Q = U x A x dT over the whole envelope. The DOE on why rules of thumb oversize
Every other BTU calculator multiplies your floor area by a number between 20 and 30. The US Department of Energy's own building science program says that practice is what produces excessively oversized systems, and oversizing is not the safe direction: an air conditioner that is too big cools the house before it has dehumidified it. This does the conduction arithmetic on your actual envelope instead, shows the rule of thumb beside it, and names every term a real Manual J adds that this does not.
Envelope conduction load
ft
ft
ft
deg F
R
R
R
sq ft
U
sq ft
R
BTU/h per sq ft
Conduction only. Infiltration, solar gain, internal gains, duct losses and latent load are not included, and they are listed below rather than left silent.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Where the heat leaves, and the rule of thumb beside it
The source also says to expect $25 to $60 per square foot, which for a 2,000 sq ft house is $50,000 to $120,000, about ten times its own system range, and it prints the gas furnace low end as $3.800.
Retrofitting an existing home costs more at every zone count than new construction in the source's table.
Worked example
A 40 by 28 ft house, 9 ft of wall, at a 60 degree design difference, with R-13 walls, an R-38 ceiling, a slab floor, 180 sq ft of U-0.30 window and 40 sq ft of R-5 door.
Walls. 136 ft of perimeter by 9 ft is 1,224 sq ft gross, less 220 of openings, so 1,004 net. U is 1/13 = 0.077, so 0.077 × 1,004 × 60 = 4,634 BTU/h.
Ceiling. 1,120 sq ft at R-38 is U-0.026: 1,768 BTU/h. Two and a half times the wall area, and a third of the heat, which is what R-38 buys you.
Windows. 180 sq ft at U-0.30 is 3,240 BTU/h. That is 180 sq ft doing nearly as much as 1,004 sq ft of wall.
Doors. 40 sq ft at R-5: 480 BTU/h.
Total.10,122 BTU/h of conduction, which is 0.84 tons.
The rule of thumb. 1,120 sq ft × 25 = 28,000 BTU/h, 177% higher.
Look at the windows. A sixth of the wall area is responsible for two fifths of the conduction, because U-0.30 is R-3.3 and the wall around it is R-13. That is the single most useful thing an envelope calculation tells you and a square-foot rule cannot see it at all. It is also why the comparison figure is on the page: 28,000 against 10,122 is not a rounding difference, and the conduction figure is a floor that infiltration and solar gain will push up before any equipment is chosen.
The formula
One expression, summed over the envelope:
Q = Σ (A / R) × ΔT
A
the area of each surface: net wall, ceiling, floor, glazing, doors
R
its R-value. U is 1/R and it is what the arithmetic uses
ΔT
design temperature difference, inside minus outside. A local figure
Σ
over every surface separately, because they have different R-values
tons
BTU/h divided by 12,000, which is a definition
Windows are entered as a U-factor rather than an R because that is what is on the label, and a manufacturer's U-factor covers the whole assembly including the frame and the edge of the glass. An R-value quoted for the center of the glass alone is a better number than the window actually delivers.
What a square-foot rule cannot see
Two houses of 1,120 sq ft. One has R-13 walls, an R-38 ceiling and U-0.30 windows. The other has R-21 walls, R-60 above, an insulated floor and U-0.20 glass. A rule of thumb gives them the same equipment, because floor area is the only thing it looks at.
The physics gives the first 10,122 BTU/h of conduction and the second 6,690. That is a third less, and it is the difference between two sizes of equipment.
The Department of Energy's Building America program is direct about the consequence: rules of thumb produce excessively oversized systems, which cost more, waste energy and cycle too often. On the cooling side it is worse than waste, because dehumidification only happens while the coil runs, so an oversized unit reaches setpoint and stops before it has taken the moisture out.
None of which means this page is a substitute for a load calculation. It is the conduction term, which is the biggest single piece and the one that is honest arithmetic. Everything else Manual J does is listed below, unbuilt and named.
The arithmetic, which is one line
Heat moves through a wall in proportion to its area, to how bad the wall is at stopping it, and to how much colder it is outside:
Q = U × A × ΔT, and U = 1 / R
R is the insulation value you already know. U is its reciprocal, the conductance, and it is what the arithmetic actually uses: an R-13 wall is U-0.077, and an R-49 attic is U-0.020. Doubling R halves U and halves the heat through that surface.
Do that for every surface of the envelope, add them up, and multiply by the design temperature difference. That is the conduction load, and it is the largest single term in a real load calculation.
The rest of the arithmetic on this page is one definition: one ton of cooling is 12,000 BTU per hour, from the heat needed to melt a short ton of ice in a day.
Why this page will not just multiply your floor area
Every other BTU calculator takes your square footage and multiplies it by something between 20 and 30. The US Department of Energy's Building America program, run by Pacific Northwest National Laboratory, says what that produces:
“Rules of thumb” are too often used to size comfort systems, which results in excessively oversized systems. This leads to increased cost, wasted energy, and too-frequent on and off cycling, which can lead to comfort and efficiency issues. A trained contractor or qualified designer will determine a home's specific heating and cooling loads to establish the right size equipment.
That is a government laboratory with nothing to sell, calling the method the rest of the internet uses the cause of oversized systems.
And oversizing is not the safe direction. A furnace that is too big heats the house to setpoint quickly and stops, so it spends its life starting and stopping, which is where the wear and most of the waste is. An air conditioner that is too big is worse: cooling and dehumidifying are different jobs, and moisture comes out of the air only while the coil is running. An oversized unit satisfies the thermostat first and leaves the house cold and damp.
So the square-foot figure is on this page only as a comparison, and the calculator prints the gap between it and the physics so you can see how far apart they are on your building.
Windows are the hole in the argument, and they are supposed to be
On the worked example, 180 sq ft of glazing does 3,240 BTU/h and 1,004 sq ft of wall does 4,634. A sixth of the area, two thirds of the heat that the wall loses.
That is not a fault in the windows, it is arithmetic. A U-0.30 window is R-3.3. The wall it sits in is R-13, four times better, and a good wall today is R-21 or more. Glass will always be the weakest part of an envelope and the gap widens every time insulation standards move.
Which makes glazing the highest-leverage number on this page. Going from U-0.30 to U-0.20 on the example takes 1,080 BTU/h out of the load, more than adding R-8 to every wall in the house.
Use the U-factor from the label, not an R-value from a brochure. The label figure covers the whole assembly, frame and glass edge included. A center-of-glass R-value describes a part of the window you cannot buy on its own.
Design temperature difference, and where the county figure is
The load scales linearly with the temperature difference you design for, so it is the single most influential input, and it is local.
Winter design temperature appears in IRC Table R301.2(1), filled in jurisdiction by jurisdiction. ENERGY STAR also publishes a heating and cooling design temperature limit for every county, and the heat load calculator looks yours up and adds air leakage to the conduction this page calculates.
What it is not is the coldest night on record. Design conditions are exceedance-based: the equipment is sized for a temperature that is beaten a small percentage of hours in a year, on the reasoning that sizing for the extreme means running badly for the other 99% of the time. That is the same logic that makes oversizing a mistake rather than a margin.
Enter the difference between your indoor setpoint and the design outdoor temperature. Seventy inside and ten outside is a 60 degree difference.
Slabs, crawl spaces and the floor term
The floor input defaults to zero and that is correct for a slab on grade, which is why it does so.
A slab does not lose heat through its area in any way this arithmetic describes. The ground under the middle of a slab reaches something close to the slab temperature and stops taking heat; the loss is at the perimeter, into the outside air, and it is calculated per linear foot of edge with an F-factor rather than per square foot with an R. That is a different term and it is not on this page.
A floor over a vented crawl space or an unheated basement is a genuine area loss and does belong here, at whatever R the floor insulation gives.
A floor over conditioned space below is not a loss at all, because both sides are at the same temperature.
What a real Manual J adds that this does not
This page computes conduction. A full ACCA Manual J computes considerably more, and the honest thing is to list exactly what is missing rather than let a number imply completeness:
Infiltration and ventilation
Air leaking through the envelope and air brought in deliberately. On a leaky house it can rival the conduction load; on a tight one it is small. It needs a blower door result or an assumed air change rate, and this page will not assume one.
Solar gain
Sun through glazing, which depends on orientation, shading, glass coating and latitude. It is a cooling load that a west-facing window imposes and a north-facing one does not, and area alone cannot see it.
Internal gains
People, lighting, cooking and appliances. Roughly the output of a small heater per occupant, which helps in winter and hurts in summer.
Duct losses
Ducts running through an unconditioned attic or crawl space lose to it. This can be a large fraction of the load and it is a design decision, not a property of the building.
Latent load
Moisture. On the cooling side a system has to remove water as well as heat, and the latent share has nothing to do with the temperature difference. It is why an oversized unit leaves a house cold and clammy: it satisfies the thermostat before it has dehumidified anything.
Manual J itself is not a free document and it is not reproduced here. What it is, per ACCA Manual J, Residential Load Calculation: ACCA Manual J is a standard for producing HVAC equipment sizing loads for single-family detached homes, small multi-unit structures, condominiums, townhouses, and manufactured homes.
So treat the figure above as a floor. It is the part of the load that is pure arithmetic, it responds to your actual building rather than its floor area, and it shows why two houses of the same size need different equipment. It is not a specification, and equipment should be selected against a real load calculation by somebody who does them.
Pacific Northwest National Laboratory for the US Department of Energy
An official US government publication. It is quoted here because it is the clearest statement, from a body with no product to sell, that the square-foot method this page refuses to use is the problem rather than the shortcut.
Air Conditioning Contractors of America, described by BASC
The standard itself is not free and is NOT reproduced here. This page carries the conduction term only and names everything Manual J adds that it does not.
The design temperature difference is not published here either. Winter design temperature is a local value, it appears in IRC Table R301.2(1) as a jurisdiction-by-jurisdiction figure, and the calculator asks you for it rather than assuming a climate.
Frequently asked questions
How many BTU do I need per square foot?
The question does not have an answer, which is the point of this page. The US Department of Energy's Building America program says rules of thumb of that kind are what produce excessively oversized systems. Two houses of the same floor area with different insulation and glazing can differ by a third in conduction load alone.
How do you calculate heating and cooling load?
For each surface of the envelope, area divided by its R-value, times the design temperature difference, then added up. That is the conduction load. A full ACCA Manual J then adds infiltration, solar gain, internal gains, duct losses and a latent load for moisture, none of which are calculated here.
How many BTU is one ton of cooling?
12,000 BTU per hour, by definition. It comes from the heat needed to melt a short ton of ice over 24 hours. So a 3 ton system is 36,000 BTU/h.
Is a bigger air conditioner better?
No, and it is worse than merely wasteful. Cooling and dehumidifying are separate jobs and moisture only leaves the air while the coil is running. An oversized unit reaches the thermostat setpoint quickly and shuts off, so the house ends up cold and damp. It also cycles on and off constantly, which is where the wear is.
Why does my window matter so much in the calculation?
Because a U-0.30 window is R-3.3 and the wall around it is R-13 or better. On the worked example, 180 sq ft of glazing loses 3,240 BTU/h against 4,634 for 1,004 sq ft of wall. Improving the glazing is usually higher leverage than adding insulation to the walls.
What design temperature should I use?
The one your jurisdiction publishes, in IRC Table R301.2(1). Where it does not, ENERGY STAR's county design temperature limits are free to read, and the heat load calculator on this site uses them. It is not the coldest night on record: design conditions are exceeded a small share of hours each year on purpose.
Do I include the floor if I have a slab?
No, leave it at zero. A slab on grade loses at its perimeter rather than through its area, and that is calculated per linear foot with an F-factor, which is a different term not included here. A floor over a vented crawl space or unheated basement does belong in the calculation.
How do I calculate BTU for a room?
Exactly as for the house, using that room's exterior surfaces only: each wall, window and ceiling area divided by its R-value, times the design temperature difference. Walls shared with other heated rooms drop out because both sides are at the same temperature. Room by room is also how a Manual J splits the load to size the ducts.
How do I size a radiator in BTU?
Match the radiator's rated heat output to the heat loss of the room it serves, which is the room calculation above. Manufacturers publish radiator output at a stated water temperature, so compare the rating at the temperature your boiler actually runs.
What size heater do I need for a garage?
The arithmetic is the same as for any room and the inputs are harsher. Enter the real R-values, which in an unfinished garage may be little more than bare framing, and count the garage door as a large area at its own low R. It can dominate the load the way windows do in a house.
What does a split system HVAC cost?
A split system has an outdoor unit and an indoor coil or air handler joined by refrigerant lines, which describes most central air conditioners and heat pumps. HomeAdvisor prices a central air conditioner at $3,900 to $7,900 installed and a heat pump at $4,200 to $7,600. HVAC estimation should still start from the load, because the load decides the size being priced.
How many watts of baseboard heat does a room need?
Divide the room's heat loss in BTU per hour by 3.412, because one watt of electric resistance heat gives 3.412 BTU per hour. Enter the room's own dimensions, walls, windows and design temperature difference above and the page gives the load in both units. A room losing 3,412 BTU/h needs 1,000 W of baseboard heat.
What does a zoned HVAC system cost?
HomeGuide's zoning guide puts two zones in an existing home at $1,700 to $2,800, three at $2,200 to $3,500 and four at $2,600 to $4,500, with each added zone $350 to $500. New construction is cheaper, $1,500 to $2,000 for two zones. Each zone gets its own thermostat and dampers, and a room by room load is how each zone's share of the total is found.
How do I convert kilowatts to BTU?
Multiply by 3,412. One watt of heat is 3.412 BTU per hour, so one kilowatt is 3,412 BTU/h, and a 12,000 BTU/h ton of cooling is about 3.5 kW of heat. That is heat moved, not electricity drawn: a 3 ton air conditioner moves about 10.5 kW of heat and draws far less, because it moves heat rather than making it.
How much does an attic fan cost?
HomeAdvisor's June 2026 guide gives $369 to $914 for an attic fan installed, and a roofer at $45 to $85 an hour if the roof has to be cut for it.
How much does it cost to install baseboard heating?
HomeAdvisor's June 2026 guide gives $420 to $1,526 for a typical project and $200 to $1,100 per heater installed, with an electrician at $75 to $250 an hour.
How much does a split system HVAC cost?
A split system has an outdoor unit and an indoor unit joined by refrigerant lines, which is how most central air and heat pump systems are built. HomeAdvisor's June 2026 guide gives $3,900 to $7,900 for central air, $4,200 to $7,600 for a heat pump and $5,000 to $12,500 for a whole new system.
How much does electric baseboard heating cost?
HomeAdvisor's June 2026 guide gives $420 to $1,526 for a typical electric baseboard heating job, $40 to $650 per heater unit and $390 to $1,190 of labor for one unit.
How much does it cost to replace an HVAC control board or line set?
HomeAdvisor's June 2026 guide gives $100 to $600 for a circuit or control board, $600 to $2,400 for coil repair and $800 to $3,000 for a compressor. It does not price a refrigerant line set replacement separately.
Check these numbers yourself
Proper Sizing HVAC System, Building America Solution CenterPacific Northwest National Laboratory for the US Department of Energy. An official US government publication. It is quoted here because it is the clearest statement, from a body with no product to sell, that the square-foot method this page refuses to use is the problem rather than the shortcut.
ACCA Manual J, Residential Load CalculationAir Conditioning Contractors of America, described by BASC. The standard itself is not free and is NOT reproduced here. This page carries the conduction term only and names everything Manual J adds that it does not.
How every figure here is verified: Sources & Method. Who builds this: Lucas Pradella, and the method is on About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This is the conduction term, not a load calculation. Infiltration, solar gain, internal gains, duct losses and latent load are not included and are listed on the page rather than left silent. ACCA Manual J is not a free document and is not reproduced here. Treat the figure as a floor and have equipment selected against a real load calculation. Sources read 8 September 2026.