HVAC Load Calculator – What BTU Size HVAC Do I Need?

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HVAC Load Calculator provides an accurate real world heat load estimate for both heating & cooling.

Additionally, the HVAC Calculator provides equipment recommendations (type heating/cooling system appropriate for your home), & calculates the cost of installing the equipment, including labor & materials!

Calculate Heat Load (BTUs), HVAC Size & Cost in VA

sq. ft.
Estimated Load Cooling / Heating: 0 BTU
Recommended Equipment
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We use a proprietary BTU calculation algorithm, that DOES NOT over-specify the unit capacity.

Most online tools give you a higher heat load estimate than you actually need for your home to sell you more expensive equipment.

How To Use HVAC Load Calculator

It is important that you enter accurate/appropriate details into the calculator. This tool brings you as close to a complicated Manual J estimation as possible. Otherwise, you may end up with a system that is too big or too small.

Step 1 (Climate Region): Select your Climate Region, using the Region Map at the top on the calculator. For example, if you live in NYC or NJ, select Region 3 (yellow). If you live TX, select Region 5 (red), etc.

Step 2 (Area Size): Enter square footage for your house/building or a specific area, for which you are doing the calculations.

This step is Critical for the accurate estimation of the annual heating/cooling load of your HVAC system! If you leave all settings as default, and only change the Region from 1 to 5 and back, you will see a huge change in Cooling/Heating BTU load.

Step 3 (Rooms / Zones): Enter number of Rooms/Zones where you want a new Heating/Cooling system installed.

If you plan to use a Central AC + Warm Air Furnace (ducted) system, or a Boiler for heating, the number of zones is not very important in terms of estimating heat load.

This value is most useful for figuring out which type of Ductless Mini-Split system to use.

Additionally, we discuss the PROS & CONS of using a multi-zone vs. installing multiple single-zone Ductless heat-pump systems, in our Mini Split DIY installation guide.

Typical Cost To Install a 4-zone Ductless Heat Pump Average: $11,890 - $18,910
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Step 4 (Space Height): Select average Ceiling Height of your house. In most cases this value should be equal to 8 ft. However if you have high ceilings, or cathedral/vaulted ceilings, the VOLUME of your space will be higher.

For cathedral/vaulted ceilings, add the lowest wall height + peak height, and divide by 2, to get the average. For example:

Your exterior wall is 8 ft. high, and the highest point on the ceiling is 12 ft. high. In that case, your average Ceiling Height is 10 ft:
(12 + 8) / 2 = 10

Step 5 (Insulation Grade): Most homes in US built between 1978 and 2000 will have 4″ studs with R-13 wall insulation, and should have R-38 roof / attic insulation. If this matches your home, leave this value as default (Avg. Insulation R-13 walls).

If you have a newer home with 6″ studs, will have R-18 insulation. In this case, select “More than Average” value.

In most cases you should not use “Very Well Insulated” value, unless you have a “Super-insulated” house.

If you have partially insulated home, select “Less than Average” or “Poorly Insulated”.

These two values are most critical in terms of heating, where your heat loss will be the highest. If your primary reason for installing a new HVAC system is cooling, we recommend using “Less than Average” value, to not over-size your cooling equipment.

Step 6 (Windows): Select the average amount of windows in your home. If you have ~1 window or fewer, for every 8 feet of exterior wall length, select “Average Amount”.

If you have more than 1 window, for every 8 feet of exterior wall length, select “More than Average”

Step 6 (Windows/Doors air tightness): Select appropriate window/door insulation level. In most cases, leave this as default “Average” value.

Average Costs For:
Most Homeowners Spent Between: Most People Spent: $4,970 - $6,430
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HVAC Load Calculation Results

Unlike other online HVAC calculators, we provide estimated heat load (system size in BTU/h) for both Heating & Cooling, as well as recommended HVAC equipment type and size!

You will get TWO results:

1) Cooling & Heating Load in BTUs – this is the actual calculated number of BTUs per hour & TONs needed to heat/cool your space.

You will get an approximate BTU / TONs load for your house, based on the information you entered in the calculator and your region.

Both heating and cooling BTU results, are calculated using our optimized BTU calculation algorithm, which is more “conservative” than most HVAC contractors and equipment sellers will give you.

On average these values will be 20-30% lower than “contractor’s guestimate”. However we recommend that you use the lower numbers, for reasons discussed above.

2) Best matching Heating / Cooling equipment type for your needs.

Our calculator tries to provide the best match / recommendation for equipment to use your particular situation, based on your Climate Region and other inputs.

Equipment Recommendation needs further clarification, as each person’s situation is different. Ideally, this calculator would be perfect for a new construction home, where you have total control of design and specifications of the type of HVAC equipment that will be used.

However, most homeowners in US are dealing with existing homes, which brings certain limitations.

First of all, if you have a Duct System in your home, a Central AC + Hot Air Furnace will be the most cost-efficient system for you.

In very hot climates, a furnace can be substituted with an electric heating coil, which would provide warm air on rare cold days/nights.

If you do not have air ducts, and live in climate zones 1, 2 or 3 – the best system for heating is a forced hot water boiler (with baseboards, wall radiators or radiant floor heat), and best cooling system is a multi-zone ductless (mini-split) AC, which are economical and extremely efficient.

In Regions 3, 4 & 5, you rarely have very cold weather. In these areas, winters are very mild, and average low temps are above 0F degrees.

Therefore, a high efficiency Ductless (mini-split) Heat Pump system can (and should) be used for both heating & cooling. It is most economical* type of heating/cooling you can get.

Ductless heat pumps can both heat and cool your house, in ambient temperatures as low as -15F degrees, and they are pretty good at doing both.

Since they can provide heating, and do so using a fairly low amount of electricity (3-4 times less than electric space heaters), you may not need to install an additional heating system, be it a furnace or boiler, saving yourself about $7,000-12,000+ in installation costs.

However, these should not be your ONLY source of heating in climate zone 1 & 2, where temperatures get very cold in the winter, and power outages are frequent, as ductless heat pumps run on electricity.

If you have a backup heating system (such as old boiler or gas/pellet stove, and can last a few days without electricity in case of a power outage, then you can use Heat Pumps for primary source of heating even in colder regions.

A big advantage is that ductless systems are “modular” and operate on zone level. So if you are spending most of the day in the living room, there is no need to cool or heat the entire house!

You only need to run 1 zone. At night, you can turn off the living room zone, and turn on the zones in bedroom(s).

Moreover, ductless systems are also about 2x more efficient than even the high efficiency modern Central AC systems, which means your electric bill will be 2x lower!

Actually even more than 2x, because of zoning, which is nearly impossible to do with central air conditioning systems.

* While most southern states have very low electricity costs (around $0.10-$0.13 per kWh), places for like California, were electric costs often exceed $0.30 per kWh, and PEAK pricing can go as high as $0.50 per kWh, a ductless AC / Heating system is ideal, as these are often 2 times more efficient than a Central AC, and you can condition only the parts of your house where you actually need cool or warm air, instead of cooling/heating your entire home, while you are sitting in the living room!

Pro Tip: If your house does not currently have air ducts, and your house is single level (ranch/cape), then air ducts and the AC + Furnace could be installed in your attic, using flexible insulated ducts.

This is a lot cheaper than traditional sheet-metal ducts that have to be installed from the basement, and extended into all your rooms, especially if your house consists of multiple levels.

In this case, installing Central AIR is significantly cheaper than Ductless Heat Pumps. However, due to massive efficiency difference, a ductless system will quickly cover the initial out of pocket cost difference, by saving an average of 40% in operating costs!

How HVAC Load Calculator Can Save You Money On Home Heating

Typical Cost To Install Central AC Average: $4,070 - $5,930
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While there are a few BTU heat load estimators available online, many homeowners are still unsure which heating and cooling system is best for them.

This is where our new algorithm can make an intelligent recommendation, which includes both system capacity (for heating & cooling), appropriate system type, and energy/fuel costs.

PRO TIP: Improving home insulation (in walls and attic), and sealing/insulating your air ducts, will have a significant effect on the BTU load of your cooling / heating system.

Savings in energy cost savings for both cooling & heating can be as high as 15-25%!

We also recommend that IF you plan to use the results of this heat load calculation for making your purchasing decisions, you SHOULD verify the results with this long-form Manual J online estimator.

Multiple heating/cooling systems: Another important feature of our calculator is the ability to estimate the cost of multiple heating / cooling systems being installed in large homes (over 3000 sq. ft.), and specifying largest possible BTU lead HVAC system(s) and then the smallest size system for the remainder of the total BTU load.

For example, if your heat load is 150K BTUs, and maximum residential Central AC size is 60K BTUs (5 Ton) then you need two 60K BTU compressors and a 30K (2.5 Ton) system.

The calculator’s algorithm will select the full size system(s) and the smallest needed system to cover the rest of required BTU load, to give you most cost efficient estimate.

HVAC System Installation Cost Estimate: the tool will estimate total installation price for your new HVAC system, which is based off equipment cost, and national average labor + overhead + profit, that plumbers/HVAC contractors charge for each type of system.

Average Costs For:
Most Homeowners Spent Between: Most People Spent: $4,970 - $6,430 (1250 - 1750 sq. ft.)
Low End
High End

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HVAC Sizing

Getting the right size HVAC system for your home/building is essential to providing enough capacity to heat or cool your living space. If your Heating or Cooling system is too small, you won’t get enough BTUs, and the space won’t be comfortable.

If you get too big of a system, you will be overpaying for the extra capacity: Bigger system = higher cost of installation. You will also be paying too much in operating costs (be it gas, electric or oil) going forward.

Most residential HVAC/Plumbing contractors do not want to spend the time to properly calculate (using Manual J method) the heat load & heat loss of your house (or individual rooms).

So instead to cover their “bases”, 99% of pros specify oversized systems (which as explained above, costs more to install and operate).

NOTE: Most contractors and equipment distributors use INFLATED BTU/h values when calculating the heat load and unit size (in tons/BTUs), primarily to cover their backs.

Our calculator uses lower BTU/h values for both Heating & Cooling, to provide a more a “real-world” heat load estimate.

However, we strongly recommend that you (or your contractor) do a Manual J heat load calculation of your home or a specific area, before making any purchasing decisions!

This calculator should be used and is intended for Informational Purposes only!

Average Costs For:
Most Homeowners Spent Between: Most People Spent: $3,630 - $4,780
Low End
High End

See costs in your area Start Here - Enter Your Zip Code

About Leo Bender

Leo has been a contractor since 2003, specializing in: roofing, siding, general contracting (GC) and residential remodeling. Leo is also a Certified HVAC, Oil & Gas Heating Technician/Installer. In addition to roofing and remodeling, Leo is passionate about Solar, green building and energy conservation, so a lot of my time and energy goes to installing energy efficient heating and cooling systems.

See more about Remodeling Calculator team here

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19 comments on “HVAC Load Calculator – What BTU Size HVAC Do I Need?

  1. EDS Tech

    Dear Leo,
    I appreciate you sharing your knowledge on HVAC load calculations. Your estimated calculating tool seems amazing.

  2. Mitesh Patel

    Hello Leo,
    Thank you for shar HVAC Load calculation related knowledge its look good and using great Estimated calculation tool.

  3. Dan S

    Hello Leo. Thanks for the great calculator. I’d like to use the result to determine how much fuel I can expect to use for a winter, ballpark. BTU is a unit of energy so already has time factored in. So if I get an estimate of 100k BTU to heat my house for a given degree day, is that for one hour, one day, one ? So if oil has 140 kBTU/gallon and 80% efficient conversion to heat, then I would burn 100k/140k/0.8 = 0.9 gallons of oil…per day?

    1. Leo B Post author


      Great question. We don’t use degree days, because it’s an outdated and inaccurate metric, which was used to calculate Oil / Coal / Propane deliveries interval, back in a day.

      We use BTUs / hour per Sq.Ft. of your house, then adjust to your climate region, and make additional adjustments for insulation, and duct/radiators heat loss. Formula is pretty complicated, but not as accurate as real Manual J heat loss calculation.

      Basically our calculator is designed to “guesstimate” your approximate fuel consumption, and based on current costs in your state, give you a ball-park cost figure.

      However, given how many unknowns and variables there are, it’s not always accurate. However, I’d say its within 5-10% spread.

      The biggest variables that can swing the result, are energy prices, which change too rapidly, and EPA stats are 2-3 months behind, and certain inaccuracy in heat loss calculation.

      To answer your question, we use 39 BTU/SF/H number to estimate your total consumption during the heating season. This is the base number, before any adjustments for length of heating season, heat loss, climate zone, etc.

      Some folks use a more aggressive 42-45 BTU/SF/H number “to cover their you know what”, because they are too lazy to do a Manual J calculation

      So in default mode, one Gallon of Oil (140K BTUs at 82% EFF) is 114,800 BTUs. Divided by 39 = 2944. So one gallon can heat one sq. ft. space for 2944 hours πŸ™‚ … or a 2950 sq. ft. home for about 1 hour… roughly speaking… And this is before any duct/radiator losses, insulation, windows, etc.

      This also give you an idea of what size Heating equipment to get, based on heat load requirement per sq. ft.

  4. Sandro

    I have a 3 to n unit for ac I wanna do a heat load calculation I want to replace cause it’s aging

  5. Matt H

    Hi, thanks for this great calculator. Quick question – the heat load required that is calculated on this website (i.e. 120K BTU Furnace) – is that the furnace output or input? For an 80% efficiency furnace: 120K BTU furnace would only output 96K BTU. Which value is being calculated?

    1. Leo B Post author

      Hi Matt,

      Thanks for the comment, I put many many hours into making this, and am very glad that people find it useful.

      To answer your question – heat OUTPUT of the furnace … Because end goal is to heat the space – not estimate how much gas or other fuel is used.

      If your input 100K BTUs, but only output 50K, while design load is 75K, you are way short on heat input πŸ™‚

      There are also input ratings on heating equipment, but those are more in relation to fire prevention and safety, and are more important for OIL BURNING DEVICES, not gas.


  6. Orlando C

    Thanks for your reply.
    My main concern was that one calculation gives me 3K BTU and the other 9K BTU.
    Also the cost of the smaller BTU unit would an issue, thanks.

    Last question:
    So if I install a 9K BTU for a 325 sq. ft. room (it is well insulated) it just means that it will cool the room faster and maintain the room temperature more efficiently than the 3K BTU unit?. Which it would be better outcome for room comfort.


    1. Leo B Post author


      On your last question – yes, a more “powerful” mini split will cool the room faster and maintain room temperature more efficiently.

      Now here is some technical data. The 9000 BTU is a rating of how much heat an air conditioner can remove per hour, at certain temperature (inside and out) and certain humidity level.

      Most ductless AC systems today have a variable speed compressor, which means the will only work as hard as they need to satisfy the thermostat.

      All ductless systems have MINIMUM capacity or minimum amount of BTUs they can provide … A typical 9000 BTU split system has 3000-5000 BTU/h minimum capacity. That means they cannot produce less than that amount without stopping the compressor completely.

      This is perfect for you – you will be getting your 3K btus, and compressor will work at its minimum load, consuming low amount of electricity.

      Again – 3K btu rated ACs do not exist – so you will have to get a bigger unit. Even the window-shaker ACs have much higher ratings than 3K btus (however those rating are worthless and misleading).

      Bottom line – for 325 s.f. room, a 6-9K BTU AC is perfect. Again 6K mitsubishi is the only single zone unit of that size, that I know of. All others start at 9K btus.

      In your case, I would recommend something like this Daikin 9K unit. It is priced on par with no-name Chinese brands, while Daikin is a very reputable manufacturer, and competes primarily with Fujitsu and Mitsubishi in quality, performance, etc. Daikin is one of 3-4 brands typically specified by architects for new condo complexes, hotels, etc., where minisplits are used.

      Good luck

  7. Orlando C

    When I fill out the short form for a Cooling only system I get about a 9K BTU system but when I use the long form I get 3K BTU system.
    it is small room 325 sq ft, 1 zone, 8 ft height, one low E 4 x 8 window, good insulation.
    When I use the other Cooling load calc I get same result about 9K BTU to 12K BTU (when I include the kitchen).

    1. Leo B Post author


      9000 BTU is pretty much the smallest size mini-split you can get. There is a 6K BTU Mitsubishi, but it’s so expensive, that I do not see a point in getting it.

      Some commercial multi-zone systems (like Fujitsu AirStage) have registers 4K btu… But these are for like 5+ zones. Not for a single room.

      Basically you can safely get a 9K unit. Fujitsu also makes 2-5 zon systems that can come with 7K btu heads.

  8. Jim Hughes

    I have seen several versions of this calculator on websites for High SEER and Service Titan. Can you tell me who produced it?

    1. Leo B Post author


      These calculators may seem similar – however they are not. High SEER and Service Titan use pretty basic methods of calculating BTU load. High SEER is actually overly aggressive and overspecifies system size, to sell more expensive equipment.

      Our calculator not only estimates BTU load, but also has an algorithm to recommend different HVAC systems based on house size, climate region, presence or absence of ductwork, and other parameters. For example, for the same house size in different regions (north vs south) our algorithm will recommend different heating system(s), and provide most economical price from standpoint of initial cost + medium term operating costs + best fit for the region. It will also provide accurate cost estimate.

      As far as “who produced it” – we develop our calculators in-house. I don’t think you can find a ready program like this on the web.

      We are planning to make these calculator(s) embeddable, so other websites can use them, in the next couple of month. After that we will also make them customizable, where users will be able to change prices, location, add & remove features, choose different design/appearance, etc.

      We are also considering adding settings like “presence of hot water radiators” (so that our algorithm would recommend Boilers over Furnaces) and other features. However, we worry that it might become too complicated for the end user.


  9. Dixit

    I do not recommend this calculator it suggested 140,000 btu for heating and 60,000 btu for cooling. When I did manual calculation room by room my house calls for 33,000 btu for heating and 19,000 btu for cooling. In HVAC bigger is not better, it has to be sized correctly.

    1. Leo B Post author


      You obviously did not use this calculator correctly. 33000 BTU heating/19000 BTU cooling is a system you would need for about 800-900 sq. ft. 140K/60K is for a whole house about 3000+ sq. ft. Therefore, it only makes sense that your inputs were off.

      What size house did you calculate? In what climate zone? What type of insulation do you have?
      All these make a difference, but even then not so much difference that you are off by 3-4 times.

      If anything, our calculator is more AGGRESSIVE in undersizing, compared to most HVAC contractors, meaning we calculate less BTU / sq. ft. than a contractor would (when they don’t want to spend time on manual load calculation). Therefore if you got a calculation that is lower than a contractor would recommend, I’d say everything is working fine. In your case I can only see this as incorrect inputs on your end.

      Cheeres, Leo

      1. Jim S

        Leo, thank you for the calculator. Very helpful.
        One question. – The calcuator doesn’t ask how many stories the house is . For the same TOTAL s.f., a 2 story house will require less heat and AC due to fewer external walls and a smaller roof. I think it can be significant.

        Also, how about the effect of a (heated) basement? – some more wall area, but the ground outside through the basement wall does not get hot in summer or cold in winter that much. …

        1. Leo B Post author

          Hey Jim,

          So while a two storey home has less roof area, it will have more wall / window area. Also roof is largely a source of heat loss in the winter, and less of a heat gain in the summer (even though it doesn’t feel like it).

          Bottom line, this calculator provides general idea, and cannot possibly account for all options and scenarios. And if we tried, it would become largely unusable for a typical homeowner, who doesn’t know much about heat loss / gain, BTUs, insulation, etc. For that, there is Manual J, which any contractor SHOULD use when sizing a new system.

          Also, consider the duct sizing, and we cannot possibly account for undersized / inadequate ducts within this calculator.