BTU Calculator - Heating, Cooling, Room Size & Cost

How to use it

1. Pick heating or cooling, then pick the space: a single room, a whole house, or a garage or shop. Garage switches the engine to the volume and temperature rise method, which is the right shape for a tall leaky building.
2. Enter length and width in their own feet and inches boxes, or switch to typing the square footage straight in. Set the ceiling height if it is not 8 ft.
3. Choose your climate zone and how well the space is insulated. Insulation picks the position inside the zone's band, so good sits at the low end and poor at the high end.
4. For cooling, set the sun exposure, how many people are usually in the room, and whether it is a kitchen. Those are ENERGY STAR's published adjustments, not our invention.
5. For a garage, set the temperature you want to hold and the coldest morning you want to hold it on. That difference drives the whole answer.
6. Read both numbers in the result: the adjusted estimate and the flat rule beside it, with the gap named.
7. Pick the equipment you are actually shopping for. The rating plate turns the load into the number on the box, plus the circuit, the size step and the runtime.
8. Open the fuel ledger and type your own prices. It shows the cost per 100,000 BTU delivered and the cost per hour to run the unit you just sized.

At a glance

• Bedroom, 12 by 15 ft, 8 ft ceiling, zone 4 with average insulation, heating: 180 sq ft at 47.5 BTU per sq ft is 8,550 BTU an hour. The flat 45 rule says 8,100, so it is 5 percent short here. That is 2,506 watts, which is two 1,500 W plug-in heaters on two separate circuits, or 11 ft of baseboard at 2,750 W, or a 12,000 BTU cold-climate mini split head. At 17 cents a kWh, resistance heat costs 43 cents an hour and a heat pump at COP 3.0 costs 14 cents.
• Two-car garage, 24 by 24 ft, 10 ft ceiling, held at 60 F on a 20 F morning, average construction: 5,760 cu ft warmed 40 degrees at 0.133 is 30,643 BTU an hour. About 24,514 of that goes through the shell and 6,129 is air changing, roughly one and a half air changes an hour. A 45,000 BTU input unit heater at 80 percent gives 36,000 of output and covers it; the 30,000 unit gives 24,000 and does not. A propane torpedo burns 0.335 gal an hour, so about 14 hours on a 20 lb tank at 99 cents an hour.
• Whole house, 1,800 sq ft, zone 5, good insulation, 96 percent AFUE furnace: 90,000 BTU of output. At 96 percent AFUE that is 93,750 BTU of input, so the next standard step is a 100,000 BTU input furnace. This is the result that carries the Manual J advisory: a tight modern house often computes 30 to 40 percent below the table method on a real load calculation.
• Living room, 12 by 14 ft, very sunny, two people, cooling: 168 sq ft sits in ENERGY STAR's 150 to 250 band at 6,000 BTU, plus 10 percent for a very sunny room is 6,600, so you buy the 8,000 BTU unit on the ladder. The flat 20 BTU per sq ft rule says 3,360, which is smaller than the smallest room air conditioner made.
• Kitchen, 300 sq ft, very sunny, four people: 8,000 base, plus 800 for sun, plus 1,200 for two extra people, plus a flat 4,000 for the kitchen is 14,000 BTU, so a 14,000 BTU unit. The adjustments more than doubled the base, which is why a kitchen sized off the square footage alone never keeps up.

How the math works

Heating a room or a house uses the area method: floor area times a BTU per square foot figure from your climate zone, scaled by ceiling height against a standard 8 ft. The zone figure is a band, not a point, and insulation picks the position in it, so zone 4 runs 45 to 50 and average insulation lands at 47.5. Cooling uses ENERGY STAR's published room air conditioner table, which reads the base capacity straight off the floor area, then applies their four adjustments in order: multiply by 0.90 if the room is heavily shaded or 1.10 if it is very sunny, add 600 BTU for each person beyond two, and add a flat 4,000 BTU if it is a kitchen. Scaling that result for ceiling height is our extension and is labelled as such, because ENERGY STAR's page says the table assumes an 8 ft ceiling and suggests looking for a higher air delivery rate rather than more BTU. A garage or shop leaves the area method entirely and uses volume times temperature rise times a loss coefficient, because a tall leaky building with a big steel door does not behave like a bedroom. That coefficient splits into the part that conducts through the shell and the part that is air changing, at 0.018 BTU per cubic foot per degree per air change, which is the volumetric heat capacity of air and the reason the familiar 1.08 times CFM times delta T works. Every load then goes through the equipment translation: watts at 3.412 BTU per watt, furnace input at your AFUE rounded up to a real size step, mini split nameplate divided by a cold-weather derate, baseboard at 853 BTU a linear foot, propane at 91,500 BTU a gallon. The fuel ledger prices 100,000 BTU delivered by each fuel at your own prices and efficiencies, then multiplies by the load to give a cost per hour. Every printed breakdown is apportioned by largest remainder, so the rows you can see always add to the number above them.

1. 1. Measure the floor, and keep feet and inches apart

   A 12 by 15 ft bedroom is 180 sq ft. Type 12 and 15 in the feet boxes; a room that is 12 ft 6 in goes in as 12 and 6, never as 12.6, because 12.6 ft is 12 ft 7 and a quarter. If you already know the square footage, switch the input over and type it straight in. For an odd-shaped room, break it into rectangles and add them up, or use the square footage calculator and bring the total back here.

2. 2. Read the zone band, and let insulation pick the position

   The heating figures are bands, not points: 30 to 35 BTU per sq ft in zone 1, 35 to 40 in zone 2, 40 to 45 in zone 3, 45 to 50 in zone 4 and 50 to 60 in zone 5. Good insulation sits at the bottom of the band, poor at the top, average in the middle. This is the single biggest lever on the page. The same 180 sq ft room needs 5,400 BTU in a hot zone with good insulation and 10,800 in a cold zone with poor insulation, which is exactly double, for the same room.

3. 3. Scale for the ceiling, because heat is sold by the cubic foot

   The per-square-foot figures assume an 8 ft ceiling. A 9 ft ceiling is 12.5 percent more air to heat, a 7 ft basement is 12.5 percent less, and the takeoff prints that as its own row, signed, so a low ceiling shows as a deduction rather than quietly disappearing. Above 12 ft this stops working, and the tool says so: a two-storey great room or a shop with 16 ft trusses wants the volume method instead.

4. 4. Cooling reads the ENERGY STAR table, then adjusts it

   The base comes straight off floor area: 100 up to 150 sq ft is 5,000 BTU, 150 up to 250 is 6,000, 450 up to 550 is 12,000, and so on up to 2,000 up to 2,500 sq ft at 34,000. Then the four official adjustments. A 300 sq ft sunny kitchen with four people goes 8,000 base, plus 800 for sun, plus 1,200 for the two extra people, plus 4,000 for the kitchen, which is 14,000 BTU: the adjustments nearly doubled it. That is why a kitchen sized off the square footage alone never keeps up in August.

5. 5. A garage leaves the area method and uses its volume

   Volume times the temperature rise you want times a loss coefficient. A 24 by 24 ft garage with a 10 ft ceiling is 5,760 cu ft; holding it at 60 F on a 20 F morning is a 40 degree rise; at 0.133 for average construction that is 30,643 BTU an hour. The area method on the same building says 576 times 45, or 25,920, which is 15 percent short, because the area method never saw the 10 ft ceiling or the steel door.

6. 6. Split the garage coefficient into shell and air

   Of that 30,643, about 24,514 conducts through the ceiling, walls, door and slab edge, and about 6,129 is outside air coming in and inside air going out, which works out to roughly one and a half air changes an hour. Air is 0.018 BTU per cubic foot per degree, so at a 40 degree rise every complete change of the air in that garage costs 4,147 BTU. Every time the overhead door goes up you throw away most of one. Weatherstrip the door bottom before you buy a bigger heater.

7. 7. Translate the load into the number on the box

   Watts is the load divided by 3.412, so 8,550 BTU is 2,506 W, and since a plug-in heater stops at 1,500 W that is two of them. Baseboard is 853 BTU a linear foot, so 8,550 needs 11 ft. A furnace is rated by input, so divide the output you need by the AFUE and round up to a real size: 90,000 output at 96 percent is 93,750 of input, which buys a 100,000 BTU furnace. Garage unit heaters have their own ladder starting at 30,000 and 45,000. A window air conditioner rounds up 5k, 6k, 8k, 10k, 12k, 14k, 18k, 24k.

8. 8. Derate the mini split before you shop it

   A mini split's nameplate is its cooling size at 47 F outdoors, and it makes less heat as the outdoor temperature falls. So the nameplate you need is the load divided by the fraction of capacity the unit keeps at your design temperature. At a conservative 70 percent, 8,550 BTU needs 12,214 of nameplate, which is the 18,000 head. At 90 percent for a genuine cold-climate unit it is 9,500, which is the 12,000 head. Same room, two sizes apart, entirely because of one number nobody prints on the box.

9. 9. Price an hour of running it

   Delivering 100,000 BTU takes 29.31 kWh of resistance heat, or 9.77 kWh through a heat pump at COP 3.0, or 1.05 therms of gas at 95 percent AFUE, or 1.15 gallons of propane at 95 percent. At 17 cents a kWh, $1.60 a therm and $2.80 a gallon that is $4.98, $1.66, $1.68 and $3.22 respectively. Multiply by the load over 100,000 for the cost per hour: the 8,550 BTU bedroom is 43 cents an hour on resistance heat and 14 cents on a heat pump. Those prices are editable placeholders, not market data.

Worked scenarios, start to finish

Real jobs with the numbers carried all the way through, so you can see how the estimate is built and check your own figures against them.

The judgement calls a pro makes

Decisions the tool cannot make for you, and how someone who does this for a living thinks them through.

How honest to be about your insulation

This is the input people flatter themselves on, and it costs the most. Good means the walls and attic are genuinely insulated to something like current guidance, the windows are double glazed, and you cannot feel air moving at the outlets on a windy day. Average means the house has insulation and nobody has ever measured it. Poor means empty stud cavities, single glazing, or you can see daylight somewhere. If you are between two, pick the worse one for equipment you cannot easily add to, and the better one if you are about to insulate anyway. Better still: work out what is actually up there with the insulation calculator first, because adding insulation is almost always cheaper per year than the next size of heater.

Whether to round up, and by how much

One step up is normal and correct, because the size ladders are coarse and the estimate is not surgical. Two steps up is a mistake with a long tail. An oversized unit satisfies the thermostat in a few minutes, shuts off, and repeats, which is called short cycling. It wastes fuel on every start, wears the ignition and the compressor, and on the cooling side it never runs long enough to pull moisture out, which is where the clammy room comes from. If you find yourself between two sizes and tempted upward, spend the difference on air sealing instead and stay on the smaller unit.

Which derate to give a mini split

This is the one number that changes the answer by two size steps, and manufacturers do not print it on the box. The nameplate is the cooling size at 47 F. On the NEEP cold-climate list a nominal 12,000 BTU head delivers anywhere from 6,300 to 16,600 BTU at 5 F, a median of 11,000, so the honest spread is enormous. Use 70 percent for a unit with no published low temperature data, which is roughly the lower quartile of even the cold-climate-listed products. Use 90 percent only when you have the manufacturer's extended capacity table in front of you and it supports it at your own design temperature.

When the garage engine beats the area method

Switch to the volume method whenever the ceiling is over about 10 ft, whenever there is a large uninsulated door, or whenever the space is not continuously heated. Those three describe most garages, shops and barns. The area method was built for a conditioned room inside a conditioned house, and it silently assumes an 8 ft ceiling, an insulated envelope and no 112 sq ft steel panel in the wall. It is not that it is inaccurate for a garage; it is that it is answering a different question.

Whether to size the heater or fix the building

Look at the loss breakdown before you look at the equipment. In a poorly insulated space the ceiling and walls carry over half the load, and both are fixable. In a well insulated one, air leakage becomes the single biggest share, and air sealing is the cheapest work on the list. The rule that holds up: the load you remove is removed for the life of the building, and it makes the equipment smaller, cheaper to buy and cheaper to run at the same time. The load you cover with a bigger heater you pay for every hour, forever.

How much to trust a rule of thumb in a garage

The three coefficients here are the values the trade uses, and they run conservative on purpose. Build the same 24 by 24 ft garage up from components and an insulated one lands nearer 0.04 to 0.07 rather than 0.08, while an uninsulated one lands at 0.14 to 0.17 against the 0.18 preset. The gap is mostly headroom for warming a cold garage up rather than just holding it, which is what a garage heater is actually asked to do. The coefficient is editable for exactly this reason. If you have done a real heat loss calculation, type your number in.

Common mistakes, and the fix

• × Using one flat BTU per square foot figure for the whole country.

  ✓ The flat 45 rule oversizes a hot-zone room with good insulation by 50 percent and undersizes a cold-zone room with poor insulation by 25 percent. Same room, same rule, wrong in both directions. Set the zone and the insulation and read the adjusted number.

• × Buying a bigger unit because bigger feels safer.

  ✓ Oversized equipment short-cycles: it wastes fuel on every start, wears out faster, and on the cooling side leaves the room cold and damp because short runs never dehumidify. One size up is fine. Two is a mistake you live with.

• × Putting two 1,500 W space heaters on the same circuit because there were two outlets.

  ✓ A 1,500 W heater draws 12.5 A, and a 15 A circuit may only carry 1,440 W continuously. Each heater needs its own circuit with nothing else on it. If you need more than one, a 240 V hardwired heater or a mini split is the right answer.

• × Sizing a mini split off the nameplate in a cold climate.

  ✓ The nameplate is the cooling capacity at 47 F. Divide the load by the fraction of capacity the unit keeps at your design temperature, then round up. For the same 8,550 BTU room that is the 18,000 head at 70 percent and the 12,000 head at 90 percent.

• × Confusing a furnace's input rating with what it actually delivers.

  ✓ A furnace is sold by input. An 80,000 BTU furnace at 80 percent AFUE puts 64,000 into the house. Work out the output you need first, divide by the AFUE, then round up to a real size step.

• × Sizing a garage on floor area.

  ✓ A garage has a tall ceiling, a huge uninsulated door and real air leakage, and the area method sees none of them. Use volume times temperature rise. On a 24 by 24 ft garage with a 10 ft ceiling the area method comes out 15 percent short.

• × Treating the number as a specification and ordering equipment off it.

  ✓ This is a refined estimating method, not an ACCA Manual J. It does not model window area and orientation, real air infiltration, or duct losses. Use it to budget and to sanity-check a quote; get a Manual J before you buy whole-house equipment.

Material and unit specifics

BTU and BTU per hour

A BTU is a quantity of heat: roughly what a kitchen match gives off. BTU per hour is a rate, and it is what every piece of equipment is actually rated in, even though the label usually just says BTU. Everything on this page is a rate. When a heater says 5,118 BTU, it means 5,118 BTU every hour it runs.

Watts, and the 3.412 that connects them

One watt is 3.412 BTU per hour, so a kilowatt hour of electricity is 3,412 BTU. That single number is why resistance heat is expensive: at 17 cents a kWh, 100,000 BTU takes 29.31 kWh and costs $4.98, while the same heat through a heat pump at COP 3.0 takes 9.77 kWh and costs $1.66. A 1,500 W plug-in heater is 5,118 BTU per hour and there is no way to make it more.

AFUE, and input versus output

Annual Fuel Utilization Efficiency is the fraction of the fuel's energy that ends up as usable heat over a season. Gas appliances are sold by INPUT: a 100,000 BTU furnace at 95 percent AFUE delivers 95,000. Standard residential inputs are 40k, 60k, 80k, 100k and 120k; garage unit heaters run 30k, 45k, 60k, 75k, 100k and 125k. Condensing furnaces at 90 percent and up need a PVC flue and a condensate drain, which is a real installation cost difference.

COP, and why a heat pump is not 100 percent efficient

Coefficient of Performance is heat delivered divided by energy in. A heat pump moves heat rather than making it, so a COP of 3.0 means three units of heat for one of electricity. COP falls as it gets colder. On the NEEP cold-climate list the median single-zone head has a COP of 3.52 at 47 F and 1.90 at its maximum output at 5 F, so the same machine costs roughly twice as much to run on the coldest morning of the year as it does in October.

Tons of cooling

One ton is 12,000 BTU per hour. The name is literal and historical: it is roughly the rate of cooling you get from a ton of ice melting over a day. Central equipment is sold in tons and half tons, room units in BTU, and mini splits in BTU that are really quarter tons. 6,000 is half a ton, 12,000 is one, 24,000 is two.

Therms, gallons and cords

A therm is exactly 100,000 BTU and is how the gas bill is denominated. A gallon of propane is about 91,500 BTU, and a 20 lb barbecue tank holds about 4.7 gallons, so it carries roughly 430,000 BTU. A cord of seasoned hardwood is about 20 million BTU, though species swing that from 12 to 30 million, and a ton of premium pellets is about 16.4 million.

Electric baseboard at 250 watts a foot

Residential baseboard is built at a standard 250 W per linear foot, which is 853 BTU per hour per foot. That fixes the length: 8,550 BTU needs 11 ft, which is more wall than most bedrooms have free. Baseboard is the cheapest heat to buy and the most expensive on the fuel table to run, and it wants a dedicated 240 V circuit sized at 125 percent of the load.

Design temperature

The outdoor temperature you size for is not the coldest it has ever been. It is roughly the 99th percentile winter temperature for your area, which means a handful of hours a year will be colder and the system will simply run continuously through them. Sizing for the record low guarantees an oversized system that short-cycles for the other 8,750 hours.

Reading your result

Read the two numbers together. The big one is the adjusted estimate for your space; the one beside it is what the flat rule of thumb would have told you, and the gap between them is the point of the page. If the flat rule reads high, the rule was about to sell you a unit half a size too big. If it reads short, it was about to leave you cold on the worst morning of the year. Then look at the rating plate, which is the number you actually shop for: watts and heater count for electric, input BTU at your AFUE for gas, nameplate after the derate for a mini split. If your load falls between two size steps, take the one above it, and take only one. Below that, the loss breakdown tells you where the load is coming from, and it is the part worth acting on: if the ceiling and walls carry half of it, insulation buys you a smaller and cheaper system permanently, whereas a bigger heater is a bill you pay every hour. Finally the fuel ledger, which prices an hour of running each option at your own numbers. On the seeded prices a heat pump runs at about a third of resistance heat and about the same as natural gas, and that ranking is usually more decisive than the equipment purchase price. None of these figures is a quote and none is a specification. For whole-house equipment, this is the number you take into the conversation, not the number you order from.

Limitations

• This is a refined estimating method, not an ACCA Manual J. Manual J models each room, each window's area and orientation, real measured infiltration and duct losses; this page models floor area, climate, insulation level and a handful of published adjustments.
• It does not model window area, orientation or shading beyond ENERGY STAR's single sun adjustment, and glass is often the largest single item in a real cooling load.
• It does not model duct losses. Ducts running through an unconditioned attic or crawlspace can lose 20 to 30 percent of what the equipment produces, which no amount of correct sizing recovers.
• The heating zone bands are estimating conventions, not code. Your local energy code and your utility's own guidance govern, and a house that has been air sealed and insulated recently can sit well below its zone band.
• Cold-climate heat pump capacity varies enormously by model. The derate here is a placeholder for the manufacturer's extended capacity table at your own design temperature, which is the only figure that actually applies to the unit you are buying.
• The garage coefficients are trade rules of thumb, not manufacturer-published constants, and they run conservative because a garage heater is usually asked to warm a cold building up rather than hold a warm one. Modine's own sizing page publishes no formula and says to get a professional heat loss calculation.
• Fuel prices and efficiencies in the ledger are editable placeholders for planning, not current market data. Electricity, gas and propane prices vary by a factor of three across the United States and change through the year.
• Unvented propane and kerosene heaters put their combustion products into the room. They are for garages, shops and job sites with genuine fresh-air ventilation, never for sleeping spaces and never unattended.
• For any whole-house equipment purchase, get a Manual J load calculation. It costs a few hundred dollars against several thousand for the equipment, and it is the only thing that turns an estimate into a specification.

Frequently asked questions

How many BTU do I need for a 12x12 room?

144 sq ft. For heating in a moderate zone 3 with average insulation that is 144 times 42.5, or 6,120 BTU an hour, which is 1,794 watts, so two plug-in heaters or 8 ft of baseboard. In a cold zone 5 with poor insulation the same room needs 8,640. For cooling, 144 sq ft sits in ENERGY STAR's first band, 100 up to 150 sq ft, so the base is 5,000 BTU, and that is also the smallest room unit made. Add 10 percent if it is very sunny and 600 BTU for each person in it beyond two.

Is a bigger furnace or air conditioner safer?

No, and it is the most expensive mistake on this page. An oversized unit reaches the set point in a few minutes, shuts off and repeats. That short cycling wastes fuel on every start, wears the ignition and the compressor, and swings the temperature more than a correctly sized unit does. On the cooling side it is worse: the unit cools the air before it has pulled the moisture out, so the room ends up cold and clammy. One size up from the estimate is normal and fine. Two sizes up is something you live with for twenty years.

How many square feet does a 12,000 BTU unit cool?

450 to 550 sq ft, per the ENERGY STAR table, in a room with average sun and no more than two people in it. Take 10 percent off that range if the room is very sunny, so about 400 to 500. If it is a kitchen, the flat 4,000 BTU kitchen adder eats a third of the unit before you start, so a 12,000 BTU unit covers more like a 300 sq ft kitchen. In heating terms a 12,000 BTU mini split head is a different animal, because its heating output at 5 F can be anywhere from 6,300 to 16,600 BTU depending on the model.

What is 1,500 watts in BTU?

5,118 BTU per hour, because one watt is 3.412 BTU per hour. That is the ceiling for anything you plug into a normal 120 V outlet, and it is why a single space heater tops out at about 108 sq ft in a cool zone with average insulation, or 85 sq ft in the cold northern tier with poor insulation. The electrical side matters as much as the heat: 1,500 W at 120 V is 12.5 amps, and a 15 amp circuit may only carry 80 percent of its rating continuously, which is 12 amps or 1,440 watts. So a 1,500 W heater is already over the continuous limit on its own and must not share a circuit with anything else.

What size heater do I need for a 2 car garage?

For a typical 24 by 24 ft garage with a 10 ft ceiling, held at 60 F on a 20 F morning with average construction, the answer is 30,643 BTU an hour, which buys a 45,000 BTU input gas unit heater at 80 percent efficiency. Insulate it well and the same building drops to about 18,400, which is a 30,000 BTU unit. Leave it uninsulated and leaky and it rises to about 41,500, which needs the 60,000. The three biggest levers, in order, are the ceiling height, the temperature rise you want and the door.

Is 45 BTU per square foot a good rule?

It is a reasonable average for a cool climate and completely wrong at the ends. A 180 sq ft room in a hot zone with good insulation needs 5,400 BTU, and the flat rule says 8,100: 50 percent oversized. The same room in a cold zone with poor insulation needs 10,800, and the flat rule still says 8,100: 25 percent short. That is why this page prints both numbers side by side rather than choosing for you. The equivalent cooling rule, 20 BTU per square foot, is worse still, because on small rooms it returns numbers below the smallest unit anyone manufactures.

How much does it cost to run an electric heater per hour?

A 1,500 W heater uses 1.5 kWh an hour, so at 17 cents a kWh it costs 25.5 cents an hour, or about $6.12 a day running continuously. Per unit of heat, resistance electricity is the most expensive fuel on the ledger: delivering 100,000 BTU takes 29.31 kWh at $4.98, against $1.66 through a heat pump at COP 3.0, $1.68 for natural gas at 95 percent AFUE and $3.22 for propane. Type your own electricity price into the ledger, because the national spread is more than three to one.

Do I need a Manual J load calculation?

For a portable heater, a window air conditioner or a single mini split head in one room, no. This estimate is the right level of precision for that decision. For a furnace, a heat pump or anything ducted, yes. A Manual J models each room, the actual window areas and orientations, measured air leakage and duct losses, and it commonly comes out 30 to 40 percent below the table method in a house built since about 2010. It costs a few hundred dollars against several thousand for the equipment, and it is the difference between an estimate and a specification.

Does ceiling height change how many BTU I need?

Yes, roughly proportionally, because you are heating a volume of air rather than a floor. The published per-square-foot figures assume an 8 ft ceiling, so a 9 ft ceiling adds about 12.5 percent and a 7 ft basement takes about 12.5 percent off. This page prints that adjustment as its own signed row so you can see it. Above about 12 ft the scaling stops holding up, because a tall space stratifies and the warm air collects where nobody is standing. At that point switch to the garage and shop mode, which works from cubic feet and a temperature rise, and consider a ceiling fan on winter reverse.