Eight hundred square feet is a full one-bedroom apartment, a small two-bedroom, a large open main floor, a big finished basement, or an accessory dwelling unit (800 is the maximum ADU size in many cities). The cooling load is 16,000 BTU/hr, or 1.33 tons, and the nearest single unit is 18,000 BTU, which is also the smallest common central air conditioner at 1.5 tons.
Treated as one zone, the decision is between 1.5 and 2 tons. Average conditions (16,000), very sunny (17,600) and four people (17,200) all fit an 18,000 BTU system. Poor insulation (18,400), a 10 ft ceiling (20,000), a kitchen (20,000) and sunny plus poor (20,200) all need 24,000 BTU, which is 2 tons. A shaded, well-insulated 800 sq ft space is 13,000 and a 14,000 BTU unit would do, though nothing that small is sold as central equipment.
Treated as two zones, which is how most 800 sq ft apartments actually live, the answer changes shape. A 550 sq ft living and kitchen area needs 11,000 plus 4,000 for cooking, so a 14,000 or 18,000 BTU unit, and a 250 sq ft bedroom needs 5,000, so a 6,000 BTU unit. The total is higher than the single-zone number because each zone rounds up on its own, and that is normal. The bedroom gets cooled with its door shut, which no single unit can manage.
Multi-zone mini-splits are built for exactly this: an 18,000 BTU outdoor unit with a 12,000 BTU head in the living area and a 9,000 in the bedroom. The mistake to avoid is oversizing the heads to 'be safe'; an 18k head in a 250 sq ft bedroom will never run below its minimum output and will cycle like a cheap window unit. For a new ADU, a 1.5-ton ducted heat pump is the common and quiet choice.
Heating 800 sq ft needs 24,000 BTU/hr in a hot climate, 32,000 in a moderate one (9.4 kW) and 44,000 in a cold one (12.9 kW). A 40,000 to 45,000 BTU furnace fits the cold case with average insulation; a 2-ton cold-climate heat pump covers it in most of the country and needs backup only on the coldest nights. Poor insulation in a cold climate pushes it to 52,800 BTU/hr.
Example: an 800 sq ft ADU in a hot climate, two people, 9 ft ceilings, average sun and insulation, no separate kitchen allowance because the kitchen is small and rarely used for heavy cooking. 800 × 20 = 16,000, times 1.125 for the ceiling and 1.10 for the climate, is 19,800 BTU/hr. That is a 2-ton (24,000 BTU) system. Drop the ceiling to 8 ft and add shade and good insulation and it is 14,300, a 1.5-ton system with margin.
Cooling BTU for 800 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 | 16,600buy 18,000 BTU | 14,400buy 18,000 BTU | 13,000buy 14,000 BTU |
| Average | 18,400buy 24,000 BTU | 16,000buy 18,000 BTU | 14,400buy 18,000 BTU |
| Very sunny | 20,200buy 24,000 BTU | 17,600buy 18,000 BTU | 15,800buy 18,000 BTU |
Common adjustments for a 800 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 16,000 | 18,000 BTU |
| 4 people instead of 2 | 17,200 | 18,000 BTU |
| 10 ft ceiling | 20,000 | 24,000 BTU |
| Kitchen in the space | 20,000 | 24,000 BTU |
| Hot climate, very sunny | 19,400 | 24,000 BTU |
Heating BTU for 800 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 28,8008.4 kW | 24,0007.0 kW | 20,4006.0 kW |
| moderate | 38,40011.3 kW | 32,0009.4 kW | 27,2008.0 kW |
| cold | 52,80015.5 kW | 44,00012.9 kW | 37,40011.0 kW |
800 sq ft sizing questions
What size air conditioner do I need for a 800 sq ft room?
About 16,000 BTU/hr in average conditions (1.3 tons), so a 18,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Is 1.5 tons enough for 800 sq ft?
In average conditions, yes: 800 sq ft is 16,000 BTU/hr, and 1.5 tons is 18,000 BTU. Poor insulation, a 10 ft ceiling, a kitchen, or a sunny exposure with poor insulation each push the load to 18,400 to 20,200 BTU/hr, where 2 tons (24,000 BTU) is the right size.
What size mini-split for an 800 sq ft apartment?
For a one-bedroom, a two-zone system with an 18,000 BTU outdoor unit, a 12,000 BTU head in the living and kitchen area and a 9,000 BTU head in the bedroom. For a truly open 800 sq ft plan, a single 18,000 BTU head works in average conditions and a 24,000 BTU head in sunny or poorly insulated ones.
How many BTU to heat 800 sq ft?
About 32,000 BTU/hr in a moderate climate with average insulation (9.4 kW) and 44,000 in a cold one. A 40,000 to 45,000 BTU furnace or a 2-ton cold-climate heat pump with backup strips covers it.