTU 17 SEER Wall-Mounted Ductless Mini-Split Inverter Air Conditioner Heat Pump System

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To answer the question, What size HVAC do I need, you first must calculate how many BTUs your home requires to stay adequately heated or cooled. HVAC pros often use a manual calculation to make this determination. This method is preferable because it uses precise measurements to calculate a much more accurate answer.

The manual calculation considers the following factors:

  • The home’s square footage
  • Climate considerations
  • Ductwork
  • Numbers of windows and style
  • Shade/sunlight
  • Insulation
  • Number of residents
  • The effect of heat-generating appliances

Many utility companies will complete an energy audit that determines BTUs needed at no cost to the customer. If your utility company doesn’t provide this service, you can hire an independent energy auditor or an HVAC dealer to conduct the audit. They will produce a report that shows how many BTUs are needed to keep your home comfortable.It’s also possible to use your home’s square footage to gain a rough estimate of BTU needs. Though more convenient, using square footage just doesn’t provide the accuracy of the manual calculation. Square footage estimates can’t account for factors like climate, window types, insulation, shade/sunlight and other factors unique to your home.


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The below chart provides a rough estimation of BTU needs based on just square footage:

House Square FootageBTUs Needed

100 – 150


150 – 250


250 – 300


300 – 350


350 – 400


400 – 450


450 – 500


500 – 700


700 – 1,000


1,000 – 1,200


1,200 – 1,400


1,400 – 1,500


1,500 – 2,000


2,000 – 2,500


Step 2

With your BTU calculation in hand, it’s time to select the best unit for your home. An HVAC dealer can advise you on the myriad of options. In some cases, your options may be limited by the type of HVAC infrastructure you have and the size of the space where it must fit. Many apartments and condos require a certain model, which unfortunately can eliminate choices that may provide more efficient heating and cooling.

Assuming your choices are not limited, the HVAC dealer can demonstrate the units that suit your BTU size. Often, BTU calculations land inbetween sizes. This should cause no concern, simply go with the bigger size. Having a unit that’s a little larger than needed is the best practice anyway. It ensures adequate heating and cooling and provides some extra capacity should the BTU change. For example, if you need more power for air conditioning because of the loss of shade trees. Also, you are covered for unusually cold- or hot spells.

Getting an excessively large unit should always be avoided. When the units are too large, they heat/cool the home too quickly, resulting in too frequent on/off cycles that cause uneven heating/cooling and higher energy bills.

Pro tip:

Limit the maximum size of the unit to 15% over the BTUs needed for cooling and 40% over the BTUs required for heating. Heat pumps, which handle both heating and cooling, should never be larger that 25% of BTUs.

Central Air Conditioning Tonnage and Size

If you need to install a central air conditioning system, determining tonnage is the first step. When it comes to air conditioning, size is a critical component in selecting the right unit. Selecting a unit that’s too big will cost you dearly every time the monthly electric bill arrives. In short, a 4-ton unit to cool a 3-ton spaceI achieves no increased performance level.

When the AC unit is too big, it cools the home too rapidly. As a result, it shuts off before completing its entire cycle. As a result, warm air seeps back in much quicker, forcing the unit on again too soon. This increased turning on and off shoots your electric bill to the moon.

An AC unit that is too small is even worse. Unless you choose a unit that is many sizes too small, the AC will keep the house nice and cool, but it will run almost constantly. Today’s central AC units are designed to gradually cool the home and then remain shut off for longer periods, which saves energy. When the unit is too small, it pumps most of the day in an effort to keep up, increasing your electric bills. On top of that, the AC components are overtaxed, causing them to wear out faster and costing even more money. The overuse is also bad for the environment.

A right-sized central AC system runs through the cycle as it was designed, resulting in even cooling and maximal efficiency.


The goal of calculating AC size is to determine what tonnage you need from your AC system. Tonnage is a measure of how many BTUs an air conditioner can cool in one hour. Each ton represents 1,000 BTUs per hour cooling capacity, so, for example, a 2-ton air conditioner cools 2,000 BTUs per hour. Residential air conditioners range from 1.5 to 5 tons.

How Much Tonnage Do You Need?

To answer the question, what size HVAC do I need when it comes to AC tonnage, use the following formula for average climates:

Square footage * 30 / 12,000 – 1

Hot and arid climates like the desert Southwest require a different calculation. For these climates, using the following formula to answer the question, “What size HVAC do I need?”

Square footage * 20 /12,000 – 0

Though these calculations are helpful in estimating the tonnage need, a professional manual J report is the best bet. The manual J report, which can be conducted by an HVAC pro through from your electric company or an HVAC dealer, takes into account important factors that are unique to your home. For example, how much shade is around your property, the thickness of installation and the type of windows you have. The manual J report will give you the tonnage number need.

Tips for Saving Energy

Getting the right sized air conditioner is a great first step, but there are many other ways to save on your utility bills. Considering that HVAC typically represents 50% to 60% of utility costs, it’s well worth the effort to shore up your HVAC system for increased performance.

Don’t Skip on Maintenance

Religiously changing the filter extends the life of your HVAC system and helps it run more efficiently. When dust and other particles collect in the coil and other areas, the system must condition more air to meet heating and cooling needs. A fresh filter keeps the system clean.

Programmable Thermostat

You may want to keep the house toasty on a cold winter’s day, but do you really need the heat blasting when you’re under the blankets? Program the thermostat to a lower temperature at night and have it automatically adjust upwards shortly before your wake up time.

Invest in Energy Efficient HVAC Equipment

If your HVAC unit is ten or more years old, new, energy efficient equipment is a great investment. The difference in efficiencies will save you every month and you won’t have to worry about costly repairs for a long time. Energy Star rated models save even more.


Conditioned air that seeps out of the house costs you money. Adding insulation to walls, windows, pipes, ducts and outlets keep the warm of cool air inside and lead your HVAC system to work less.

Utility bills leave many households tapped out at the end of the month. Stop the cycle of energy inefficiency that has the money seeping out of your bank account by installing new, energy efficient HVAC equipment. Before selecting which HVAC unit to buy, make sure to get a manual J report or accurate BTU estimate. With the accurate BTU or tonnage number, you will know exactly what size unit you need. By having the right-sized unit and practicing energy saving techniques, your home will be more comfortable than ever for less cost.

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