If you only remember one thing from this page, make it this: a room air conditioner needs roughly 20 BTU per hour for every square foot of floor, and then you nudge that number up or down for sun, insulation, climate, ceiling height, people, and cooking. A 300 sq ft bedroom lands around 6,000BTU. A 500 sq ft living room lands around 10,000. That single rule gets most people within one unit size of the right answer, and the rest of this guide is about the adjustments that close the gap, plus the one mistake almost everyone makes, which is buying bigger "to be safe."
Start with the floor area, measured honestly
Measure the room in feet, length times width. A 12 × 14 bedroom is 168 sq ft. For an L-shaped room, split it into two rectangles and add them. For an open-plan space, the area that matters is everything the cold air can reach without a door in the way, so a living room that flows into a dining nook and kitchen is one big room for sizing purposes, even if you think of it as three.
Multiply by 20. That is your starting point in BTU per hour: roughly what a room with 8 ft ceilings, average windows, average insulation, and two people gains through the walls, glass, and roof on a hot afternoon. Energy Star, the big-box retailers, and most contractors' quick quotes all start from the same place.
The Energy Star chart, and why it flattens out
Energy Star publishes a table of room sizes and recommended BTU that you will see printed on the side of nearly every window unit box. Here it is, next to what our calculator returns for the top of each range with default settings.
| Room area | Energy Star | Calculator (top of range) |
|---|---|---|
| 100–150 sq ft | 5,000 BTU | 3,000 BTU |
| 150–250 sq ft | 6,000 BTU | 5,000 BTU |
| 250–300 sq ft | 7,000 BTU | 6,000 BTU |
| 300–350 sq ft | 8,000 BTU | 7,000 BTU |
| 350–400 sq ft | 9,000 BTU | 8,000 BTU |
| 400–450 sq ft | 10,000 BTU | 9,000 BTU |
| 450–550 sq ft | 12,000 BTU | 11,000 BTU |
| 550–700 sq ft | 14,000 BTU | 14,000 BTU |
| 700–1,000 sq ft | 18,000 BTU | 20,000 BTU |
| 1,000–1,200 sq ft | 21,000 BTU | 24,000 BTU |
| 1,200–1,400 sq ft | 23,000 BTU | 28,000 BTU |
| 1,400–1,500 sq ft | 24,000 BTU | 30,000 BTU |
Below 700 sq ft the two columns agree closely, which is the range where window and portable units live. Above that, Energy Star's numbers climb more slowly than 20 BTU per square foot, because a large open room has proportionally less exterior wall and window per square foot of floor. Our calculator stays at 20 to be conservative; by the time you are cooling 1,000 sq ft you are shopping for a mini-split or central system anyway, where a proper load calculation matters more than any chart.
The chart also has fine print most people skip: subtract 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU per person beyond two, and add 4,000 BTU for a kitchen. Those are exactly the adjustments the calculator applies.
What actually moves the number
Take a 300 sq ft room as the test case. With average settings it needs 6,000 BTU. Here is what each factor does on its own, and what happens if every bad factor stacks up.
| Change | Effect | Estimate |
|---|---|---|
| Baseline (average everything) | ×1.00 | 6,000 |
| Very sunny, west or south glass | ×1.10 | 6,600 |
| Heavily shaded | ×0.90 | 5,400 |
| Poor insulation | ×1.15 | 6,900 |
| Good insulation | ×0.90 | 5,400 |
| Hot climate | ×1.10 | 6,600 |
| 10 ft ceiling | ×1.25 | 7,500 |
| Four people instead of two | +1,200 | 7,200 |
| Open to a kitchen | +4,000 | 10,000 |
| All of the above at once | stacked | 15,600 |
Sun is about which way the big windows face. A room with a west-facing sliding door is "very sunny" whether or not the rest of the room is dark; a north-facing room under a mature tree is "shaded." Insulation is about the age of the house and the windows: pre-1980 walls with single-pane glass are poor, a 2000s build with double-pane low-E glass is good, and most houses in between are average. Our insulation guide walks through how to judge yours in ten minutes.
Ceiling height is the one people forget. The rule of thumb assumes 8 ft; a 10 ft ceiling is 25% more air to cool, so the calculator scales the estimate by height ÷ 8. A resting adult gives off about 400 BTU/hr plus a good deal of moisture, hence 600 per extra person. Kitchens get a flat 4,000 BTU for the refrigerator, oven, and range. Climate matters less than you might think for a room unit (×1.10 for the hot South, ×0.90 for the cool North) because a room air conditioner is sized for the afternoon peak, and peak afternoons in Minneapolis are not that different from peak afternoons in Atlanta. They are just rarer.
Three worked examples
A 12 × 14 bedroom with a west window. That is 168 sq ft, so 3,360 BTU base, times 1.10 for the afternoon sun, which rounds to 3,700 BTU. The smallest unit sold is 5,000 BTU, so you buy the 5,000 and it will be fine. Do not talk yourself into an 8,000 because it was on sale; a 5,000 in this room is already about 35% over the load. See the bedroom AC size page for the full range of bedroom layouts.
A 500 sq ft living room open to the kitchen, four people watching TV. 10,000 base, plus 1,200 for the two extra people, plus 4,000 for the kitchen, which is 15,200 BTU. That is between the 14,000 and 18,000 unit sizes. Because it is closer to 14,000 and the kitchen load only exists while someone is cooking, the 14,000 unit (or a 12,000 BTU mini-split, which usually delivers a bit more than its nominal rating) is the sensible pick. The living room and kitchen pages have more variations.
A 200 sq ft sunroom with a 10 ft ceiling and thin walls. 4,000 base, times 1.25 for the ceiling, times 1.10 for sun, times 1.15 for poor insulation, which rounds to 6,300BTU. The floor area says "small room, 5,000 BTU unit," but the reality is a choice between a 6,000 that runs flat out on the worst afternoons and an 8,000 with a little margin. In a glass room, take the 8,000. Glass rooms are where the adjustments earn their keep; the sunroom page covers the harder cases.
Turning the estimate into a unit you can buy
Room air conditioners come in steps: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, and 24,000 BTU, with mini-splits adding 9,000 and 36,000 to that list. Your estimate will almost never land exactly on one. The rule is to pick the smallest unit that meets or slightly exceeds the estimate. If the estimate is 6,600, buy the 8,000. If it is 8,200, the 8,000 is fine; a rule of thumb has a ±10% band built in, and 200 BTU is noise.
When the estimate sits deep between two sizes, say 9,000 with 8,000 and 10,000 on either side, go down if the room is shaded, gets used mainly in the evening, or is open to the rest of a cooled house, and go up if it faces west or you run a lot of electronics. The one thing you should not do is jump two sizes. An air conditioner that is 50% too big cools the air quickly, shuts off before it has pulled much moisture out, and leaves the room cold and clammy. The oversizing guide explains why that happens and how much extra is too much.
Window, portable, or mini-split?
The BTU number is the same regardless of unit type, but how much of the rated cooling reaches the room is not. A window unit delivers what the box says. A single-hose portable delivers roughly 60–70% of its ASHRAE rating, which is why the newer SACC number on the label is so much lower; shop portables by SACC, and see the window vs. portable guide for a conversion table. Mini-splits are the reverse case: their inverter compressors throttle down to a quarter of full output, so a slightly oversized mini-split behaves far better than a slightly oversized window unit. The mini-split guide covers per-zone and multi-zone sizing.
When 20 BTU per square foot is not enough
The rule of thumb is for one room, or one open area, served by one unit. It is not for a whole house. Central air and ducted heat pumps should be sized with an ACCA Manual J load calculation, which accounts for every window's orientation and glazing, the actual insulation levels, the air leakage of the building, and the local design temperature. Contractors who skip Manual J and size by square footage routinely oversize by 30–50%, which is the most common reason new central systems disappoint their owners. If a quote for a whole-house system does not mention a load calculation, ask for one.
For a single room, the rule of thumb can miss in three situations: an unusual amount of glass (more than about a quarter of the wall area), a lot of equipment running (servers, grow lights, a large aquarium), or a room above an uninsulated garage or under an uninsulated roof. In those cases set the room to "very sunny" and "poor insulation" even if it technically isn't, and you will land in the right place.
What to do next
Measure the room, note which way the windows face, and decide whether the house is older or newer than about 1990. Then run those numbers through the free BTU calculator; it applies every adjustment above and tells you which unit size to shop for and why. For a quick reference without typing, the pre-computed pages cover most cases: 300 sq ft, 500 sq ft, 1,000 sq ft, or the room pages for a master bedroom, home office, or apartment. Buy the smallest unit that meets the number, and spend the money you saved on blackout curtains for the west window.