Size Matters: Choosing the Right Size HVAC System for Your Home

For the first-time homeowner, navigating in the world of HVAC systems can be daunting. A problem with air conditioning and heat could be easily fixed by a landlord, but in your home, you’re expected to know the steps you need to take to keep your home the perfect temperature. You may be tempted to look only to price and quality, but there’s so much more that goes into the right-sized HVAC system for your home.

In this article, we’ll be going over how size really does matter when it comes to HVAC, how to pick out the right option for your home, and how to do the math to ensure you’re getting the biggest bang for your buck.

HVAC Explained

HVAC stands for heating, ventilation, and air conditioning–the three major components of treating air indoors. All three are necessary, and all three are built into a home’s central air system. In the summer months, you’ll be reliant on the compressors within your air conditioning to provide cool air and help filter out pollen and other irritants that are common during this time of year. In the winter, a heating element or furnace will keep out the bitter cold and help your family stay cozy as the temperatures drop..

Ventilation is probably the least-discussed aspect of the HVAC system but one of the most important. Proper ventilation prevents stale air from becoming a breeding ground for irritants, mold, and other such hazards. Proper ventilation can have a profound impact on your respiratory health.

Let’s take a closer look at how two of these systems work and what you need to know about them in order to make an informed decision about your next system.


Furnaces and heaters within your HVAC system are reliant on either electricity or natural gas in order to keep your house warm in the winter. The measurement of how powerful these heaters are is often stated by their heating and energy output calculated in British Thermal Units (BTUs). One BTU is equal to the energy needed to raise the temperature of one gram of water by one degree Celsius.

That means it takes a lot of BTUs to heat an entire home. Furnaces range anywhere from 18,000 BTUs all the way up to 200,000 BTUs or greater. To calculate your BTU needs, you’ll need to take a look at the climate in which your home exists, the square footage of your home, and the efficiency of the furnace you’ve purchased. We’ll get into this math a bit later on.

Air Conditioners

Air conditioners rely on compressors, fans, and coolant to take the heat out of your home and released the cooled air back inside. In the summer months, air conditioners are going to be highly important to keeping your home cool, and their power is similarly measured in BTUs.

Calculating BTU ratings for air conditioners are going to be a fair bit simpler than calculating them for furnaces. We’ll give you a table to work with below, but for now, know that you’ll need your home’s square footage and an idea of how high your ceiling may be.

Residential Vs. Commercial Units

We want to stress that the math below is calculated for residential units. Residential and commercial HVAC systems have completely different methods of operating, and the differences make them impossible to compare directly. Generally speaking, residential HVAC systems are built to work as an independent system. If you need better cooling capacity, you’ll often need to replace the system. For commercial units, though, HVAC systems are built in a modular fashion so you can daisy chain units to fit your cooling and heating needs. Likewise, commercial units are often situated at the top of a building while residential units are located on the ground floor, either to the side of or behind a home. If you want the power of a commercial unit but don’t have a flat rooftop, this could present a major problem.

The Simple Math

All that being said, let’s take a look at how to calculate your home’s HVAC system needs. We’ll start with air conditioning:

First, calculate the square footage of your home that will need to be cooled by the system, and multiply by 25. The rule of thumb is that it takes about 25 BTUs to cool one square foot of home, so that’ll give you a rough estimate of how many BTUs you’ll need to cool the entirety of your house.

The rule of thumb is that it takes about 25 BTUs to cool one square foot of home

Next, you’ll want to calculate for special conditions, such as high volume rooms where more than two people reside for extended periods of time, kitchens, or rooms that are notably sunny or shade-filled. You’ll want to add another 500 BTUs for every high volume room, add or subtract 10% of a room’s BTU value for sunny or shady rooms, and add on another 4,000 BTUs for a kitchen. Finally, you may find that air conditioner strength is measured in tons instead of BTUs. 1 ton is equivalent to 12,000 BTUs, so conversion should be rather simple.

These are rough estimates should give you a ballpark estimate of how much cooling potential you need. For various home sizes, here are some good averages to work from (excluding any special conditions):

Square FootageBTU Needed
500 – 1,000 square feet12,500 – 25,000 BTUs
1,000 – 1,500 square feet25,000 – 37,500 BTUs
1,500 – 2,000 square feet37,500 – 50,000 BTUs
2,000 – 2,500 square feet50,000 – 62,500 BTUs
2,500+ square feet62,500 BTUs and up

For heat, calculating a ballpark estimate is going to be a fair bit more complicated but is still doable with some time and a calculator. First, you’ll need to find your heating factor–or, how many BTUs you need to heat one square footage of your home. With air conditioning, this was a flat 25 BTUs, but for heat you’ll need to choose between one of four numbers depending upon your climate and the age of your home.

Older homes are less efficient (and therefore require more BTUs) than newer homes. We’ll signify this as “new” or “old.” Likewise, cooler climates require more BTUs to heat than warmer climates. We’ll signify this as “warm” or “cold.” To find your heating factor, choose from one of the following four options:

Home TypeNew/WarmOld/WarmNew/ColdOld/Cold
Heating Factor30 BTUs35 BTUs50 BTUs60 BTUs

You’ll then want to multiply your heating factor by your square footage to calculate the best number of BTUs for your home. Before you head out to purchase that a heater, though, you need to finally calculate for efficiency.

To keep things simple, most furnaces are about 80% effective, which means that you’ll need to purchase a furnace with more BTUs than you need in order to have the correct number for your unit’s efficiency. To account for this, divide your BTU needs by 0.8. If you’re not precisely sure of your square footage and need something to reference quickly, try out the following values based on square footage:

Square FootageNew/Warm BTUOld/Warm BTUNew/Cold BTUOld/Cold BTU

Final Thoughts

Estimating the best size for your home’s HVAC system can be tricky, and we highly recommend looking into a professional estimate for your home before making a large purchase based on some of the numbers above. We hope, though, that these ballpark figures have given you a better idea of the budget you need and the BTUs required to properly maintain your home’s temperature no matter the time of year.

Further Reading