What Size AC for 1,800 Sq Ft?
A 1,800 sq ft space needs about 36,000 BTU/h of cooling (3 tons), so the nearest size is 36,000 BTU. This assumes an 8 ft ceiling and average sun. It is a rule of thumb (20 BTU per sq ft), not an engineered load.
AC size for 1,800 sq ft by situation
| Scenario | BTU/h | Tons | Suggested size |
|---|---|---|---|
| Heavily shaded (−10%) | 32,400 | 2.7 | 36,000 BTU |
| Average sun, 8 ft ceiling | 36,000 | 3 | 36,000 BTU |
| Very sunny (+10%) | 39,600 | 3.3 | 3.5-ton system (42,000 BTU) |
| Kitchen (+4,000 BTU) | 40,000 | 3.33 | 3.5-ton system (42,000 BTU) |
| 9 ft ceilings (+12.5%) | 40,500 | 3.38 | 3.5-ton system (42,000 BTU) |
| 10 ft ceilings (+25%) | 45,000 | 3.75 | 4-ton system (48,000 BTU) |
| Hot climate (+10%) | 39,600 | 3.3 | 3.5-ton system (42,000 BTU) |
Rule-of-thumb figures for a two-person space; each extra person over 2 adds about 600 BTU.
How it is calculated
1,800 sq ft × 20 BTU × (8 ÷ 8) = 36,000 BTU/h
36,000 ÷ 12,000 = 3 tons
Sunny: 36,000 × 1.1 = 39,600 · Kitchen: 36,000 + 4,000 = 40,000
Central AC: 36,000 ÷ 12,000 = 3 → 3 tons (rounded up to a half ton)
At 1,800 sq ft, insulation, windows, duct leakage and local climate swing the real answer far more than the square-footage rule. A Manual J load calculation by an HVAC contractor is recommended before buying a central or multi-zone system.
Which type of AC for 1,800 sq ft?
- Central AC – Rule-of-thumb tonnage is 3 tons, which rounds up to a 3-ton system. In a hot climate the same math gives 3.5 tons.
- Ductless multi-zone – Use one indoor head per zone (bedrooms, living area) sized room by room instead of one 36,000 BTU figure for the whole house.
- Window units – Not practical as the main cooling at 1,800 sq ft; they only make sense as supplements for a hot room.
Frequently asked questions
How many BTU do I need for 1,800 square feet?
About 36,000 BTU/h for an 8 ft ceiling in average conditions (1,800 × 20). That is 3 tons, so the closest standard size is 36,000 BTU.
Is 36,000 BTU enough for 1,800 sq ft if the room is sunny?
A very sunny space needs about 39,600 BTU, so 3.5-ton system (42,000 BTU) is recommended – a step different from the 36,000 BTU for average sun.
What if the 1,800 sq ft space has 10 ft ceilings?
The load rises 25% to 45,000 BTU, so choose 4-ton system (48,000 BTU) instead of 36,000 BTU. Volume matters, not just floor area.