Furnace Size Calculator

Calculate the Heat Load (BTU size) of a High-Efficiency Furnace for your home. This calculator is based on simplified Manual J heat load calculation method, and estimates size of a Furnace based your home’s heat loss, Wall, Ceiling, Windows & Ductwork insulation levels, Duct heat loss, Furnace efficiency, indoor set temperature, etc.

We also use location-based local climate conditions, home size, such as Heating Degree Days, and Furnace Design Temperature (lowest winter temps in your state). To get the most accurate results, carefully adjust all settings. Results are only an estimate, and you should always perform a Manual J calculation to Size the Furnace for your home.

Calculate Furnace Size (BTU load) in Texas

Climate Region / Zone: Your estimated location is Texas - Climate Region #5

Your location on the climate zone determines the heat load needed for your furnace.

Some states have multiple climate zones/regions. Please adjust your climate region as needed - see map below:

For More Accurate Calculation, Select Your State:

Climate conditions, winter low temperatures, and as a result, Furnace Heat Load or Size (measured in BTUs/h) can differ greatly, even within the same Climate Zone/Region. For more accurate furnace size estimate, select your state below.

Note – some large states have multiple climate zones. For example California, Arizona, New Mexico, etc, can range from extremely hot (RED – zone 5), to a fairly cold (GREEN – zone 2) climates. Since these are large states, average winter temperature may not accurately represent your local conditions. In such case, you may use the Climate Zone selector to adjust your heat load estimate.

Why it is Important to Have Better Insulation?

Insulation is often overlooked by homeowners. Yet it can save you tens of thousands of dollars over a decade! Consider a 2000 sq. ft. house in Illinois or Massachusetts.

A super-insulated home (30R walls / 60R attic / 0.21U Triple Glaze windows) would need 47800 - 48200 BTUs per hour (50K BTU furnace).

Same size home with lowest insulation grade, will need 152200 - 154700 BTUs of heating per hour. That is 3 times more energy usages, and a furnace size that's 3 times bigger.

The difference is about 100K BTUs per hour or 1 Therm of Natural Gas. Consider that Natural Gas in IL costs $2.40/therm. In MA, the cost $1.94/therm

Asuming that a furnace runs 15-20 minutes / hour, the EXTRA cost would be $14.4 - $19.2 / per day in IL, and $11.64 - $15.52 per day in MA.

PRO TIP: Use our BTU cost calculator to compare costs of different heating fuels (Gas, Oil, Propane, Electricity & Wood Pellets) in your state.

Why are Ducts Important? Avoid up-to 45% Heat Losses.

Did you know that you can loose as much as 45% of your heat in the ducts? While this is rare, a 20-40% losses in the ducts are very common.

This means that if you spend $2000 / year on heating, $400-900 is typically wasted in you ducts!

Worst-case scenario: Imagine un-insulated metal duct running through un-insulated crawl space or attic. Let’s say outside temperature is 5°F. Temperature inside your uninsulated (vented) attic/crawl-space is probably around 15°F. And now imagine ALL THE HOT AIR made by your furnace, is passing through your thin wall metal ducts.

Let’s calculate duct heat loss: Assume you have about 100 ft. of 12″ square duct – that’s 400 sq. ft. of surface. Now the R-value of 26 ga. steel is about 1/2R … Let’s call it 1R, which means its U-factor is also 1U (U = 1/R-value).

The supply air temperature is about 140°F, and if attic temp is 15°F, Delta T (ΔT) is 140-15 = 125°F.

Here is how much heat you are loosing:

LOSS (BTUs/hr) = 1U * 400 s.f. * ΔT 125°F * 1 Hour = 50,000 BTUs / Hr!

If you have a 95% AFUE, 100,000 BTU furnace, half is lost in the ducts, and another 5% or 5,000 BTUs is lost as exhaust gas. Your total heat loss is 55%, and you only have 45,000 BTUs left for heating your home (before accounting for heat loss associated with leaky windows, poor wall and attic insulation, etc.

How much is 50,000 BTUs anyway? Here is a simple answer. 50,000 BTUs is equivalent of TEN (10) 1500W space heaters running on full blast!

That’s how much heat you would be losing in the example above!

Now imagine you have 10R insulation around your duct. Now the formula will look like this:

LOSS (BTUs/hr) = 1/10R (0.1U) * 400 s.f. * ΔT 125°F * 1 Hour = 5,000 BTUs / Hr!

Now your combined heating system loss is just 10%, and you retain 90% or 90,000 BTUs to actually heat the house. Of course it’s best to insulate the attic, then have a ΔT of say 20-50°F instead of 125°F. But the same goes for insulating everything in your home!

By hyper-insulating your ducts, you can reduce your heat loss as much as 90%, and reduce your heating bill by as much as 40% each year, for as long as you live in the house!

How to get Accurate Furnace Size (BTU Load) Estimate

It is important to have a properly sized Furnace, to save money on heating costs, and not be too cold in the winter.

A furnace that’s too large, will use more gas/heating oil to run. A furnace that’s too small, may not produce enough heat during the coldest days of the winter – when outside temps are in single digits, or even below 0°F.

Calculating furnace size involves many variables, such as insulation level of your walls, attic, windows, basement, U-factor and R-values, air tightness, square footage of your windows and exterior walls, heat loss per hour, leaky air ducts, etc.

Ideal method to calculate a furnace size is to perform a Manual J calculation. However, even HVAC professionals often struggle with this lengthy and complicated method, with 100s of data points.

NOTE: We made our Furnace Size Calculator almost as good as Manual J, but very easy to use, with just 14 inputs/selectors!

What these “lazy heating contractors” do instead, is use your square footage, and a “rule of thumb” number – like 45-50 BTUs per square foot. Then, to “cover their butts”, they upsize your furnace another 10000 BTUs – just to not get an “it’s too cold” phone call from the homeowner.

This “lazy method”, does not take into account the insulation and air tightness of your home, or your ductwork.

PRO TIP: Insulation is extremely important! A super-insulated home can use up-to 75% less energy, and a much smaller furnace to provide enough heating, even in the cold climates. For example, a 2000 s.f. home in northern New Hampshire may only need a 50,000 BTU furnace.

The same size home with little insulation & leaky single pane windows, may need more that 150,000 BTU furnace!

Ducts are Very Important: You can lose as much as 45-50% of your total heat through your ducts! At the same time, properly designed, and well insulated / sealed ducts, may have heat loss as little as 4-5%!

Bottom line – it’s best to have a fairly accurate estimate of a furnace size, than a rule of thumb + 10K BTUs. Here is how to get the accurate size estimate:

• Step 1: Select your Climate Zone. This will be preselected based on your location. Confirm that you have correct zone, by looking and climate zone map.
• Step 2: Enter your home size (conditioned/heated living space).
• Step 3: Enter Indoor Target Temperature – this is how warm you want your home to be. Default is 72°F.
• Step 4: Select furnace efficiency. Furnaces in 90+ % AFUE range are considered “High Efficiency”. Note – while High Efficiency furnaces use less gas, and retain more heat, they are more expensive to install ad require more maintenance. Often it’s more economical to use a standard efficiency furnace (80-86% AFUE).
• Step 5: Select fuel type. In most cases, you will want Natural Gas. All oil furnaces are “standard efficiency” and have around 85% AFUE ratings.
• Step 6 (IMPORTANT): As mentioned above – you can loose as much as 45-50% of your heat through ducts. Furnace size calculation is heavily influenced by duct type, insulation and location in your house.
• Step 7 (IMPORTANT): Select duct insulation level. Most flex ducts will have 6R value, which is the code minimum in most states. Metal ducts can be insulated or un-insulated. If you selected “un-insulated” type in step 7, this setting will “disappear”.
• Step 8 (IMPORTANT): Select duct location in your home. This selector is critical, as ducts located in un-insulated attic or crawl space will have immense heat losses. Consider this – ALL of your warm air moves through ducts BEFORE entering your living space. With un-insulated metal ducts located in un-insulated attic or crawl space, you can lose as much as 40% of your heat!
• Step 9 *: Select House Insulation Level. This is a “parent” selector for the next 3 settings (Walls, Attic & Windows insulation). This means that each level in this option will change the following options. However, by clicking on “Show More Options”, you can make individual adjustments to Walls, Attic & Windows insulation grades. Every time you change “House Insulation” setting, Walls, Attic & Windows settings will also change.
• Step 10 (Optional): Adjust the insulation level of your walls. Standard 2×4 walls with fiberglass insulation will have 13R value. 2×6 walls with fiberglass insulation will have 19R value. 2×6 with closed cell spray foam insulation will have around 30R value.
• Step 11 (Optional): Select attic insulation level.
• Step 12 (Optional): Select windows quality (insulation level) and U-factor.
• Step 13 (Optional): Select amount of windows in your home.

BOOM – YOU ARE DONE! Now click “calculate” and you will have an estimated furnace size, including duct and insulation losses, etc.