What Size Air Conditioner Do I Need for a 400 Sq Ft Room?
400 sq ft is the published line between 9,000 and 10,000 BTU/hr.
Full breakdown: BTU per square foot.
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Capacity from the chart
| Area to be cooled | Capacity needed | Source |
|---|---|---|
| 350 up to 400 sq ft | 9,000 BTU/hr | ENERGY STAR |
| 400 up to 450 sq ft | 10,000 BTU/hr | ENERGY STAR |
There is no 400 sq ft row of its own. The square footage is the last foot of 350 up to 400 and the first foot of 400 up to 450. Those capacities differ by 1,000 BTU/hr. Ten percent of 9,000 is 900 BTU/hr, and ten percent of 10,000 is 1,000 BTU/hr, so on the upper row the sun adder happens to equal the gap between the rows. That coincidence is not a reason to apply both.
How to size it
Choose 9,000 BTU/hr when the 400 sq ft room is heavily shaded and has two or fewer people. Choose 10,000 BTU/hr when it is very sunny or more than two people use it regularly. Run one of the four adjustments against the row you chose.
| Adjustment | On the 9,000 row | On the 10,000 row | Source |
|---|---|---|---|
| Heavily shaded, reduce capacity 10% | 9,000 − 10% = 8,100 BTU/hr | 10,000 − 10% = 9,000 BTU/hr | ENERGY STAR |
| Very sunny, increase capacity 10% | 9,000 + 10% = 9,900 BTU/hr | 10,000 + 10% = 11,000 BTU/hr | ENERGY STAR |
| Each additional person beyond two | 9,000 + 600 BTU/hr = 9,600 BTU/hr | 10,000 + 600 BTU/hr = 10,600 BTU/hr | ENERGY STAR |
| Unit used in a kitchen | 9,000 + 4,000 BTU/hr = 13,000 BTU/hr | 10,000 + 4,000 BTU/hr = 14,000 BTU/hr | ENERGY STAR |
10% of 9,000 BTU/hr is 900 BTU/hr (9,000 × 10%).
10% of 10,000 BTU/hr is 1,000 BTU/hr (10,000 × 10%).
Shaded path: 9,000 − 10% = 8,100 BTU/hr. Sunny path: 10,000 + 10% = 11,000 BTU/hr. One person beyond two on the upper row, without also adding sun: 10,000 + 600 = 10,600 BTU/hr. If this room is where the unit serves a kitchen, add 4,000 BTU/hr after the row: 13,000 from 9,000, or 14,000 from 10,000. That 14,000 result is arithmetic on this page. It is not the chart row for a larger floor area.
Refrigerant is a separate label on the unit, not a sizing factor. ENERGY STAR's lower-GWP refrigerants for room air conditioners are R-32 (HFC-32), R-454B, and R-290. The current specification it cites took effect Oct 30, 2023. None of those names moves a 400 sq ft room from 9,000 to 10,000.
The chart still assumes an 8-foot ceiling. Above that, the published advice is a higher CADR, in ENERGY STAR's wording, rather than a homemade percent.
ENERGY STAR's purchase warning fits the temptation to buy 10,000 for a shaded room: "An oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling."
A ductless head is the other single-room option. See the mini-split room size guide before you assume the window is the only opening that can cool 400 sq ft.
10,000 BTU window air conditioners on Amazon Window air conditioner support brackets on Amazon
Related questions
- What size air conditioner for a 300 sq ft room
- What size air conditioner for a 500 sq ft room
- What size air conditioner for a kitchen
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 do I need for a 400 sq ft room?
350 up to 400 sq ft needs 9,000 BTU/hr, and 400 up to 450 sq ft needs 10,000 BTU/hr. Heavily shaded rooms stay with 9,000. Very sunny rooms, and rooms with more than two people regularly, start at 10,000.
What is 10% of each 400 sq ft row?
Ten percent of 9,000 is 900, so a shaded start at 9,000 becomes 8,100 BTU/hr. Ten percent of 10,000 is 1,000, so a sunny start at 10,000 becomes 11,000 BTU/hr. Pick the row first. The percent does not replace it.
Do kitchen use and refrigerant type change the row?
A unit used in a kitchen adds 4,000 BTU/hr after the row choice: 13,000 from 9,000, or 14,000 from 10,000. Refrigerant names do not change the row. ENERGY STAR lists lower-GWP refrigerants for room air conditioners as R-32 (HFC-32), R-454B, and R-290, with the current spec effective Oct 30, 2023.
Should a 400 sq ft room use a central system chart?
Not this one. The table is a room air conditioner chart on an 8-foot ceiling. Whole-house equipment needs a load calculation on the HVAC load calculator. Higher than 8-foot ceilings: consider a room air conditioner with a higher CADR.