Use this furnace BTU calculator to estimate how much heat output your home needs and which input rating to look for. Just enter your square footage, IECC climate zone and the furnace’s AFUE.
Ranges checked on November 4, 2026. Not sure of your zone? Find your county on the IECC climate zone map from PNNL first.
Furnace BTU calculator
How the furnace BTU calculator works
First, it multiplies your square footage by Fixr’s BTU-per-square-foot range for your climate zone. As a result, you get the heat output, in BTU per hour, that the furnace should deliver.
| Climate zone | BTU per sq ft | 2,000 sq ft home |
|---|---|---|
| 1 (hottest) | 30–35 | 60,000–70,000 BTU/h |
| 2 | 35–40 | 70,000–80,000 BTU/h |
| 3 | 35–45 | 70,000–90,000 BTU/h |
| 4 | 45–50 | 90,000–100,000 BTU/h |
| 5 | 50–60 | 100,000–120,000 BTU/h |
| 6 | 55+ | 110,000+ BTU/h |
| 7 (coldest) | 60+ | 120,000+ BTU/h |
Then it converts that output into an input rating. Furnaces are often listed by input, meaning the gas or oil they burn, while your home needs output, the heat they deliver. Since AFUE is the share of fuel that becomes heat, input equals output divided by AFUE.
For example, a 2,000-square-foot home in zone 4 needs about 90,000 to 100,000 BTU/h of output. With a 96% AFUE furnace, that’s an input rating of about 94,000 to 104,000 BTU/h. With an 80% AFUE furnace, however, it’s about 113,000 to 125,000 BTU/h.
An electric furnace, by contrast, is rated at 100% AFUE, according to HomeGuide. So its input and output are the same.
Why the furnace BTU calculator gives a range
After all, Fixr’s figures are rules of thumb, and HomeGuide gives an even wider range of 30 to 60 BTU per square foot. Moreover, the Department of Energy warns that rules of thumb too often lead to oversized systems, which cost more, waste energy and cycle on and off too often.
DOE also notes that heating loads in energy-efficient homes can be so small that they fall below manufacturers’ smallest sizes. In other words, a well-insulated home may need much less than the chart suggests.
Still, sizing errors cut both ways. According to Fixr, an oversized furnace cycles on and off rapidly, which hurts efficiency and can lead to early breakdowns. Meanwhile, an undersized one works overtime and wears out sooner.
That’s why the final size should come from an ACCA Manual J load calculation. Also, our Manual J load calculation guide explains what a proper one includes.
What to do next
- First, compare quotes using output BTU, not only input BTU.
- Don’t simply match your old furnace’s size, since it may have been oversized.
- If your load falls between sizes, ask about a two-stage or modulating furnace, which can run at lower output most of the time.
- Next, pick an efficiency level with our 80% vs 96% AFUE furnace comparison.
- Then price the job with our furnace replacement cost guide.
For a deeper look at sizing, read our guide on what size furnace you need.
Furnace BTU calculator FAQ
What size do I need for 2,000 square feet?
Based on Fixr’s ranges, you need from about 60,000 BTU/h of output in the warmest zone to 120,000 BTU/h or more in the coldest. In a moderate zone 4 climate, for instance, that’s about 90,000 to 100,000 BTU/h.
Is the rating input or output?
In fact, spec sheets show both. Input is the fuel the furnace burns, and output is the heat it delivers. So to compare models, use output, or multiply the input by the AFUE.
What is my climate zone?
Look up your county on the PNNL map of IECC climate zones. Zone 1 is the hottest, and higher numbers are colder.
Does the furnace BTU calculator work for heat pumps?
Not directly, because heat pumps are sized in tons, and DOE notes that cooling loads are often larger than heating loads. Instead, use our AC and heat pump size calculator.
Sources and review
Sources: Fixr, furnace replacement cost and sizing (January 28, 2026); HomeGuide, new furnace cost (December 31, 2025); HomeGuide, electric furnace cost; DOE Building Science Education, proper sizing of HVAC systems; PNNL, IECC 2021 climate zone map; Carrier, high-efficiency gas furnaces (October 13, 2026). All checked on November 4, 2026.
Written by: HVAC Switch. Last updated: November 4, 2026. Read our editorial policy to learn how we research prices, or contact us if you spot an error.