Note: This post only discusses heating BTU. If you are looking for a calculator for cooling, check another post: AC Btu Calculator – What Size Air Conditioner Do I Need?
Our goal in this guide is to help you determine the right size furnace for your heating requirements. It is based on the needed heating BTU per square foot for most homes and your climate. Our heating BTU calculator takes those very important factors into account, so, for example, you can answer questions like, “how many BTU do I need to heat 1500 square feet? Or 2000 square feet? More?
The BTU per square foot heating rule of thumb varies widely based on your climate and the size of the house or space you want to heat.
How Many BTU of Heating Do I Need?
That’s an important question to get a right answer to. A furnace that is too small won’t adequately heat your home on the coldest winter days. Rooms furthest from the furnace might become uncomfortably cool spots in your house.
A furnace that is too large will waste energy and create warm spots at grates near the furnace before the cycle ends.
The generally accepted BTU per square foot heating rule of thumb ranges from 30 BTU per square foot to 60 BTU per square foot. That’s a wide difference, essential one furnace being twice as large as the other, so the size factors are discussed to help you narrow down the right BTUs needed for your home.
This list showing heating BTU per square foot is taken from our PickHVAC Furnace Reviews, Prices and Buying Guide:
- Zones 1 & 2 (hot): 30-35 Btu/sq. ft.
- Zone 3 (warm): 35-40 Btu/sq. ft.
- Zone 4 (moderate): 45 Btu/sq. ft.
- Zone 5 (cool): 50 Btu/sq. ft.
- Zone 6 (cold): 55 Btu/sq. ft.
- Zone 7 (very cold): 60 Btu/sq. ft.
The Zones are shown in the map below. There are factors other than climate, and they are used in our heating BTU calculator below to give you the most accurate information.
What about heating BTU per square foot for boilers? It’s about the same, and all the details are discussed in our PickHVAC Boiler Reviews, Prices and Buying Guide.
Heating BTU Calculator
OK, there is more information below.
Let’s start with this very useful map. It was developed by the International Code Council / International Energy Conservation Code and adopted by the US Department of Energy’s Office of Energy Efficiency & Renewable Energy.
We mention that to represent it as accurate and very useful for our purposes.
Heating BTU Calculator
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Calculate heating BTU needed in four easy steps.
1. Climate Zone: Find your location on the map. Pro tip: If you’re on or near the border between Zones, choose the higher number/cooler Zone.
2. Home Size: Enter the square footage of your home. If you don’t know it, the information might be found on a blueprint or house drawing, in closing documents or similar. If you don’t have any of those, the easiest way to determine it is to measure rectangles within your home – multiplying width x length for each if there are more than one – and adding their totals.
3. Insulation: There are two keys to insulation – How well your home is sealed and the type and thickness of the insulation. In terms of being sealed, is your home’s exterior covered in house wrap like Tyvek? It’s also called vapor barrier. It is now code, so if your home doesn’t have it, vapor barrier/wrap will have to be installed the next time you replace the siding.
Are the windows and doors newer and in good condition? If you answer “yes” to the house wrap and window/doors questions, then choose Good or Average in the Insulation box. If you don’t know, if your home is older and hasn’t been remodeled, or if you can feel drafts coming in around windows and doors, choose Poor.
Money Saving Tip: Adding attic insulation to an attic that is inadequately insulated is the #1 best way to lower your heating bills and get the best return on investment. In fact, the prestigious Remodeling Magazine annual survey consistently shows that attic insulation has a 105% to almost 120% return on investment (Cost vs. Value). In other words, the money spent will be quickly gained back through lower energy bills.
The image below shows how much attic insulation your home should have.
4. Sun Exposure: Climate and the trees around your home factor into this box. If your home is surrounded by tall trees that shade the walls and the roof – and few are – then choose Heavily shaded. Most homes, in this regard, are either Average or Very Sunny.
But “Very Sunny” means something very different in Denver (245 days with sun) than it does in Portland, Oregon (142 days with sun), even if there are no shade trees around the house. Therefore, consider the map below. The deeper the color, the more sunny the climate. In Minnesota, a home with no shade would still only be Average for sun exposure, for example.
Chart of Recommended Heating BTU Per Square Foot
This chart begins small – space heater size, not furnace size – and progresses to homes in the average to large size category. So it answers common search queries like:
- how many btu do I need to heat 1500 square feet
- how many btu do I need to heat 2000 square feet
- how many btu do I need to heat 2500 square feet
- how many btu do I need to heat 3000 square feet
|Room/Area Size:||Examples:||Ave. Recommended BTU:|
|100-200 sq ft||Small den or office||4,000-8,000 BTU|
|200-300 sq ft||Den, office, small bedroom||7,000-15,000 BTU|
|300-400 sq ft||Efficiency apartment||12,000-18,000 BTU|
|400-500 sq ft||Studio/1-bedroom apartment, Tiny house||15,000-24,000 BTU|
|500-600 sq ft||Small 2-bedroom apartment, Tiny House||18,000-30,000 BTU|
|600-700 sq ft||Small bungalow, 1 or 2-bdrm townhouse, cottage||22,000-36,000 BTU|
|800-900 sq ft||Small home, 2-3 bdrm apartment or townhouse||30,000-45,000 BTU|
|900-1,000 sq ft||Small home, 2-3 bdrm apartment or townhouse||32,000-48,000 BTU|
|1,000-1,200 sq ft||Small home, 3- bdrm townhouse, home addition||35,000-50,000 BTU|
|1,200-1,400 sq ft||Small/Average home, large townhouse or apartment||44,000-70,000 BTU|
|1,400-1,600 sq ft||Average home, large townhouse or home addition||52,000-80,000 BTU|
|1,600-1,800 sq ft||Average home, large townhouse or apartment||58,000-90,000 BTU|
|1,800-2,000 sq ft||Average home, large townhouse, guest house||66,000-105,000 BTU|
|2,000-2,200 sq ft||Average home, 4-bdrm apartment||80,000-115,000 BTU|
|2200-2,400 sq ft||Average to large home, very large townhouse||95,000-130,000 BTU|
|2,400-2,600 sq ft||Average to large home, very large townhouse||110,000-150,000 BTU|
|2,600-2,800 sq ft||Large home, one zone in a very large home||125,000-160,000 BTU|
|2,800-3,000 sq ft||Large home, one zone in a very large home||135,000-160,000+ BTU|
At some point, depending on the climate, you have to begin to think about more than one heating device - furnace, boiler, heat pump, etc., for your home. You can still use this chart for very large homes by considering the size of the zones in your home.
For example, if the bedroom and office area zone is 1,800 square feet and the living areas and kitchen are 2,400 square feet, add together the recommended BTUs for each. You’ll need between about 180,000 and 280,000 total BTUs for your home based on the climate and house factors considered in the calculator.
Pro Tip: For smaller rooms/zones/apartments, a ductless mini split heat pump is another really good option. They’re efficient and come with potential lower installation costs than furnaces.
BTUs: Definition and Ratings
A BTU is a measurement of heat – specifically, the heat needed to raise the temperature of one pound of water 1 degree Fahrenheit. The full name is British thermal unit.
It is an exact measurement that can be applied to most heating equipment. The correct rating is BTU/hour, or BTU/h, meaning how much heat does the equipment create in one hour of constant operation.
Furnaces rarely run for an hour straight, but that’s how they are rated.
Did you Know? BTU ratings are also used for air conditioning. The rating for ACs isn’t, obviously, the amount of heat the equipment creates, but the heat that it removes from a room each hour.
And for heat pumps, it is the same as for air conditioners – the amount of heat moved from outside to inside during heating.
Our guide How Many BTU Air Conditioner Do I Need? explains how BTUs apply to air conditioners and, to some extent, heat pumps.
How Many BTU Do Furnaces Make? Standard Boilers?
It’s a wide range. The smallest gas furnaces start at about 30,000 BTU per hour, or BTU/h.
While BTU/h, how many BTUs of heat the unit can create in one hour, is the technical specification, most heating equipment such as propane, oil and gas furnaces are rated simply by BTUs or BTU.
The most common sizes are from about 70,000 to 120,000 BTU/h.
The largest gas furnaces create about 140,000 BTU/h.
Oil furnaces have a slightly different range from about 60,000 to 160,000 BTU/h.
What About Wood Stove/Boilers and Combi Boilers?
Both of these heating equipment types provide both hot water for heat and water used to heat water in your hot water tank. The second half of that sentence is a mouthful – wood-burning combi boilers heat water – but that hot water doesn’t flow directly into your hot water pipes. Instead, it circulates through a pipe in your hot water tank to provide you with what is called “domestic” hot water. This type of heating equipment is growing in popularity, especially where free or cheap wood is readily available. See our Wood Furnace/Boiler Reviews and Buying Guide for details.