A 350 square foot room is a big one for a house: a primary suite with a sitting area, a 14 by 25 family room, a converted single-car garage (14 by 24 is 336), or a two-room home office. The cooling load is 7,000 BTU/hr, and the nearest unit is 8,000 BTU, which gives you 1,000 BTU of headroom in an average room.
That headroom disappears quickly. A very sunny room is 7,700, a poorly insulated one is 8,000 on the nose, and four people push an average room to 8,200. Any of those, or a 10 ft ceiling (8,800), or a hot climate with strong sun (8,500), means a 10,000 BTU unit. Sunny and poorly insulated together is 8,900, also 10,000. So 8,000 BTU is right for a room that is average or better, and 10,000 BTU for one that is not.
Converted garages deserve their own warning. A slab floor, a thin garage door and walls that were never insulated mean you should use the poor insulation row, and if the door faces west you should also use very sunny. That is 8,900 BTU/hr, and a lot of people put an 8,000 BTU unit in a converted garage because that is what 350 sq ft 'should' need, then wonder why it runs all day. Insulate the door and buy 10,000.
A 9,000 BTU mini-split is a natural fit for 350 sq ft. Because it is an inverter unit it can run at 9,000 on a hot afternoon and idle at 3,000 in the evening, so it handles the sunny and shaded versions of the same room equally well. Window units cannot do that, which is why the 8,000-or-10,000 decision matters more for them.
Heating 350 sq ft with average insulation runs 10,500 BTU/hr in a hot climate, 14,000 in a moderate one (4.1 kW) and 19,300 in a cold one. A converted garage in a cold climate with poor insulation is 23,100 BTU/hr, which is a 7.5 kW electric unit heater or a small gas unit heater, not a plug-in anything.
Example: a 14 by 25 family room, four people watching TV, average sun and insulation, moderate climate. 350 × 20 = 7,000, plus 600 for each person beyond two, is 8,200 BTU/hr. That is a 10,000 BTU unit. With only two people in the room it drops back to 7,000 and an 8,000 BTU unit.
Cooling BTU for 350 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 | 7,200buy 8,000 BTU | 6,300buy 8,000 BTU | 5,700buy 6,000 BTU |
| Average | 8,000buy 8,000 BTU | 7,000buy 8,000 BTU | 6,300buy 8,000 BTU |
| Very sunny | 8,900buy 10,000 BTU | 7,700buy 8,000 BTU | 6,900buy 8,000 BTU |
Common adjustments for a 350 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 7,000 | 8,000 BTU |
| 4 people instead of 2 | 8,200 | 10,000 BTU |
| 10 ft ceiling | 8,800 | 10,000 BTU |
| Kitchen in the space | 11,000 | 12,000 BTU |
| Hot climate, very sunny | 8,500 | 10,000 BTU |
Heating BTU for 350 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 12,6003.7 kW | 10,5003.1 kW | 8,9002.6 kW |
| moderate | 16,8004.9 kW | 14,0004.1 kW | 11,9003.5 kW |
| cold | 23,1006.8 kW | 19,3005.7 kW | 16,4004.8 kW |
350 sq ft sizing questions
What size air conditioner do I need for a 350 sq ft room?
About 7,000 BTU/hr in average conditions (0.58 tons), so a 8,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Is 8,000 BTU enough for 350 sq ft?
In an average or shaded room, yes: the load is 6,300 to 7,000 BTU/hr. If the room is very sunny, poorly insulated, or regularly holds four people, the load climbs to 7,700 to 8,900 BTU/hr and you should buy 10,000 BTU.
What size mini-split for a 350 sq ft room?
A 9,000 BTU single-zone mini-split. It covers the 7,000 BTU/hr average load with margin, handles the 8,900 BTU/hr sunny, poorly insulated case, and turns down to around 3,000 BTU when the room is mild, so it does not short-cycle.
How many BTU to heat a 350 sq ft converted garage?
With average insulation, about 14,000 BTU/hr in a moderate climate and 19,300 in a cold one. If the walls and door were never insulated, the cold-climate figure is 23,100 BTU/hr, which calls for a 7.5 kW electric unit heater or a gas unit heater.