What Size Air Conditioner Do I Need for a 300 Sq Ft Room?
300 sq ft is the line between 7,000 and 8,000 BTU/hr, and occupancy is often what decides it.
Full breakdown: BTU per square foot.
Affiliate disclosure: as an Amazon Associate, we earn from qualifying purchases — at no cost to you.
Capacity from the chart
| Area to be cooled | Capacity needed | Source |
|---|---|---|
| 250 up to 300 sq ft | 7,000 BTU/hr | ENERGY STAR |
| 300 up to 350 sq ft | 8,000 BTU/hr | ENERGY STAR |
Rooms around this floor area are often the ones that break the two-person assumption: a primary bedroom, a home office that hosts a third person, or a den. ENERGY STAR's people rule is specific. More than two people regularly in the room means add 600 BTU/hr per additional person. It does not mean "add 600 once and also jump a row." If the extra person is the reason you already chose 8,000, the adder goes on top of 8,000.
How to size it
Decide the row from shade, sun, and whether more than two people use the room. Then run the matching adjustment. The two capacities are 1,000 BTU/hr apart (8,000 − 7,000), which is more than the 700 BTU/hr shade cut on the lower row.
| Adjustment | On the 7,000 row | On the 8,000 row | Source |
|---|---|---|---|
| Heavily shaded, reduce capacity 10% | 7,000 − 10% = 6,300 BTU/hr | 8,000 − 10% = 7,200 BTU/hr | ENERGY STAR |
| Very sunny, increase capacity 10% | 7,000 + 10% = 7,700 BTU/hr | 8,000 + 10% = 8,800 BTU/hr | ENERGY STAR |
| Each additional person beyond two | 7,000 + 600 BTU/hr = 7,600 BTU/hr | 8,000 + 600 BTU/hr = 8,600 BTU/hr | ENERGY STAR |
| Unit used in a kitchen | 7,000 + 4,000 BTU/hr = 11,000 BTU/hr | 8,000 + 4,000 BTU/hr = 12,000 BTU/hr | ENERGY STAR |
10% of 7,000 BTU/hr is 700 BTU/hr (7,000 × 10%).
10% of 8,000 BTU/hr is 800 BTU/hr (8,000 × 10%).
Heavy shade and two or fewer people: 7,000 minus 10% = 6,300 BTU/hr. Very sunny, before any extra person: 8,000 plus 10% = 8,800 BTU/hr. One person beyond two, on the upper row and without stacking sun: 8,000 + 600 = 8,600 BTU/hr. A kitchen use is the large step, 4,000 BTU/hr, so 11,000 from the low row or 12,000 from the high row. Take that step only when the unit is actually used in a kitchen.
None of those results assume a ceiling above 8-foot. If yours is higher, ENERGY STAR's instruction is to select a room air conditioner with a higher CADR. There is no published percent per extra foot on this chart.
If the wall will not take a window unit, use the mini-split room size guide as the ductless alternative for this room.
8,000 BTU window air conditioners on Amazon Window air conditioner support brackets on Amazon
Related questions
- What size air conditioner for a 250 sq ft room
- What size air conditioner for a 400 sq ft room
- What size air conditioner for a 200 sq ft room
All of these questions are listed on the topic map.
Sources
- ENERGY STAR, Room Air Conditioners — https://www.energystar.gov/products/room_air_conditioners — accessed Oct 2, 2026.
Frequently asked questions
What size air conditioner for a 300 sq ft room?
250 up to 300 sq ft is 7,000 BTU/hr. 300 up to 350 sq ft is 8,000 BTU/hr. A heavily shaded room can stay at 7,000. A very sunny room, or more than two people regularly, should start at 8,000.
How do I add a third regular occupant at 300 sq ft?
The chart covers up to two people. Each person beyond two adds 600 BTU/hr. If occupancy is why you chose the 8,000 row, one extra person is 8,000 + 600 = 8,600 BTU/hr. Do not also switch rows a second time.
What is the shaded result on the lower row?
Start at 7,000 and reduce capacity 10%. Ten percent of 7,000 is 700, so 7,000 − 700 = 6,300 BTU/hr. That is the heavy-shade case, not a reason to quote the 6,000 row from a smaller area.
Is a window unit the only option at 300 sq ft?
This page is the room air conditioner chart, based on an 8-foot ceiling. A ductless alternative is the mini-split room size guide. A whole-house central system should get a load calculation on the HVAC load calculator. Higher ceilings: select a room air conditioner with a higher CADR.