How to Calculate Heater Size

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When the time comes to replace a heating system, you will need to know how to calculate heater size. Although it can be a major investment, getting a more efficient unit nearly always saves money in the long run.


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Choosing a properly sized heater is one of the best ways to reduce your energy costs. Once you know the size you'll need, you should also look for a high-efficiency rating, or AFUE. In colder climates, choose ones with AFUEs of 90 percent or more.

How to Calculate Heater Size

To calculate heater sizes for smaller structures, you will first need to measure and calculate the building's length by width by ceiling height. This will reveal its size in cubic feet. So, if a garage is 16 x 24 x 14 feet, your heater has to cover 5,376 cubic feet.

You will also need to determine the space's insulation factor, which determines how many BTUs your heater needs. Some homes have old, inefficient insulation, while others are well-insulated. Insulation factors are rated in this way: 7 is poor insulation, 5 is average insulation and 4 is excellent house insulation.

Take your cubic feet measurement and multiply by insulation factor accordingly. If the garage's insulation is excellent, you multiply 5,376 by 4 to determine that you need a heater with 21,504 BTU. If it is poor, you would need a heater with 37,632 BTU.

Calculate Heater Size for a Home

Obviously, getting the right heater size is important. If it is too small, you will have higher energy bills and uneven heating. On top of that, you will be more likely to feel uncomfortable, and the furnace may break down sooner. Similar problems occur when heaters are too big. You will also feel uncomfortable, there will be cold and hot spots in the house and the unit will not be efficient.

To calculate heater size for a home, you first need to find out the recommended heating factor. This is based on where you live. The farther you are from the equator, the more BTU your home needs. The United States is divided into several climate zones by the International Energy Conservation Code (IECC). Each zone has its own recommended heating factor per square foot:


  • Zone 1: 30 to 35 BTU
  • Zone 2: 35 to 40 BTU
  • Zone 3: 40 to 45 BTU
  • Zone 4: 45 to 50 BTU
  • Zone 5: 50 to 60 BTU
  • Zone 6: 55 BTU
  • Zone 7: 60 or more BTU

Completing the Calculations

Zone 1 includes the warmest regions of the nation, and zones 7 and 8 are the northernmost and coldest (zone 8 is in parts of Alaska). You can identify your zone using an IECC climate map.

You also need to know the square footage of your home. Try to locate your deed or title, which should be among the paperwork from when the home was purchased. These documents will likely include the square footage of the building. If you cannot locate the paperwork, you can do a property search online. Websites like often have this information. Alternatively, you can visit your county's property appraiser website or township office to see if they have the information available.

Finally, you could measure the length and width of your home outside and multiply them to determine the square footage. Don't forget to include additional stories if applicable. Once you have the square footage, multiply it by the heating zone factor. A 2,500-square-foot home in zone 3 would need a 100,000 to 112,500 BTU heater.