What Size Air Conditioner Do I Need for 800 Sq Ft?
800 sq ft is inside the 18,000 BTU/hr row, which runs from 700 up to 1,000 sq ft.
Full breakdown: BTU per square foot.
The site's separate whole-space walkthrough is what size AC unit for 1,000 sq ft. This page stays on the ENERGY STAR room-AC row for 800 sq ft.
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 |
|---|---|---|
| 700 up to 1,000 sq ft | 18,000 BTU/hr | ENERGY STAR |
An area this large is still, on this chart, "area to be cooled" by a room air conditioner. That is a different question from sizing a central system for a house. Keep the two apart. If the tape says 800 sq ft and the ceiling is the chart's 8-foot standard, 18,000 BTU/hr is the row. The neighboring lower capacity, 14,000, applies only through 700 sq ft. The next capacity, 21,000, starts at 1,000 up to 1,200 sq ft.
How to size it
Modify 18,000 rather than shopping a different row. Shade and sun are 10% in opposite directions. People add 600 BTU/hr each beyond two. A kitchen adds 4,000 BTU/hr. Those last two can sit on top of a percent when both facts are true.
| Adjustment | On the 18,000 row | Source |
|---|---|---|
| Heavily shaded, reduce capacity 10% | 18,000 − 10% = 16,200 BTU/hr | ENERGY STAR |
| Very sunny, increase capacity 10% | 18,000 + 10% = 19,800 BTU/hr | ENERGY STAR |
| Each additional person beyond two | 18,000 + 600 BTU/hr = 18,600 BTU/hr | ENERGY STAR |
| Unit used in a kitchen | 18,000 + 4,000 BTU/hr = 22,000 BTU/hr | ENERGY STAR |
10% of 18,000 BTU/hr is 1,800 BTU/hr (18,000 × 10%).
Shaded 800 sq ft is 18,000 − 10% = 16,200 BTU/hr, which is still above 14,000. Sunny 800 sq ft is 18,000 + 10% = 19,800 BTU/hr, which is still below 21,000. One extra person is 18,000 + 600 = 18,600 BTU/hr. Kitchen use is 18,000 + 4,000 = 22,000 BTU/hr. A sunny kitchen is 18,000 + 10% + 4,000: 19,800 + 4,000 = 23,800 BTU/hr. Quote that stack only for a space that is both very sunny and a kitchen.
A unit that is too big for the shaded case cools the air and stops before the humidity is gone. ENERGY STAR's description of the result is that "cool, moist air will make the room feel damp and clammy."
If the ceiling is higher than 8-foot, the chart's own note is to select a room air conditioner with a higher CADR. Do not convert that sentence into a percent the table does not print.
18,000 BTU window air conditioners on Amazon Window air conditioner support brackets on Amazon
Related questions
- What size air conditioner for 700 sq ft
- What size air conditioner for 1,200 sq ft
- What size air conditioner for 600 sq ft
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 800 sq ft?
The ENERGY STAR row is 700 up to 1,000 sq ft at 18,000 BTU/hr. 800 sq ft is inside it, so the chart capacity is 18,000 BTU/hr before adjustments.
Will heavy shade move 800 sq ft down to 14,000 BTU/hr?
No. Heavy shade reduces 18,000 by 10%. Ten percent of 18,000 is 1,800, so the shaded target is 16,200 BTU/hr. The 14,000 row is only for 550 up to 700 sq ft.
What do extra people and a kitchen do to 18,000 BTU/hr?
One person beyond two adds 600 BTU/hr: 18,000 + 600 = 18,600 BTU/hr. Kitchen use adds 4,000: 18,000 + 4,000 = 22,000 BTU/hr. A very sunny room adds 10%, which is 19,800 BTU/hr.
How is this different from the 1,000 sq ft guide?
This page is only the room air conditioner chart, on an 8-foot ceiling. The 1,000 sq ft AC guide is a separate walkthrough. A central system should get a load calculation on the HVAC load calculator. Higher ceilings: select a higher CADR.