How Window AC Sizing Works
A window air conditioner cools a single room with no duct losses, so the sizing math is the same room cooling-load calculation used by our main air conditioner tonnage calculator. We start with your room's square footage, apply a baseline of about 22 BTU per square foot, then adjust for ceiling height, climate zone, sun exposure, insulation, occupants, and appliances.
The difference is the final step. Window units are sold in fixed BTU sizes, so we round your calculated load up to the nearest available capacity and report the result in BTU (the number printed on the box) rather than tonnage.
Want the full formula? See our methodology page. For a whole-house ducted system, use the split AC tonnage calculator, or for a ductless single-zone unit try the mini split size calculator. And if you are weighing a window unit against a central system, the SEER savings calculator shows what an efficiency step is actually worth per year in your climate.
Window AC Size Chart by BTU and Room Size
People arrive here going one of two directions. Either you know the room and want the BTU number, or you have already seen a unit on a shelf and want to know whether it will actually cool your room. The window AC size calculator above answers the first one. The chart below and the sections under it answer the second, using the same bands the calculator uses.
One thing to know before you read it. These bands are narrower than the coverage printed on the box, and that is deliberate. Manufacturer ratings assume a bare room in mild weather. Ours assume a room with people, a television, lights and a fan in it, which is what rooms actually contain.
| Window AC Size (BTU) | Tonnage | Room Size It Cools | Typical Room |
|---|---|---|---|
| 5,000 BTU | 0.4 Ton | 135 – 155 sq ft | Box room, small home office. Smallest unit sold. |
| 6,000 BTU | 0.5 Ton | 170 – 195 sq ft | Standard bedroom |
| 8,000 BTU | 0.67 Ton | 240 – 280 sq ft | Large bedroom, small studio |
| 10,000 BTU | 0.83 Ton | 310 – 360 sq ft | Studio apartment, small living room |
| 12,000 BTU | 1 Ton | 385 – 440 sq ft | Living room, bedroom with a sitting area |
| 14,000 BTU | 1.17 Tons | 455 – 525 sq ft | Sunny or top floor living room |
| 15,000 BTU | 1.25 Tons | 490 – 565 sq ft | Large living room. Often needs its own circuit. |
| 18,000 BTU | 1.5 Tons | 600 – 690 sq ft | Open plan main room. Few models made. |
| 24,000 BTU | 2 Tons | 815 – 935 sq ft | Whole small apartment. Largest window unit made. |
| Note: bands assume 8 ft ceilings, average insulation, moderate sun, and a room with two people and normal electronics in it. Hot and humid climates sit at the low end of each band, mild climates at the high end. | |||
5000 BTU Air Conditioner Room Size: The Smallest Window Unit Sold
A 5,000 BTU window AC covers about 135 to 155 sq ft. It is the floor of the entire category, and that creates a situation people run into constantly. If your room is smaller than this, you cannot buy down. One owner posted about a 66 sq ft room and worried that a 5,000 unit rated for 150 sq ft would be far too big for it. It is oversized, and so is everything else on the shelf. In a room under roughly 120 sq ft, buy the 5,000 and run it on the lowest fan speed, because nothing smaller is manufactured.
6000 BTU Air Conditioner Room Size and the Overkill Question
A 6,000 BTU window AC covers roughly 170 to 195 sq ft. This is the size that produces the most second-guessing, usually from someone with a 120 sq ft room asking whether 6,000 is overkill. By the bands above it is, by about 50 sq ft. Does that matter much? At this end of the ladder, honestly not very. The gap between 5,000 and 6,000 is small enough that an inverter model will modulate straight through it. Two owners reported 220 and 226 sq ft bedrooms staying cool on 6,000 running on low, which is above the band and still works, because a bedroom at night has no sun load and almost nothing generating heat in it.
8000 BTU Air Conditioner Room Size, and Why 350 Sq Ft Is Optimistic
An 8,000 BTU window AC covers about 240 to 280 sq ft. You will also see this exact unit advertised as covering 350 sq ft, and that gap is where a lot of disappointment begins. Two owner reports sit either side of it. One sized a 282 sq ft room with 8 ft ceilings and low sun, landed on 8,000, and it worked. The other put 8,000 into a 290 sq ft studio and described it as horrible and barely cooling. Eight square feet did not cause that. What separates those two rooms is what else is inside them, which is the subject of the studio section further down.
10000 BTU Air Conditioner Room Size in Real Apartments
A 10,000 BTU window AC covers roughly 310 to 360 sq ft on our bands. Owner reports run well above that, and this is the size where the gap between the math and real life opens widest. One owner cools a 676 sq ft open plan space in Florida on 10,000 and says it is comfortable on the low setting. Another reports 10,000 easily cooling an entire 1,000 sq ft top floor apartment. Those are owner reports rather than our calculation, and they work because open space with few interior doors behaves nothing like the same floor area cut into rooms. Size for the room the unit sits in, not the floor area on the lease.
12000 BTU Air Conditioner Room Size Where a Window Unit Hits 1 Ton
A 12,000 BTU window AC covers about 385 to 440 sq ft, and it is exactly 1 ton. So this is the rung where window units and central systems finally get described in the same language, which matters when you are comparing options. A contractor talking about 1 ton and a box on a shelf reading 12,000 BTU are the same quantity of cooling doing completely different jobs. One serves a room through a window, the other serves a house through ducts. If you need to move between the two labels at any size, our BTU to ton conversion chart lists every common step.
14000 BTU Air Conditioner Room Size for Sunny and Top Floor Rooms
A 14,000 BTU window AC covers roughly 455 to 525 sq ft. The reason this step exists at all, wedged between 12,000 and 15,000, is that sun and floor level can move a room across a boundary without the floor area changing at all. Our engine adds 10% for heavy sun exposure and 5% for a top floor. Run the numbers on a mild climate and a 420 sq ft top floor room facing the afternoon sun comes out at 14,000, while the identical room on the ground floor with shade comes out at 12,000. When you are stuck between two sizes, the tiebreaker is sun and floor level, not square footage.
15000 BTU Air Conditioner Room Size and the Circuit It Needs
A 15,000 BTU window AC covers about 490 to 565 sq ft. This is also roughly where the electrical question starts mattering more than the sizing question. Units at and above this capacity commonly want a dedicated circuit rather than sharing a bedroom outlet with everything else on that wall, and some ship with a plug shape that will not fit the receptacle under your window. Check that on the spec sheet before you buy, because it turns a same-afternoon install into an electrician visit. Our AC wattage calculator gives you the running watts, the amps, and the startup surge for any BTU rating.
18000 BTU Air Conditioner Room Size Near the Window Unit Ceiling
An 18,000 BTU window AC covers roughly 600 to 690 sq ft. Two things change once you reach this size. The unit gets heavy enough that the window frame and the sill stop being an afterthought and become part of the decision, and the number of models on the market drops off sharply. Retail ranges are deep at 5,000 through 12,000 and thin above 15,000. So the practical question quietly shifts from what size do I need to what can I actually get delivered.
24000 BTU Air Conditioner Room Size: The Largest Window AC Made
A 24,000 BTU window AC covers about 815 to 935 sq ft, and it is the top of the ladder. There is no step above it. The calculator on this page flags that for you rather than hiding it: when your room load comes out past 24,000, the result tells you the room has outgrown the category instead of quietly showing you a size nobody manufactures. That warning is the single most useful thing this page can give you, so the next section is about what to do when you get it.
When Your Room Needs More Than a Window AC Can Deliver
Here is the situation that sends people round in circles, and almost nothing online addresses it directly. You measure your living room, put the numbers into a sizing tool, and it tells you that you need 20,000 to 24,000 BTU. Then you go shopping and every window unit sold for a room that size is rated 14,000. So which one is lying?
Neither, quite. One owner hit this exact wall with a 700 sq ft living room: the calculator returned 20,000 to 24,000 and the retail listings for that space said 14,000. On our bands a 700 sq ft room genuinely does need 24,000, so the calculator was right and the retail coverage rating was optimistic. Another owner was told plainly that 1,000 sq ft needs around 20,000 BTU in total and that window units are not sold that big.
So what do you do about it? There are three honest answers and not one of them is a bigger window unit.
- Split the load across two units. Two 12,000 BTU units in two windows will cool 700 sq ft better than one 24,000 unit ever could, because you are putting the air where it needs to be instead of asking one fan to throw it the length of the room.
- Cool the room you sit in, not the floor plan. If the space measures 700 sq ft only because a dining area opens off it, size for where people actually are and let the rest coast a few degrees warmer.
- Step out of the category. Past roughly 900 sq ft the honest answer is a ductless head or a central system. Our mini split size calculator covers the first and the split AC tonnage calculator covers the second.
The rule: if the calculator hands you more than 24,000 BTU, stop looking at window units altogether, because the number you need is not made in that shape.
Is a Bigger Window AC Better?
Every window AC discussion has this question in it somewhere, usually phrased as whether there is any harm in going one size up just to be safe. Short cycling and the cold, clammy room it produces are covered in the FAQ below. What is worth adding here is the part that actually settles it, because "never oversize" is not quite true either.
Going one size up is the right call when the room carries something the square footage does not capture:
- A west or south facing window taking afternoon sun. Our engine adds 10% for heavy sun exposure.
- A top floor sitting under a hot roof, worth another 5%.
- A kitchen in the same room. That one is not a percentage, it is a flat 4,000 BTU, because appliances throw off real heat. A refrigerator alone is 600 BTU and it runs whether you are home or not.
- More than two people in the room regularly, at 400 BTU each.
Going one size up is a mistake when none of those apply. A 120 sq ft bedroom on the ground floor with a shaded window does not need 6,000 BTU because somebody said bigger is safer, and the owner who paused to ask that question about a 120 sq ft room was right to pause. Size up for a reason you can name out loud. If you cannot name one, take the smaller unit.
Studios and Open Plan Rooms Need More Than Their Square Footage Suggests
A studio breaks the chart, and the 8,000 BTU section above is where you can watch it happen. One owner put 8,000 into a 282 sq ft room and it worked fine. Another put 8,000 into a 290 sq ft studio and called it barely cooling. Eight square feet is not the difference. The difference is that a studio is a bedroom, a living room and a kitchen all sharing one volume of air.
Our engine treats that literally, and the numbers are blunt. A bedroom adds nothing on top of the envelope load. A living room adds 1,000 BTU. A kitchen adds 4,000. So a 290 sq ft studio counted honestly as a living room rather than a bedroom comes out at 10,000 BTU instead of 8,000, which is exactly the size that owner turned out to need. Count it as a kitchen, because the stove genuinely is in the same room, and it climbs further still.
For a studio, size it as the room type that generates the most heat, not the one you spend the most hours in. The same logic applies to any open plan space where a cooking area has no door on it.
Can One Window AC Cool Two Rooms?
Short answer, badly. A window AC is a fan sitting in a wall, and a doorway is a far smaller opening than the room behind it.
The clearest case from real installs is 350 sq ft split across two rooms on an 8,000 BTU unit. On floor area alone that is not wildly off. 350 sq ft sits just above our 8,000 band, so you would expect it to be marginal rather than a failure. It failed, and the advice that owner got back was 12,000 or more, and possibly two units.
Now set that against the 676 sq ft open plan space in Florida running comfortably on 10,000. Nearly twice the floor area on a smaller unit, and it works. The only thing that changed is whether the air can move.
So the number to put into the calculator is the room the unit sits in, plus any space that opens directly onto it with no door in between. Anything behind a doorway needs its own unit.
Window Air Conditioner Size by Square Feet
The bands above run by unit size. Below we have gone the other way for the room sizes people ask about most, and at the larger end the answer stops being a single unit.
Window Air Conditioner for 500 Square Feet
500 sq ft comes out somewhere between 12,000 and 15,000 BTU, and which end you land on depends entirely on what is in the room. Work from floor area alone and 500 sq ft is about 12,000. Put a living room's worth of people, a television and lights into it and our engine lands on 15,000, because those add roughly 2,250 BTU before the climate factor even applies. A hot and humid climate takes it to 18,000 or beyond. That is the whole reason a coverage rating printed on a box and a calculator result disagree, and it is worth knowing which one you are reading.
The other useful question at this size is whether 500 sq ft is really one room. If it is a single open living room, one unit handles it. If it is a living room with a bedroom off it, the two-room section above applies and you will be better served by a smaller unit in each.
Window Air Conditioner for 700 Square Feet
700 sq ft comes out at about 24,000 BTU, the largest window unit made, and in a hot climate it needs more than the category sells. This is the precise case one owner walked into when the calculator said 20,000 to 24,000 and every retail listing for that space insisted 14,000 was enough. Trust the calculator. Better still, split it. Two 12,000 BTU units in two windows will cool 700 sq ft more evenly than one 24,000 unit, and if one dies in August you are not left with nothing.
Window Air Conditioner 800 Square Feet: Where Two Units Beat One
At 800 sq ft you are at the edge of what one window unit manages even on paper, and past it in any warm climate. A 24,000 BTU unit reaches about 935 sq ft under mild conditions, so 800 technically fits inside that. In practice a room of that size almost never has the shape for it, because one fan cannot throw air thirty feet and around a corner. Two units sized for the halves of the space is the right answer here nearly every time, and it usually costs less than the single largest unit as well.
Window Air Conditioner for 1000 Square Feet
1,000 sq ft is past the ceiling. The load works out near 27,000 BTU in mild conditions and higher in a hot climate, while the largest window unit made is 24,000. One owner reports a single 10,000 BTU unit easily cooling a 1,000 sq ft top floor apartment, which sounds like a flat contradiction, and it is worth explaining why it is not. That unit is not holding 1,000 sq ft at a setpoint. It is cooling the room it is in and letting cooler air drift through an open plan with no doors to stop it.
If your 1,000 sq ft has bedrooms with doors on them, one unit will not do it. A second owner working the same problem was told to plan for roughly 20,000 BTU in total and that no window unit is sold that big. So the practical answer at this size is two or three units, or a ductless system. If you would rather not put anything in the window at all, the portable AC size calculator sizes a free-standing unit instead, though note that portables are rated on a different standard and lose some capacity to the exhaust hose.