At 300 square feet you are in a 15 by 20 primary bedroom, an average living room, a sunroom, the main area of a studio apartment, or a 12 by 25 garage. This is the size most sizing charts use as their example, and the base answer is 6,000 BTU/hr, which maps to a 6,000 BTU unit with no margin at all.
Because of that, 8,000 BTU is what most 300 sq ft rooms should get. A very sunny room needs 6,600, a poorly insulated one 6,900, and both together 7,600; four people add 1,200 and a 10 ft ceiling adds 25 percent. All of those land on 8,000 BTU. The 6,000 BTU unit is the right call only when the room is shaded or well insulated, which takes it to 5,400 or 4,900. This is also the smallest room where a 9,000 BTU mini-split head makes sense; it will modulate down to about 3,000 BTU and never short-cycle.
Sunrooms are the special case. A 300 sq ft room with glass on two or three sides should be sized as very sunny with poor insulation, 7,600 BTU/hr, and in a hot climate 8,300, which is a 10,000 BTU unit. The common error is buying the 6,000 BTU unit the floor area suggests and watching it run all afternoon without catching up. Exterior shades or solar screens can pull a sunroom back one full size.
If you are going portable, you need a 12,000 BTU ASHRAE model, which delivers roughly 8,000 BTU SACC. A 10,000 ASHRAE portable (about 6,500 SACC) is marginal at 300 sq ft.
For heat, 300 sq ft is past the single space heater everywhere in the US: 9,000 BTU/hr in a hot climate, 12,000 in a moderate one (3.5 kW) and 16,500 in a cold one. A 240 V 4 kW wall heater handles the moderate case, a 5 kW unit the cold one. A 12,000 BTU heat pump mini-split covers both cooling and moderate-climate heating, which is why they are so common in primary bedrooms of this size.
Example: a 15 by 20 primary bedroom in a newer house with good insulation, two people, west-facing windows, moderate climate. 300 × 20 = 6,000, times 1.10 for sun and 0.90 for insulation, is 5,940, rounded to 5,900 BTU/hr. A 6,000 BTU window unit fits. Take away the good insulation and it is 6,600, an 8,000 BTU unit.
Cooling BTU for 300 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 | 6,200buy 8,000 BTU | 5,400buy 6,000 BTU | 4,900buy 5,000 BTU |
| Average | 6,900buy 8,000 BTU | 6,000buy 6,000 BTU | 5,400buy 6,000 BTU |
| Very sunny | 7,600buy 8,000 BTU | 6,600buy 8,000 BTU | 5,900buy 6,000 BTU |
Common adjustments for a 300 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 6,000 | 6,000 BTU |
| 4 people instead of 2 | 7,200 | 8,000 BTU |
| 10 ft ceiling | 7,500 | 8,000 BTU |
| Kitchen in the space | 10,000 | 10,000 BTU |
| Hot climate, very sunny | 7,300 | 8,000 BTU |
Heating BTU for 300 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 10,8003.2 kW | 9,0002.6 kW | 7,7002.3 kW |
| moderate | 14,4004.2 kW | 12,0003.5 kW | 10,2003.0 kW |
| cold | 19,8005.8 kW | 16,5004.8 kW | 14,0004.1 kW |
300 sq ft sizing questions
What size air conditioner do I need for a 300 sq ft room?
About 6,000 BTU/hr in average conditions (0.50 tons), so a 6,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Is 8,000 BTU too much for a 300 sq ft room?
No. Average conditions need 6,000 BTU/hr, and a very sunny or poorly insulated room needs 6,600 to 7,600, so 8,000 BTU is the right size for most 300 sq ft rooms. 6,000 BTU only fits when the room is shaded or well insulated. 10,000 or 12,000 BTU would be oversized.
What size AC for a 300 sq ft sunroom?
Treat it as very sunny with poor insulation: about 7,600 BTU/hr in a moderate climate and 8,300 in a hot one. Buy an 8,000 BTU unit in most of the US and a 10,000 BTU unit in the South, or a 9,000 to 12,000 BTU mini-split.
Can one space heater warm 300 sq ft?
Not reliably. The room needs about 12,000 BTU/hr in a moderate climate, and a 1,500 W heater delivers 5,100. Even in a hot climate the load is 9,000 BTU/hr. Use a 240 V wall heater of 3 to 4 kW or a heat pump mini-split.