Furnace BTU Calculator: How Many Btu Furnace Do I Need?

Gas Furnace Sizes

This furnace size calculator uses your house size, climate (see the map below) and other important information to tell you what size furnace your home needs to keep it warm without wasting energy.

  • Adequately heated in even the most extreme cold
  •  Sized to not waste energy or equipment costs by buying a furnace that is too big

Our calculator is more accurate than others because it factors your home’s insulation (can be estimated) and the level of furnace efficiency you want. Both factors make a difference that a simple size/climate gas furnace size calculator doesn’t consider.

There’s more information below including an explanation of the inputs used (Size, Climate, Insulation and AFUE Efficiency) plus a huge table with sample results for homes with various inputs.

Furnace Sizing Calculator

Use the calculator, or for a quick answer, see the examples below that answer common questions like:

  • What size furnace do I need for a 1,000 square foot home?
  • How many BTU do I need for a 2,000 square foot home?
  • What size furnace do I need for a 3,000 square foot house?

We take the size, climate and insulation value, and give you the most accurate furnace btu calculation results you’ll find online. If you want to double-check your results against ours, see the Table below that uses average-sized homes in a range of Zones, etc.

Furnace BTU Calculator

If you’re familiar with the input information on furnaces including AFUE and what it means for BTU output, then you might have all the information you need to effectively use the calculator. Great!

  • But if this information is new to you, the information below will help you in three ways. First, you’ll understand furnace sizing and why it is important.
  • Secondly, you’ll get an accurate result when sizing your furnace.
  • Thirdly, if you have a furnace salesperson in to give you a quote, you’ll be able to knowledgeably discuss furnace sizing with them, so you will feel in charge of the process rather than wondering what they are talking about!

5 Step Gas Furnace Sizing Calculator

These brief instructions give details for providing accurate inputs into the sizing calculator.

Following that, we “do the math” for readers that prefer a quick, accurate answer to “what size furnace do I need?”

Step 1 – Climate Zone

This is straightforward. Find where you live, and input the zone.

What if I live near the edge of a zone? If you keep your thermostat low during the winter and wear an extra sweater for warmth, choose the cooler zone (higher number) for your location. If you like it toasty inside, use the warmer zone (lower number)

For example, let’s say you live in Memphis, TN, which is in the northern tier of Zone 3 in western TN. If your goal is an efficient furnace that cuts energy costs, and you’re used to wearing an extra layer in winter, use Zone 4 for your calculation. Otherwise, stick with Zone 3.

Step 2 – Home Size

Type in the number of square feet in your home – all heated areas. If you have a heated basement, include the square footage. If it is unheated, then don’t. Pretty simple.

Calculating square feet: If you are unsure of your home’s square footage, you can locate it on a blueprint/house drawing or in closing documents.

For exact measurements, you have two options that will work well enough.

First – The most accurate way is to calculate the square footage of each room and add the numbers together. Measure the length x width of each. For example, a room 24 feet long and 15 feet wide is 360 square feet.

Secondly – A method that also yields an accurate calculation is to measure the footprint of your home. A simple rectangle is easiest. For example:

47 feet long x 32 feet wide = 1,500 square feet

If it’s a two-story with that footprint, double the calculation to 3,000 square feet.

When there are 6 or more corners on the house, the calculation is a little trickier. Break down the home into rectangles. Measure/calculate the sections separately, and add them together.

Consider a six-sided home. Let’s say the main body is 47 x 32, as in the example above. That’s 1,500 square feet. And there is an addition or extension that is 25 x 20, or 500 square feet. Add the sections for a total of 2,000.

Step 3 – Insulation Condition

The best way to think about this factor is for us to describe each option:

Good: There are a couple options here.

First, the home was built after 2010, and you’re sure it has a wrap like Tyvek or similar. Blown-in insulation in the attic is at least a foot deep. The doors and windows are in good condition – you don’t feel any drafts and it’s never extra-cold when you stand next to them.

Secondly, if you have an older home, but you have added attic insulation, replaced the windows and/or doors in the last 15 years with energy-efficient units and perhaps even added house wrap during a siding project, then the insulation quality would be considered Good.

Average: The home was built between about 1990 and 2010, and not much has been done to it. Perhaps the windows have been replaced, but you’re unsure of their quality. You’re not sure if it has house wrap.

Poor: Your home is older, and it’s a little drafty around windows and doors. Not much has been done to the house.

Step 4 – Sun Exposure

Is your home exposed to the sun, or is it shaded?

That’s pretty straightforward. You might also want to consider this question: How much sun does your climate get?

A totally exposed home in northern Michigan will get less warmth from the sun in winter, because winters are cloudy there, than a partially exposed home will get in Minnesota where there are many more days of winter sun.

The main issue is shade – some/little/none. But think about days of sunshine during winter too.

Step 5 – Efficiency Rating

You’ll be shopping for furnaces in two tiers: 80% and 90% to 99%. There are no mid-80% AFUE furnaces.

Why does this matter? Because you want to know how much heat gets into your ductwork.

80% furnace: 80% of the heat is transferred into ductwork while 20% is wasted out the exhaust vent.

95% furnace: 95% of heat is transferred, and 5% is lost.

If you take 100,000 BTU furnaces at these efficiency levels, you’ll get very different results:

  • 80% = 80,000 BTU transferred to heat your home
  • 95% = 95,000 BTU transferred

If you need 90,000 BTU of heat, according to our gas furnace calculator, then the 80% furnace won’t do the job. The 95% furnace might be a little big, but it will definitely heat your home.

So, use the calculator and input different efficiency ratings based on the furnaces you might be considering. In a warm climate, experiment with 80% and 90% to 92%. You probably don’t need a more efficient furnace.

If your climate is cold, an 80% furnace might not even be available. So, consider furnaces of 90% and higher AFUE.

Furnace Input/Output: This is a confusing topic that we’ve alluded to in the content above.

Homeowners tend to think in terms of output – how many BTUs do I need for a warm house on the coldest day of winter?

Manufacturers list their furnaces by input – how many BTUs of heat does this furnace create? This doesn’t consider the waste – 5% in a 95% AFUE furnace and 20% in an 80% AFUE furnace, for example.

The manuals supplied by manufacturers list both for any given furnace, but if you browse their websites, most furnaces are listed by input – amount of heat created – and you have to do the math to get output.

The equation is:

Input in BTUs x % AFUE = Output BTUs


  • 100,000 BTUs x 80% = 80,000 BTUs
  • 100,000 BTUs x 90% = 90,000 BTUs
  • 100,000 BTUs x 97% = 97,000 BTUs

Know your Numbers! Is the furnace you’re considering listed by input or output? It can be difficult to tell. Sometimes there is no indication in the furnace’s model name; other times the information is right there.

The answer is in the documents, often a PDF file that lists Input and Output in separate columns. Remember that Output is the essential number once you have determined how many BTUs furnace you need.

Here are two examples:

The Goodman GCVM97 gas/propane furnace is advertised as a, “Modulating, Variable-Speed ECM Gas Furnace Up to 98% AFUE Heating Input: 60,000–120,000 BTU/h”

Note “Input: 60,000 – 120,000 BTU/h” If you look at the PDF document, you see that there are four models – 60,000, 80,000, 100,000 and 120,000 BTUs Input.

You have to do the math, multiplying input by .98 efficiency.

The Carrier/Bryant OVM oil furnace is available in several models/sizes, which are also named according to Output. The specifications PDF for the OVM98, for example, shows 98,000 BTU input and 80,000 BTU output.

Results – Recommended Furnace Size

Our results are the easiest to understand because we list them by the furnace AFUE you choose and the number of BTUs required for your home. Plus, we give you an estimated furnace size because manufacturers produce furnaces in increments of 10,000 to 20,000 BTU. For example, if you need 94,000 BTUs to heat your home, your choices, depending on the furnace brand, might be 90,000, 96,000 or 100,000 BTUs.

It’s generally better to go slightly bigger rather than a little too small. The smaller furnace would adequately heat your home on all but the coldest days, but it still makes sense to ensure your home is cozy every day of a cold winter.

Table of Furnace Sizing by Square Footage

OK, we’ve taken the time to input data for common home sizes, climates, etc. This Table is designed for homeowners unsure about whether they “got it right.”

>Use the Pick HVAC Gas Furnace Sizing Calculator for most accurate results.

>Use this Furnace Sizing Table to check your results compared with the most common results.

Home SizeHot ClimateModerate ClimateCold Climate
1,000 sq ft35,000 BTU44,000 BTU55,000 BTU
1,200 sq ft41,000 BTU50,000 BTU66,000 BTU
1,400 sq ft48,000 BTU59,000 BTU77,000 BTU
1,500 sq ft51,000 BTU63,000 BTU82,000 BTU
1,600 sq ft54,000 BTU67,000 BTU88,000 BTU
1,800 sq ft61,000 BTU75,000 BTU99,000 BTU
2,000 sq ft68,000 BTU83,000 BTU110,000 BTU
2,200 sq ft75,000 BTU92,000 BTU121,000 BTU
2,400 sq ft81,000 BTU104,000 BTU131,000 BTU
2,500 sq ft85,000 BTU105,000 BTU137,000 BTU
2,800 sq ft95,000 BTU124,000 BTU153,000 BTU
3,000 sq ft102,000 BTU125,000 BTU163,000 BTU

The Table uses the most common home sizes, 96% furnace AFUE efficiencies and Average for Insulation and Sunlight. The results show the most common furnace size for each scenario. Again, for the most accurate results, use the Pick HVAC Furnace Sizing Calculator.


Q: What size furnace do I need for a 1000 sq ft home?

A 35,000 to 60,000 BTU furnace.

Q:  what size furnace do i need for a 1500 square foot home

A 60,000 to 80,000 BTU furnace.

Q: What size furnace do I need for a 2000 sq ft home?

A 70,000 to 110,000 BTU furnace.

Q: What size furnace do I need for a 3000 sq ft house?

A 100,000 to 160,000-plus BTU furnace.

These are quick answers. Use the content above to get the most accurate gas furnace sizing calculator available anywhere. Use the Pick HVAC Gas Furnace Sizing Calculator, and when you get estimates from furnace contractors in your area, you’ll have a very clear understanding of what to expect.

How Many Square Feet Will 30,000 – 120,000 BTU Furnace Heat?

If you already bought a specific size furnace and want to know the square footage it can heat, this furnace sizing by btu to square footage Table can give you all common answers :

Furnace CapacitySquare Footage(Hot Climate)Square Footage(Moderate Climate)Square Footage(Cold Climate)
30,000 BTU1,000 sq ft650 sq ft540 sq ft
40,000 BTU1,320 sq ft880 sq ft720 sq ft
45,000 BTU1,500 sq ft1,000 sq ft810 sq ft
50,000 BTU1,650 sq ft1,100 sq ft900 sq ft
60,000 BTU2,000 sq ft1,320 sq ft1,090 sq ft
70,000 BTU2,320 sq ft1,550 sq ft1,270 sq ft
80,000 BTU2,650 sq ft1,760 sq ft1,450 sq ft
90,000 BTU3,000 sq ft2,000 sq ft1,630 sq ft
100,000 BTU3,320 sq ft2,210 sq ft1,810 sq ft
120,000 BTU4,000 sq ft2,650 sq ft2,180 sq ft

The aboved table is also calculated based on the 96 AFUE.

For example, a 90,000 BTU furnace is sufficient for heating a 3,000 sq ft home in Miami. If you’re living in Seattle, a 90,000 BTU furnace would only heat a 2,000 sq ft home.

How Many Furnace Btu Per Square Foot Do I Need?

You may also want an avearge furnace BTU per square foot rule of thumb to help you calculate the furnace size without an online calculator. This table can help you get an estimated BTU for gas or propane furnace easily.

Climate ZoneAverage Furnace Btu per Square Foot
Zone 130 Btu/sq. ft.
Zone 235 Btu/sq. ft.
Zone 340 Btu/sq. ft.
Zone 445 Btu/sq. ft.
Zone 550 Btu/sq. ft.
Zone 655 Btu/sq. ft.
Zone 760 Btu/sq. ft.

About Furnace Largest Size or BTU

The Pick HVAC Furnace Sizing Calculator gives results with BTU sizes over 200,000 BTUs.

Do they make furnaces that size? No – Not for residential use. Most top brands – Carrier, Trane, Lennox, Rheem and all the others, make furnaces with heating capacities of 40,000 or 50,000 BTUs on the low end to 120,000 to 140,000 BTUs on the upper end.

So, what is the answer? Many large homes, especially where winters are cold, have two furnaces. If your 3,000 square foot home in Nebraska needs 225,000 BTUs of heat to keep it warm, you might need to divide the space up and use a 140,000 BTU furnace for the main 2,000 square feet of the house and a second 90,000 BTU furnace for the “bedroom wing,” upper floor or other section of the home.

Our Furnace BTU Calculator tells you how much heat – how many BTUs – you need to keep your home warm in winter. For large homes, how that heat is produced, through one furnace or two, is best determined by considering the floor plan of your home and what space is occupied most frequently and during what times of the day.

Heating with two furnaces is a challenge, but with programmable thermostats and WiFi thermostats, you can maintain optimal comfort in your home and keep your heating costs as low as possible, or as we say at Pick HVAC, ALAP!

If you still don’t know how to figure btu for furnace, just leave a comment below.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

Share Your HVAC Quote/Cost

Share Your HVAC Quote/Cost

We rely on readers like you to share your HVAC system cost or quote. It really helps other visitors to estimate the cost of a new HVAC unit.

ie: Tranx XR13, Lennox xp15
Include Ductwork Replacement? *

Leave a Comment

DMCA.com Protection Status