A 12 by 12 bedroom is 144 square feet, a 10 by 15 is 150, and between them they cover most bedrooms built in the US since the 1950s. Dorm doubles, guest rooms and one-person home offices land here too. The load is 3,000 BTU per hour, and the answer is the same in almost every case: a 5,000 BTU window unit.
The reason 5,000 BTU works so consistently is headroom. Push everything the wrong way at once, a very sunny room with poor insulation in a hot climate, and the load is still only 4,200 BTU/hr. Four people instead of two adds 1,200 BTU and lands at the same 4,200. Nothing short of a 10 ft ceiling plus a kitchenette moves this room out of the 5,000 BTU class.
The mistake people make at 150 sq ft is buying an 8,000 or 10,000 BTU unit, usually because the price gap to 5,000 is small or because they want it to cool faster. It does cool faster, for about six minutes, then shuts off before it has removed any humidity. Damp sheets and a musty smell by August are the usual result. If you are choosing a portable instead of a window unit, an 8,000 BTU ASHRAE portable delivers about 5,000 BTU SACC, which happens to be a perfect fit.
Heat is where 150 sq ft gets less forgiving. The room needs about 6,000 BTU/hr in a moderate climate, and a 1,500 W plug-in heater tops out near 5,100 BTU/hr. That is enough in a hot climate (4,500) or a moderate one with good insulation (5,100), but a cold-climate bedroom at 8,300 BTU/hr needs a 240 V wall heater, a 2,000 W baseboard run, or a 9,000 BTU heat pump mini-split that handles both seasons.
Here is a typical case. A 12 by 12 dorm room, two students, two laptops, a mini fridge, and a south-facing window. 150 × 20 = 3,000 BTU, times 1.10 for the sun exposure, is 3,300 BTU/hr; the electronics add a few hundred more. A 5,000 BTU window unit has 1,500 BTU to spare and will cycle at a sane pace.
Cooling BTU for 150 sq ft by sun exposure and insulation
Two occupants, 8 ft ceiling, moderate climate. Each cell shows BTU/hr needed and the unit size to buy.
| Sun exposure | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| Heavily shaded | 3,100buy 5,000 BTU | 2,700buy 5,000 BTU | 2,400buy 5,000 BTU |
| Average | 3,400buy 5,000 BTU | 3,000buy 5,000 BTU | 2,700buy 5,000 BTU |
| Very sunny | 3,800buy 5,000 BTU | 3,300buy 5,000 BTU | 3,000buy 5,000 BTU |
Common adjustments for a 150 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 3,000 | 5,000 BTU |
| 4 people instead of 2 | 4,200 | 5,000 BTU |
| 10 ft ceiling | 3,800 | 5,000 BTU |
| Kitchen in the space | 7,000 | 8,000 BTU |
| Hot climate, very sunny | 3,600 | 5,000 BTU |
Heating BTU for 150 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 5,4001.6 kW | 4,5001.3 kW | 3,8001.1 kW |
| moderate | 7,2002.1 kW | 6,0001.8 kW | 5,1001.5 kW |
| cold | 9,9002.9 kW | 8,3002.4 kW | 7,0002.1 kW |
150 sq ft sizing questions
What size air conditioner do I need for a 150 sq ft room?
About 3,000 BTU/hr in average conditions (0.25 tons), so a 5,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Is 5,000 BTU enough for a 12 × 12 bedroom?
Yes, with room to spare. A 12 × 12 room (144 sq ft) needs about 3,000 BTU/hr in average conditions and no more than 4,200 even when it is sunny, poorly insulated and in a hot climate. A 5,000 BTU unit is the right size, not the minimum.
Should I buy 8,000 BTU for a 150 sq ft room to cool it faster?
No. An 8,000 BTU unit is nearly three times the load. It will cool the air in minutes and shut off before it dehumidifies, so the room ends up cold and clammy. Faster cooling in a bedroom this size is not worth the humidity problem.
What size heater for a 150 sq ft bedroom?
About 6,000 BTU/hr in a moderate climate, so a 1,500 W heater (5,100 BTU/hr) is slightly under unless the room is well insulated. In a cold climate the room needs 8,300 BTU/hr, which calls for a 240 V wall or baseboard heater, or a heat pump mini-split.