Four hundred square feet is a 20 by 20 living room, a standard two-car garage, a studio apartment, or a large kitchen-diner. It is the size where the rule of thumb and the real world start to disagree, because the base answer of 8,000 BTU/hr assumes two people and no cooking, and most 400 sq ft spaces have more of both.
Start with the geometry of the classes. 8,000 BTU is exactly the load, so an 8,000 BTU unit has no margin. Very sunny is 8,800, poor insulation is 9,200, four people is 9,200, a 10 ft ceiling is 10,000: all of those are a 10,000 BTU unit. Sunny and poorly insulated together is 10,100, and a kitchen in the space is 12,000, both of which mean a 12,000 BTU unit. Most 400 sq ft rooms end up at 10,000 or 12,000 BTU, and the 8,000 the chart says is a floor, not an answer.
Studio renters hit this the hardest. They look up 400 sq ft, see 8,000 BTU, and buy an 8,000 BTU window unit for a space with a kitchen along one wall and a person home all day. Add the 4,000 BTU cooking allowance and the load is 12,000 BTU/hr. The 8,000 unit runs continuously from noon on and never gets below 78. If you have a kitchen, size for it.
This is also where a 12,000 BTU (1 ton) mini-split becomes the comfort choice rather than a luxury. Its quieter indoor unit and steady low-speed running suit a room you live in all day. If you are staying with window units, the 10,000 and 12,000 BTU sizes still run on a standard 115 V outlet.
For heating, a 400 sq ft room needs 12,000 BTU/hr in a hot climate, 16,000 in a moderate one (4.7 kW) and 22,000 in a cold one. A two-car garage with poor insulation in a cold climate is 26,400 BTU/hr; a 7.5 kW electric heater (about 25,600 BTU/hr) is marginal there, and a 30,000 BTU gas unit heater or 10 kW electric is the comfortable pick. In a moderate climate a 5 kW garage heater handles it.
Example: a 20 by 20 top-floor studio with a kitchen, one person, average sun, poor insulation under the roof, moderate climate. 400 × 20 = 8,000, times 1.15 for insulation is 9,200, plus 4,000 for the kitchen is 13,200 BTU/hr. That is a 14,000 BTU unit, nearly double the 8,000 the floor area alone suggested.
Cooling BTU for 400 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 | 8,300buy 10,000 BTU | 7,200buy 8,000 BTU | 6,500buy 8,000 BTU |
| Average | 9,200buy 10,000 BTU | 8,000buy 8,000 BTU | 7,200buy 8,000 BTU |
| Very sunny | 10,100buy 12,000 BTU | 8,800buy 10,000 BTU | 7,900buy 8,000 BTU |
Common adjustments for a 400 sq ft room
| Scenario | Cooling BTU/hr | Unit to buy |
|---|---|---|
| Average (baseline) | 8,000 | 8,000 BTU |
| 4 people instead of 2 | 9,200 | 10,000 BTU |
| 10 ft ceiling | 10,000 | 10,000 BTU |
| Kitchen in the space | 12,000 | 12,000 BTU |
| Hot climate, very sunny | 9,700 | 10,000 BTU |
Heating BTU for 400 sq ft by climate and insulation
| Climate | Poor insulation | Average insulation | Good insulation |
|---|---|---|---|
| hot | 14,4004.2 kW | 12,0003.5 kW | 10,2003.0 kW |
| moderate | 19,2005.6 kW | 16,0004.7 kW | 13,6004.0 kW |
| cold | 26,4007.7 kW | 22,0006.4 kW | 18,7005.5 kW |
400 sq ft sizing questions
What size air conditioner do I need for a 400 sq ft room?
About 8,000 BTU/hr in average conditions (0.67 tons), so a 8,000 BTU unit. The table above covers sunnier and worse-insulated rooms.
Is 8,000 BTU enough for 400 sq ft?
Only for a shaded or average room with two people and no kitchen. The load is exactly 8,000 BTU/hr in average conditions, so there is no margin. Most 400 sq ft rooms are better served by a 10,000 BTU unit, and a studio with a kitchen needs 12,000 BTU.
What size AC for a 400 sq ft studio apartment?
12,000 BTU in most cases. The 8,000 BTU/hr floor-area load plus the 4,000 BTU kitchen allowance is 12,000 BTU/hr. On a sunny top floor it can reach 13,000 to 14,000, so a 14,000 BTU unit or a 12,000 BTU mini-split with a little margin is a sound choice.
How many BTU to heat a 400 sq ft two-car garage?
About 16,000 BTU/hr in a moderate climate and 22,000 in a cold one with average insulation. An uninsulated garage in a cold climate needs 26,400 BTU/hr, which is a 10 kW electric heater or a 30,000 BTU gas unit heater.