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How to Size a Furnace for a House: A Brief Guide

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Winters in Bowie, MD, require a furnace that keeps your home comfortable and warm all season. When it’s time to install a furnace, it’s essential to choose the right one for your home. Not only should you consider energy efficiency and cost, but most importantly, select the right size furnace for your home. At S&L Air Conditioning And Heating, we’re Bowie’s trusted furnace installation professionals.

We assist our customers in choosing the furnace that checks all of the boxes for their home, helping you avoid the pitfalls of selecting the wrong size furnace for your space. Not everyone knows how to size a furnace for a house, so below, we’ve compiled a few tips to offer guidance when calculating the correct furnace size for your home. 

The Importance of Furnace Size

Purchasing the right size furnace for your home ensures your comfort and helps keep your energy bills low. A too-small furnace won’t adequately heat your house, and the constant running to keep up with the temperature causes extensive wear and tear and reduces energy efficiency. A too-large furnace may unevenly heat your home, with some areas uncomfortably hot and others not warm enough. Too-large furnaces offer less energy efficiency and may wear out more quickly than a properly sized heater. 

How Do I Calculate Furnace Size?

The key to calculating furnace size is understanding British Thermal Units, or BTUs. BTUs measure your furnace’s heat output, so the higher the BTU rating on your furnace, the higher the heat capacity. Using BTU calculations to determine furnace size helps you choose the right furnace for your needs. When choosing your furnace, you’ll select the size using calculations of BTUs in two areas: 

Total Square Footage of Your Home

When you’re figuring out how to size a furnace for a house, begin by calculating the square footage of your home. Most people know the estimated square footage, but be aware that some rooms may not be included in your home’s “official” square footage because of low ceilings. In this case, you’ll want to measure those rooms and add them to the total. 

Once you’ve calculated the square footage, multiply it by 30. That’s the general estimate for the amount of BTUs per square foot it takes to heat your home. So if you live in a house with 2,000 square feet, you need a minimum of a 60,000 BTU furnace.   

Climate Zone

The BTU rating on your furnace differs depending on the climate zone you live in. Someone living in the Northern United States needs a furnace with a higher heat output than someone residing in the Southern United States. Use a climate zone map to help you calculate the correct size furnace for your home according to your location. 

Other Factors To Consider

Energy Efficiency Rating

Newer furnaces take into account the user’s desire for heating efficiency and reduced energy bills. For best results, choose a furnace within your budget that offers the highest energy efficiency rating. Typical furnaces usually have an energy efficiency rating of around 80%, while the top energy-efficient furnaces offer up to 93% efficiency. 


Old windows and window frames tend to be drafty and poorly insulated, letting heated air escape. Newer windows offer better quality seals and insulation and other features that help your home retain heat. The number of windows and the direction they face also affect the heat in your home, so consider those aspects as well. 

Age of Your Home

Older homes typically have better quality insulation, therefore requiring less BTUs to heat. Quality insulation keeps the heated air inside where it belongs and prevents cold outside air from seeping in.

S&L Air Conditioning And Heating | Furnace Experts in Bowie

At S&L Air Conditioning And Heating, we understand that even if you know the basics for sizing a furnace for your home, you may still feel it’s daunting to find the right one. If you’re unsure how to size a furnace for a house, our furnace professionals help you select the right unit, from size to energy efficiency and everything in between. Our technicians undergo background checks and training to ensure you’re getting the best in furnace repair, maintenance, and installation services. The S&L Air Conditioning And Heating professionals proudly share their time and experience with the Bowie community and recently received the “Best in Bowie” award in the HVAC contractor category. Call us today at (301) 923-4571 to schedule an appointment, and let us assist with all of your HVAC needs. 

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