How to use the BTU calculator
This BTU calculator answers the question most people ask before buying a window, portable or through-the-wall air conditioner: what size do I need? Air conditioner capacity is rated in British thermal units per hour (BTU/h, often written simply as BTU), and the right size depends mostly on the floor area of the room. The calculator looks the area up in the sizing table published by ENERGY STAR, the federal efficiency program, applies the adjustments ENERGY STAR gives for sun, shade, people and kitchens, and shows the result in BTU per hour, watts, kilowatts and tons. All of the math happens in your browser.
- Enter the room. Choose Length × width and type the two sides, or choose Floor area and type the square feet or square meters directly. Every length has its own unit, and you can type
12' 6",150 inor3.6 m. - Describe the room. Set the ceiling height, whether the room is heavily shaded, average or very sunny, how many people regularly use it, and tick the box if it is a kitchen.
- Add more rooms. Use Add room to size each room that gets its own air conditioner. Each room is sized on its own, and the results also show the total.
- Read the results. The results show the capacity needed, the size to look for, the same capacity in watts, kilowatts and tons, and a table that explains every step. Copy the results, download them as a CSV file or print them.
Tick Also estimate heating (rough) for a ballpark heating figure, and switch Mode to Convert units to turn any BTU/h figure into watts, kilowatts or tons, or back. Switch Units to Metric to see square meters. The calculator remembers your last entries on this device.
How to measure a room for an air conditioner
The ENERGY STAR table goes by floor area, so a tape measure is all you need. ENERGY STAR says to "Always measure in feet, converting the inches to fractions of a foot", so 12 ft 6 in is 12.5 ft; the calculator does that conversion for you, whatever unit you type.
- Square and rectangular rooms. Multiply the length by the width. A 12 × 15 ft room is 180 sq ft.
- Triangular areas. Multiply the length by the width and divide by two.
- L-shaped and odd rooms. Split the floor into rectangles and triangles, work out each part and add them up, then enter the total with Floor area. The square footage calculator adds up odd shapes for you.
- Open plans. Count only the area the unit has to cool. A room air conditioner cools the space it sits in; it does not push cool air through closed doors into other rooms.
How to size a room air conditioner
Sizing a room air conditioner takes a table lookup and up to three adjustments. The calculator does every step for each room:
floor area = length × width
capacity = ENERGY STAR table capacity for that area
× 0.9 if heavily shaded, × 1.1 if very sunny
+ 600 BTU/h for each person over two
+ 4,000 BTU/h if the room is a kitchen- Measure the floor area. Multiply the length of the room by its width, in feet. For an L-shaped room, split it into rectangles and add them up.
- Find the row in the table. Look up the area in the ENERGY STAR table and read the capacity needed, in BTU per hour.
- Adjust for sun or shade. Add 10 percent for a very sunny room, or take off 10 percent for a heavily shaded one.
- Add for people and cooking. Add 600 BTU per hour for each person over two who regularly uses the room, and 4,000 BTU per hour if the room is a kitchen.
- Pick a unit size. Choose a unit at or just above the result. A much bigger unit cools in short bursts and leaves the room damp.
Example: a 12 × 15 ft bedroom has 180 sq ft of floor. That falls in the row for 150 up to 250 sq ft, so it needs 6,000 BTU/h. With average sun and two people there is nothing to adjust: 6,000 BTU/h, which is 1,758 watts, 1.76 kW or 0.50 tons.
| Area to be cooled (sq ft) | Area (m²) | Capacity needed (BTU/h) | Capacity (kW) |
|---|---|---|---|
| 100 up to 150 | 9.3 to 13.9 | 5,000 | 1.47 |
| 150 up to 250 | 13.9 to 23.2 | 6,000 | 1.76 |
| 250 up to 300 | 23.2 to 27.9 | 7,000 | 2.05 |
| 300 up to 350 | 27.9 to 32.5 | 8,000 | 2.34 |
| 350 up to 400 | 32.5 to 37.2 | 9,000 | 2.64 |
| 400 up to 450 | 37.2 to 41.8 | 10,000 | 2.93 |
| 450 up to 550 | 41.8 to 51.1 | 12,000 | 3.52 |
| 550 up to 700 | 51.1 to 65 | 14,000 | 4.10 |
| 700 up to 1,000 | 65 to 92.9 | 18,000 | 5.28 |
| 1,000 up to 1,200 | 92.9 to 111.5 | 21,000 | 6.15 |
| 1,200 up to 1,400 | 111.5 to 130.1 | 23,000 | 6.74 |
| 1,400 up to 1,500 | 130.1 to 139.4 | 24,000 | 7.03 |
| 1,500 up to 2,000 | 139.4 to 185.8 | 30,000 | 8.79 |
| 2,000 up to 2,500 | 185.8 to 232.3 | 34,000 | 9.96 |
The calculator reads "100 up to 150" as 100 sq ft or more but under 150, so an area right on a boundary, such as 150 sq ft, goes in the row that starts there. The table starts at 100 sq ft: a smaller room gets its smallest capacity, 5,000 BTU/h. It ends at 2,500 sq ft; a larger space is beyond what a room air conditioner is meant for, and the calculator asks for a professional load calculation instead.
Adjusting for sun, shade, people and kitchens
The table is a starting point for an ordinary room. ENERGY STAR gives four adjustments, and the calculator applies each one you choose: "If the room is heavily shaded, reduce capacity by 10 percent. If the room is very sunny, increase capacity by 10 percent. If more than two people regularly occupy the room, add 600 BTUs for each additional person. If the unit is used in a kitchen, increase capacity by 4,000 BTUs."
ENERGY STAR lists the percentage first, so the calculator applies the 10 percent to the table capacity and then adds the amounts for people and for a kitchen. For the 12 × 15 ft example room:
- Very sunny: 6,000 + 10 percent = 6,600 BTU/h; look for 7,000 BTU/h.
- Heavily shaded: 6,000 − 10 percent = 5,400 BTU/h; look for 6,000 BTU/h.
- Very sunny, used by four people: 6,600 + 2 × 600 = 7,800 BTU/h; look for 8,000 BTU/h.
- A 10 × 12 ft kitchen: 120 sq ft is 5,000 BTU/h in the table, + 4,000 for the kitchen = 9,000 BTU/h.
"Regularly occupy" means the people who are usually in the room when the air conditioner runs, such as a family in a living room in the evening, not the occasional guest.
Sizing several rooms
A room air conditioner cools one room, so each room that gets its own unit is sized on its own. Add a row for each, with its own sun, people and kitchen settings. The calculator shows a size for every room and adds the estimates up, which is useful for comparing rooms, but the total is not the size of one unit and not the size of a whole-house system.
| Room | Area | Settings | Estimate | Look for |
|---|---|---|---|---|
| Bedroom, 12 × 15 ft | 180 sq ft | average sun, 2 people | 6,000 BTU/h | 6,000 BTU/h |
| Living room, 16 × 20 ft | 320 sq ft | very sunny, 3 people | 9,400 BTU/h | 10,000 BTU/h |
| Kitchen, 10 × 12 ft | 120 sq ft | kitchen, 2 people | 9,000 BTU/h | 9,000 BTU/h |
Together the three rooms come to 620 sq ft and 24,400 BTU/h, or 2.03 tons: a figure to compare, not a size to buy.
Which size to buy
Units are sold in fixed capacities, so the calculator also shows a size to look for: the smallest capacity in the ENERGY STAR table that is at or above your estimate. Treat it as a starting point. The capacities on sale vary, so check what is available and choose the closest one at or above the estimate, rather than the biggest you can find.
Bigger is not better. ENERGY STAR warns, "If the unit is too large, it will cool the room before it has a chance to remove the humidity," and "An oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling." A unit that is too small, on the other hand, runs constantly on hot days and may not reach the temperature you set.
High ceilings
ENERGY STAR notes, "The capacities are calculated based on an 8-foot ceiling." A room with a higher ceiling holds more air for the same floor area, so the table may understate what it needs. ENERGY STAR gives no correction factor, so the calculator does not invent one: it keeps the table figure and shows a note when a room's ceiling is higher than 8 ft, so you can treat the estimate as a minimum. Rooms with tall windows, a vaulted ceiling under a hot roof or an unusual layout are good reasons to ask a professional.
Whole-house sizing needs a Manual J load calculation
This calculator sizes room air conditioners. It is not a way to size central air conditioning, a heat pump, a ductless system or a furnace for a home.
Whole-house equipment is sized from a heating and cooling load calculation that accounts for the insulation, windows, air leakage, orientation, local climate and ducts of the actual house. In the United States the recognized method is ACCA Manual J, published by the Air Conditioning Contractors of America; ACCA describes Manual J as the national ANSI-recognized standard for producing HVAC equipment sizing loads, and says building codes in most places require a load calculation done with it.
The U.S. Department of Energy explains why square-foot shortcuts are a problem: “‘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 compromise comfort and efficiency.” Ask a qualified HVAC contractor for a Manual J calculation before replacing or adding a central system.
A rough heating estimate
Tick Also estimate heating (rough) for a ballpark figure for warming one room, for example when choosing a space heater. The calculator uses a simple rule:
volume (cu ft) = floor area × ceiling height
heating (BTU/h) = volume × temperature rise (°F) × factorThe temperature rise is the room temperature you want minus the coldest the room would get without heat. The factor starts at 0.133 BTU/h per cubic foot per °F, a rule of thumb used in heater sizing guides. We could not find an engineering or government source for it, so the field is editable and the result is labeled rough: a well-insulated, airtight room needs less, and a drafty room with large windows needs more. A temperature rise entered in °C is converted at 1 °C = 1.8 °F, since it is a difference, not a reading.
Example: the 12 × 15 ft room with an 8 ft ceiling holds 1,440 cu ft. Warming it by 40 °F gives 1,440 × 40 × 0.133 = 7,661 BTU/h, about 2,245 watts. For comparison, a 1,500 W electric heater puts out 5,118 BTU/h.
Real heat loss depends on the walls, windows, insulation and air leakage, which this rule cannot see. Never size a furnace, boiler or heat pump from it: those need the Manual J calculation described above.
BTU to watts, kilowatts and tons
A BTU (British thermal unit) is an amount of energy; air conditioner and heater capacities are a rate, BTU per hour. The calculator uses the International Table BTU, which NIST defines as exactly 1.055 055 852 62 kilojoules. Divided by the 3,600 seconds in an hour:
- 1 BTU/h = 0.29307107 watts, and 1 watt = 3.412 BTU/h.
- 1 kilowatt = 1,000 watts = 3,412 BTU/h.
- 1 ton of refrigeration = 12,000 BTU/h = 3.517 kW. The ton comes from the cooling of melting ice and has nothing to do with weight.
| BTU/h | Watts | Kilowatts | Tons |
|---|---|---|---|
| 5,000 | 1,465 | 1.47 | 0.42 |
| 6,000 | 1,758 | 1.76 | 0.50 |
| 8,000 | 2,345 | 2.34 | 0.67 |
| 10,000 | 2,931 | 2.93 | 0.83 |
| 12,000 | 3,517 | 3.52 | 1.00 |
| 18,000 | 5,275 | 5.28 | 1.50 |
| 24,000 | 7,034 | 7.03 | 2.00 |
| 36,000 | 10,551 | 10.55 | 3.00 |
These watts are the cooling or heating output, not the electricity an air conditioner draws. An air conditioner moves several times more heat than the electric power it uses, so its power draw, printed on its label, is much lower than its capacity in watts. An electric resistance heater is the exception: its watts in and its heat out are the same.
For metric users, the calculator converts every length to feet (meters ÷ 0.3048) and every area to square feet (square meters ÷ 0.09290304) before it looks up the table, and shows the results in square meters and cubic meters when Units is set to Metric.
How we calculate
Every figure in the tool and on this page comes from the same code, and the examples above are checked against it automatically.
- Area. Length × width after converting to feet (inches ÷ 12, centimeters ÷ 30.48, meters ÷ 0.3048), or the floor area you type (square meters ÷ 0.09290304).
- Table. The ENERGY STAR row with lower bound ≤ area < upper bound; the last row includes 2,500 sq ft. Under 100 sq ft takes the first row; over 2,500 sq ft gets no estimate.
- Adjustments. Table capacity × (100 − 10) ÷ 100 if heavily shaded or × (100 + 10) ÷ 100 if very sunny, then + 600 × (people − 2) when more than two people regularly use the room, then + 4,000 for a kitchen.
- Look for. The smallest capacity in the ENERGY STAR table at or above the estimate, or a note when the estimate is above 34,000 BTU/h.
- Several rooms. Each room on its own; the total is the sum of the room estimates.
- Heating. Floor area × ceiling height × temperature rise (°F; °C × 1.8) × your factor.
- Units. Watts = BTU/h × 1,055.05585262 ÷ 3,600; kilowatts = watts ÷ 1,000; tons = BTU/h ÷ 12,000.
The calculator does not account for climate, insulation, window area, appliances or the efficiency of a particular unit, and it does not estimate running costs. It sizes a room air conditioner the way ENERGY STAR describes; anything beyond that needs a professional.
Your measurements stay on your device
The calculator runs entirely as JavaScript in your browser tab. Nothing you enter is uploaded: the page's Content-Security-Policy allows connections only back to this site, and the tool has no code that sends anything anywhere. It remembers your last rooms and settings in your browser's local storage, on your device only. There is no account and no cookie.
Frequently asked questions
How many BTU do I need for my room?
Measure the floor area, look it up in the ENERGY STAR table, then adjust: take off 10 percent for a heavily shaded room, add 10 percent for a very sunny one, add 600 BTU per hour for each regular occupant over two and 4,000 BTU per hour for a kitchen. The calculator does all of this for each room you add.
How many BTU do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 sq ft, which falls in the table's first row, 100 up to 150 sq ft: 5,000 BTU per hour with average sun and up to two people. If it is very sunny, add 10 percent for 5,500 BTU per hour, and look for a 6,000 BTU unit.
What size air conditioner do I need for 500 square feet?
500 sq ft falls in the row for 450 up to 550 sq ft, so ENERGY STAR's table gives 12,000 BTU per hour, which is 1.00 ton or 3.52 kW, before any adjustment for sun, people or a kitchen.
How many BTU per square foot do I need?
There is no single figure, because the table is not a straight line. A 150 sq ft room gets 6,000 BTU per hour, or 40 BTU per square foot, while a 1,500 sq ft space gets 30,000, or 20 per square foot. Use the table rather than a per-square-foot rule.
What happens if my air conditioner is too big?
It cools the room quickly and shuts off before it has removed much moisture, so the room can feel cold and clammy. ENERGY STAR also notes that an oversized unit costs more to buy and wastes energy without cooling any better. Choose a size at or just above your estimate.
How many watts is a 5,000 BTU air conditioner?
A cooling capacity of 5,000 BTU per hour equals 1,465 watts, or 1.47 kW, of cooling. That is the heat it removes, not the electricity it uses: the power draw on its label is much lower.
How many BTU is a ton of air conditioning?
One ton of refrigeration is 12,000 BTU per hour, or 3.52 kW. A 24,000 BTU unit is a 2-ton unit.
Does ceiling height matter?
Yes. ENERGY STAR's capacities assume an 8-foot ceiling, and a taller room holds more air. ENERGY STAR gives no correction factor, so treat the table figure as a minimum for a high ceiling, and the calculator flags any room taller than 8 ft.
How many BTU do I need to heat a room?
As a rough guide, multiply the room's volume in cubic feet by the temperature rise in °F and by a factor, 0.133 by default. A 12 × 15 ft room with an 8 ft ceiling, warmed by 40 °F, comes to about 7,661 BTU per hour. It is a rule of thumb, not a load calculation.
Do I need a Manual J calculation?
For a window, portable or through-the-wall unit in one room, the ENERGY STAR table is the usual guide. For central air, a heat pump, a ductless system or a furnace, yes: whole-house equipment should be sized from an ACCA Manual J load calculation by a qualified professional, and building codes in most places require one.
Can one air conditioner cool two rooms?
A room unit cools the room it is in. If two rooms are open to each other, measure them as one area; if a door or wall separates them, size each room on its own and plan a unit for each.
Sources
- ENERGY STAR: Room Air Conditioners: the sizing table by area to be cooled, the adjustments for shade, sun, people and kitchens, the 8-foot ceiling basis and the advice against oversizing.
- U.S. Department of Energy, Building Science Education: HVAC Proper Sizing of HVAC Systems, on rules of thumb, oversizing and Manual J load calculations.
- Air Conditioning Contractors of America: Manual J Residential Load Calculation, the ANSI-recognized standard for residential equipment sizing loads.
- NIST Special Publication 811, Appendix B.8, Factors for units listed alphabetically: the BTU per hour in watts and the ton of refrigeration (12,000 BTU per hour).
- NIST Special Publication 811, Footnotes: the International Table BTU is exactly 1.055 055 852 62 kilojoules.
- National Institute of Standards and Technology: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement: 1 foot = 0.3048 meter and 1 inch = 2.54 centimeters, exactly.
This is an educational tool, not professional HVAC or engineering advice. The results are estimates for room air conditioners from the ENERGY STAR table; check the capacity and instructions of the unit you buy, and have a qualified professional size central air, heat pumps and furnaces with a Manual J load calculation.