What changes the size for a 400 ft² room
400 ft² is about 20 × 20 ft, or 40 × 10 ft, or 33 × 12 ft. The base capacity is for an average room, and the table adjusts it for the things that add or remove heat:
| Conditions | BTU needed | Shop for |
|---|---|---|
| Average room (2 people, 8 ft ceiling) | 9,000 BTU | 10,000 BTU |
| Heavily shaded room | 8,100 BTU | 10,000 BTU |
| Very sunny room | 9,900 BTU | 10,000 BTU |
| 4 people regularly in the room | 10,200 BTU | 12,000 BTU |
| It is a kitchen | 13,000 BTU | 14,000 BTU |
| 10 ft ceilings | 11,300 BTU | 12,000 BTU |
What kind of air conditioner is that?
This is a medium window or portable air conditioner. Most run on a standard 120 V outlet, but check the unit's amp rating against the circuit it will share.
Bigger is not better: an oversized unit cools the room quickly, shuts off, and leaves it clammy. See the full sizing guide for window, portable and mini-split options.
Want to change anything else? The calculator above is the full AC Size Calculator (BTU): edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
What size air conditioner for 400 square feet?
About 9,000 BTU per hour, so shop for a 10,000 BTU unit. That is 0.75 tons of cooling, based on Energy Star room-size guidance for an average room.
Is a 10,000 BTU air conditioner enough for 400 sq ft?
Yes for an average room with 8 ft ceilings and two people. A very sunny room would need about 9,900 BTU, and a kitchen about 13,000 BTU, so size up in those cases.
What if my 400 sq ft room has high ceilings?
With 10 ft ceilings, the same 400 ft² needs about 11,300 BTU (12,000 BTU), because there is more air to cool.
Related sizes and guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.