Furnace BTU Size Calculator: Everything You Need to Know

| Last Updated: March 13, 2021

Are you looking to how to size a furnace for your home?

This BTU sizing calculator will help you determine the best size of furnace for your specific needs. Buying a furnace is a big decision and buying an incorrect size could be costly.

Furnace BTU Size Calculator

Note: Our calculator is a very good representation of the correct sizing, but for it to be 100% accurate you would need to use a professional HVAC installer.

What are BTUs?

Since our calculator spits out the sizing in BTUs, it is important that you understand the meaning behind this unit of measurement. See the definition by Wikipedia below

The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

Wikipedia

Factors to Consider When Sizing a Furnace

As with just about anything in life, there are nuances that could change the sizing calculations. The list below are variables to consider when calculating your furnace size.

  • How well is your house insulated?
  • Where do you live? Your geographic region impacts sizing
  • Furnace efficiency
  • Lots of windows or other glass areas in your home? Glass area will have higher heat leakage vs. a home with less windows.

Common House Sizes and Their Requirements

Below are some common sizes of houses and their respective BTUs required for a furnace to heat it. Please note that the zone where you are located based on the map above will determine exact BTU amounts needed. The reason being warmer states will require less energy to get the house temperature to a comfortable level. Where as a colder state such as Montana will require much more energy due to the outside temperature being further away from the average inside temperature of 70-80 degrees.

For the sake of these examples, I will be using the location of Montana, which is considered Zone 5.

What Size Furnace Do I Need for 3,000 Square Foot Home?

You will need a furnace with BTUs in the range of 105,000 to 180,000 BTUs.

The size furnace you need for a 3,000 square foot home for each zone is below:

  • Zone 1: 105,000 BTUs
  • Zone 2: 120,000 BTUs
  • Zone 3: 135,000 BTUs
  • Zone 4: 150,000 BTUs
  • Zone 5: 180,000 BTUs

BTUs required for a 3000 square foot home in Montana (zone 5)

What Size Furnace Do I Need for 2,000 Square Foot Home?

You will need a furnace with BTUs in the range of 70,000 to 120,000 BTUs.

The size furnace you need for a 2,000 square foot home for each zone is below:

  • Zone 1: 70,000 BTUs
  • Zone 2: 80,000 BTUs
  • Zone 3: 90,000 BTUs
  • Zone 4: 100,000 BTUs
  • Zone 5: 120,000 BTUs

BTUs required for a 2000 square foot home in Montana (Zone 5)

What Size Furnace Do I Need for 1,000 Square Foot Home?

You will need a furnace with BTUs in the range of 35,000 to 60,000 BTUs.

The size furnace you need for a 1,000 square foot home for each zone is below:

  • Zone 1: 35,000 BTUs
  • Zone 2: 40,000 BTUs
  • Zone 3: 45,000 BTUs
  • Zone 4: 50,000 BTUs
  • Zone 5: 60,000 BTUs

BTUs required for a 1000 square foot home in Montana (Zone 5)

Video Overview

Check out the video below that does a great job of explaining quickly what size of furnace you need

https://www.youtube.com/watch?v=o5k3SduPy8g

My name is Bob Wells and I am a retired HVAC tech from Washington state. I am currently retired and no longer do much with HVAC, however, I feel like I have a lot of knowledge in the subject and I wanted to create a website where I could talk about what I've learned and help upcoming HVAC techs. Find more info about me and HVAC Training 101.