AC BTU calculator: room size to air conditioner size
Draw your room, and the calculator sizes the air conditioner the way ENERGY STAR does: base capacity by floor area, then adjusted for sun, people, a kitchen and ceiling height.
Plan on about 20 BTU per square foot. A 400 sq ft room needs about 10,000 BTU; 800 sq ft about 18,000 BTU (1.5 tons). Add 10% for a sunny room and 4,000 BTU for a kitchen.
How many BTU do I need?
Start from floor area using the ENERGY STAR chart, then adjust: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU for each person beyond two, and +4,000 BTU for a kitchen.
The ceiling factor is our addition: the ENERGY STAR chart assumes a standard 8 ft ceiling, so we scale by room volume when yours is higher. That part is an estimate.
Why bigger is not better
An oversized AC cools the air fast and shuts off before it pulls the moisture out, leaving the room cold and clammy while it cycles on and off. An undersized one runs nonstop on hot afternoons. Sizing correctly gets you both comfort and the lowest bill.
ENERGY STAR BTU chart by room size
| Area | Metric | Capacity |
|---|---|---|
| 100–150 ft² | 9–14 m² | 5,000 BTU |
| 150–250 ft² | 14–23 m² | 6,000 BTU |
| 250–300 ft² | 23–28 m² | 7,000 BTU |
| 300–350 ft² | 28–33 m² | 8,000 BTU |
| 350–400 ft² | 33–37 m² | 9,000 BTU |
| 400–450 ft² | 37–42 m² | 10,000 BTU |
| 450–550 ft² | 42–51 m² | 12,000 BTU |
| 550–700 ft² | 51–65 m² | 14,000 BTU |
| 700–1,000 ft² | 65–93 m² | 18,000 BTU |
| 1,000–1,200 ft² | 93–111 m² | 21,000 BTU |
| 1,200–1,400 ft² | 111–130 m² | 23,000 BTU |
| 1,400–1,500 ft² | 130–139 m² | 24,000 BTU |
| 1,500–2,000 ft² | 139–186 m² | 30,000 BTU |
| 2,000–2,500 ft² | 186–232 m² | 34,000 BTU |
How many square feet does an AC cool?
Read the ENERGY STAR chart the other way around: find the unit size you are looking at and see the room size it is meant for.
- 5,000 BTU (0.42 ton): about 100–150 sq ft (9–14 m²)
- 6,000 BTU (0.5 ton): about 150–250 sq ft (14–23 m²)
- 8,000 BTU (0.67 ton): about 300–350 sq ft (28–33 m²)
- 10,000 BTU (0.83 ton): about 400–450 sq ft (37–42 m²)
- 12,000 BTU (1.0 ton): about 450–550 sq ft (42–51 m²)
- 14,000 BTU (1.17 ton): about 550–700 sq ft (51–65 m²)
- 18,000 BTU (1.5 ton): about 700–1,000 sq ft (65–93 m²)
- 24,000 BTU (2.0 ton): about 1,400–1,500 sq ft (130–139 m²)
Go one size up for a very sunny room or a kitchen, and one size down for a heavily shaded room.
How much does it cost to run an air conditioner?
An 8,000 BTU window AC (EER 11) draws about 727 watts. Run 8 hours a day at the 2026 U.S. average of about 18¢/kWh, that is about 1.05 a day, or roughly 31 a month.
Three things move the number: your electricity rate (it varies by more than 2× between states), how many hours you run it, and efficiency. Inverter units throttle down instead of cycling off and on, so they typically use less energy over a season than single-speed units of the same size.
Questions homeowners ask
How many BTU do I need for 400 square feet?
About 9,000–10,000 BTU. ENERGY STAR lists 9,000 BTU for 350–400 sq ft and 10,000 BTU for 400–450 sq ft. Go up 10% for a sunny room.
How many BTU do I need for 800 square feet?
About 18,000 BTU (1.5 tons) for 700–1,000 sq ft.
How many BTU per square foot for air conditioning?
Roughly 20 BTU per square foot for a standard room with an 8 ft ceiling. Small rooms run a little higher per square foot, large open areas a little lower.
How many square feet does an 18,000 BTU AC cool?
About 700 to 1,000 square feet under average sun and occupancy.
How do I convert BTU to tons?
Divide by 12,000. One ton of cooling = 12,000 BTU per hour, so 24,000 BTU = 2 tons and 36,000 BTU = 3 tons.
How much does it cost to run an AC for 1 hour?
Divide BTU by EER to get watts, then multiply by your rate. A 12,000 BTU unit at EER 11 uses about 1.1 kWh per hour: about 0.20 at the 2026 U.S. average (≈18¢/kWh).
Estimates for planning only. Prices vary by region, home, and contractor. Gas, refrigerant, and high-voltage work must be done by a licensed technician.