Too small runs all day; too big never dries the room.
By David Liu, founder of DIY9lives. Last updated November 2, 2026.
An air conditioner's capacity is measured in BTUs per hour, and getting it wrong is expensive in both directions. Too small and the unit runs continuously without reaching the setpoint. Too big and it reaches the temperature quickly, shuts off, and leaves the humidity where it was. This guide works the sizing the way the BTU calculator does, then compares the result against the manufacturer chart.
The usual starting point is about 20 BTU per square foot of floor area for an ordinary room with an 8 foot ceiling. A 300 square foot room starts at 6,000 BTU.
That baseline assumes a normal ceiling, average insulation, and a moderate climate. Every one of those assumptions has an adjustment, and the adjustments are where the estimate becomes useful.
| Condition | Adjustment |
|---|---|
| Ceiling taller than 8 feet | Scale the base by height divided by 8 |
| Sunny room, west or south facing | Add about 10% |
| Heavily shaded room | Reduce about 10% |
| Each person beyond two | Add about 600 BTU |
| Kitchen | Add about 4,000 BTU |
A 300 square foot room with a 10 foot ceiling, two occupants, and afternoon sun works out as 6,000 × 1.25 = 7,500, plus 10% for the sun, which is 8,250. There is no standard unit at 8,250, so the next standard size up is 10,000.
The kitchen figure is not a small correction. Cooking appliances and a refrigerator dump heat into the room continuously, which is why a kitchen the same size as a bedroom needs a materially bigger unit.
Energy Star's room air conditioner guidance gives a chart by floor area: 250 to 300 square feet maps to 7,000 BTU, and 300 to 350 maps to 8,000. It then tells you to add 10% for a sunny room, subtract 10% for deep shade, add 600 BTU per additional person, and add about 4,000 BTU if the unit is in a kitchen.
Those adjustments are essentially the ones above, which is reassuring. The 20 BTU per square foot baseline is a little lower than the chart at the small end, which is why the calculator rounds up to the nearest standard size rather than down. Buying the smaller unit to save money on a chart boundary is the false economy in this whole subject.
An air conditioner does two jobs: it lowers the temperature and it removes moisture. Moisture removal happens while the unit is running, because the cold coil condenses water out of the air. A unit that is too large reaches the setpoint in ten minutes and stops, having removed very little water. The room is the right temperature and feels damp.
The second cost is wear. A unit that starts and stops constantly cycles its compressor far more often than one that runs steadily, and the starting current is much higher than the running current. Short cycling is harder on the equipment than long runs.
The modern counterpoint is that inverter-driven units modulate rather than switching on and off, which softens this trade-off. A window unit at a fixed speed does not, and that is what most of this sizing advice is about.
An open-plan room is one space, not two. A kitchen open to a living room needs the combined area and the kitchen adder, or the air will simply migrate to the part of the house you are not sitting in.
Closed doors change the answer in the other direction. A bedroom with the door shut is its own zone; a bedroom with the door open to a warm hallway is competing with the whole floor.
Windows are the other variable the area figure cannot see. A room with a large west-facing window gains far more heat in the afternoon than the same area with a north-facing one, which is what the 10% adjustment is standing in for. Heavy blinds, exterior shade and a film on the glass all reduce the load.
Take the floor area times 20, adjust for ceiling height, sun, people and cooking, then buy the smallest standard size at or above your number. For a typical 300 square foot bedroom with two people and an 8 foot ceiling, that is 6,000 BTU; with a tall ceiling and afternoon sun, it is 8,000. The difference between those two answers is the price of measuring properly rather than guessing from the floor area alone.
The capacity arithmetic is the same for all three, but the installation decides how much of it reaches the room. A window unit sits in the opening and dumps its heat outside, which is the most efficient arrangement of the three. A portable unit sits on the floor and exhausts through a window hose, and the hose radiates heat back into the room; a single-hose portable is measurably worse than the same rated capacity in a window unit, which is why the ratings are not directly comparable in practice.
A ductless mini-split is a different category. It is professionally installed, costs several times more, and is far quieter and more efficient, with an inverter compressor that modulates instead of cycling. For one room used every day for years, it is often the better purchase; for a bedroom used in July and August, a window unit is hard to beat on cost.
Whatever the type, the placement rules are similar: shade the outdoor side if you can, keep the airflow path clear, and do not put the unit where it blows directly onto a person sitting still. A cold draught at the setpoint temperature is the most common complaint about an otherwise correctly sized unit, and it is a placement problem rather than a capacity one.
Cooling two rooms from one unit works only when the air can move between them. A unit in a bedroom with the door shut cools the bedroom; with the door open it cools a share of the hallway and the room beyond, which means the sized capacity is now serving more volume than you measured. Add the areas together and size for the total, or accept that the far room will be warmer.
Fans help more than people give them credit for. A ceiling fan moves air across the skin, which changes how the temperature feels without changing it, and that often lets you set the unit a couple of degrees higher. It does not cool the air, and it does nothing for humidity, which is the part of comfort an oversized unit fails at.
Run the calculator for each room separately before you decide to share a unit. The answer for the combination is often a size that neither room needs on its own.
Last updated: November 2, 2026. Estimates only. Not tax, legal, financial, or professional advice. See the terms.