Nine hundred square feet is a large one-bedroom or small two-bedroom apartment, the main floor of a small house (30 by 30), a compact ranch, or the upstairs of a Cape. The cooling load is 18,000 BTU/hr, which is 1.5 tons on the nose, and that makes 900 sq ft the first size where the decision is mostly about central equipment.
The 1.5-ton versus 2-ton question turns on what else is in the space. A shaded 900 sq ft floor (16,200) or a well-insulated one fits 1.5 tons comfortably. Add a kitchen (22,000), a third and fourth person (19,200), strong sun (19,800), poor insulation (20,700) or a 10 ft ceiling (22,500), and each one alone pushes you to 2 tons (24,000 BTU). Since almost every 900 sq ft home has a kitchen, 2 tons is what most of them get, and it is the right answer. A hot climate with sun and poor insulation together (25,000) is the only combination that reaches 2.5 tons.
Contractors sometimes quote 2.5 or even 3 tons for 900 sq ft 'to be safe'. That is 30,000 to 36,000 BTU for a load that tops out around 25,000 in the worst case. An oversized central system short-cycles just like an oversized window unit, and in a humid climate the house ends up cool and sticky. If a quote is more than half a ton above the table for your row, ask for the load calculation.
For ductless, 900 sq ft splits well into a 24,000 BTU outdoor unit with a 12,000 BTU head in the open living and kitchen area and 6,000 or 9,000 BTU heads in two bedrooms. A single 18,000 BTU head only works if the plan is genuinely open.
Heating 900 sq ft with average insulation runs 27,000 BTU/hr in a hot climate, 36,000 in a moderate one (10.6 kW) and 49,500 in a cold one (14.5 kW). A 40,000 BTU furnace covers the moderate case; a cold climate wants 60,000 BTU of furnace or a 2-ton cold-climate heat pump with 10 kW of backup strips. With poor insulation in a cold climate the load is 59,400 BTU/hr.
Example: a 30 by 30 main floor with a kitchen, three people, average sun and insulation, moderate climate. 900 × 20 = 18,000, plus 600 for the third person and 4,000 for the kitchen, is 22,600 BTU/hr. That is a 2-ton system. Take the same floor in a shaded, well-insulated house and it is 14,600 + 4,600 = 19,200, still 2 tons but with real margin, and a modulating 1.5-ton heat pump would also be defensible.
Cooling BTU for 900 sq ft by sun exposure and insulation
Two occupants, 8 ft ceiling, moderate climate. Each cell shows BTU/hr needed and the unit size to buy.
| Sun exposure | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| Heavily shaded | 18,600buy 24,000 BTU | 16,200buy 18,000 BTU | 14,600buy 18,000 BTU |
| Average | 20,700buy 24,000 BTU | 18,000buy 18,000 BTU | 16,200buy 18,000 BTU |
| Very sunny | 22,800buy 24,000 BTU | 19,800buy 24,000 BTU | 17,800buy 18,000 BTU |
Common adjustments for a 900 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 18,000 | 18,000 BTU |
| 4 people instead of 2 | 19,200 | 24,000 BTU |
| 10 ft ceiling | 22,500 | 24,000 BTU |
| Kitchen in the space | 22,000 | 24,000 BTU |
| Hot climate, very sunny | 21,800 | 24,000 BTU |
Heating BTU for 900 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 32,4009.5 kW | 27,0007.9 kW | 23,0006.7 kW |
| moderate | 43,20012.7 kW | 36,00010.6 kW | 30,6009.0 kW |
| cold | 59,40017.4 kW | 49,50014.5 kW | 42,10012.3 kW |
900 sq ft sizing questions
What size air conditioner do I need for a 900 sq ft room?
About 18,000 BTU/hr in average conditions (1.5 tons), so a 18,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Do I need 1.5 or 2 tons for 900 sq ft?
The floor area alone is 18,000 BTU/hr, exactly 1.5 tons, but a kitchen (+4,000), extra people, sun or poor insulation each push it past 19,000 BTU/hr. Most 900 sq ft homes are best served by 2 tons (24,000 BTU); 1.5 tons fits a shaded, well-insulated space or one without a kitchen.
Is 3 tons too much for 900 sq ft?
Yes. Even a hot, sunny, poorly insulated 900 sq ft space only reaches about 25,000 BTU/hr, so 3 tons (36,000 BTU) is roughly 45 percent oversized. It will short-cycle and leave the air humid. Ask for a Manual J if a quote comes in that high.
How many BTU to heat 900 sq ft?
About 36,000 BTU/hr in a moderate climate with average insulation (10.6 kW) and 49,500 in a cold one. A 40,000 BTU furnace fits the moderate case, a 60,000 BTU furnace or a 2-ton cold-climate heat pump the cold one.