How to use this BTU calculator
- Enter the room’s length and width, or switch to Total floor area if you already know it. For an L-shaped room, work out the area with our square footage calculator first.
- Enter the ceiling height. Leave it at 8 ft (2.4 m) for a standard room.
- Choose the sun exposure. “Very sunny” means large windows facing the afternoon sun; “heavily shaded” means trees or buildings block most direct sun.
- Enter how many people regularly use the room at the same time, and whether it is a kitchen.
- Read the result. Buy the recommended unit size, which is the nearest common size at or above your figure.
How the calculation works
This calculator follows the sizing guidance that ENERGY STAR publishes for room air conditioners. It starts with the cooling capacity from the ENERGY STAR chart for your floor area, then adjusts it:
2. Ceiling above 8 ft: × (ceiling height ÷ 8) (our approximation)
3. Heavily shaded: × 0.9 · Very sunny: × 1.1
4. + 600 BTU/h for each regular occupant beyond two
5. + 4,000 BTU/h if the room is a kitchen
Tons = BTU/h ÷ 12,000 · kW = BTU/h ÷ 3,412
The ceiling-height step is not part of the ENERGY STAR chart. It is a simple proportional estimate for taller rooms.
Worked example: a sunny 14 × 18 ft living room
- Floor area: 14 × 18 = 252 sq ft, which falls in the 250–300 sq ft row: 7,000 BTU/h
- Very sunny: 7,000 × 1.1 = 7,700 BTU/h
- Three people regularly use it, one more than two: + 600 = 8,300 BTU/h (about 0.69 tons or 2.4 kW)
- Nearest common unit at or above that: buy a 9,000 BTU air conditioner
In metric: a 4 × 5 m bedroom (20 m², about 215 sq ft) with a 2.4 m ceiling, average sun and two people needs6,000 BTU/h, or about 1.8 kW.
ENERGY STAR room air conditioner sizing chart
Cooling capacity for a room with an 8 ft (2.4 m) ceiling, before any adjustments. Square metres and kW are converted and rounded.
| Area (sq ft) | Area (m²) | Capacity (BTU/h) | ≈ kW |
|---|---|---|---|
| 100 to 150 | 9.3 to 13.9 | 5,000 | 1.5 |
| 150 to 250 | 13.9 to 23.2 | 6,000 | 1.8 |
| 250 to 300 | 23.2 to 27.9 | 7,000 | 2.1 |
| 300 to 350 | 27.9 to 32.5 | 8,000 | 2.3 |
| 350 to 400 | 32.5 to 37.2 | 9,000 | 2.6 |
| 400 to 450 | 37.2 to 41.8 | 10,000 | 2.9 |
| 450 to 550 | 41.8 to 51.1 | 12,000 | 3.5 |
| 550 to 700 | 51.1 to 65.0 | 14,000 | 4.1 |
| 700 to 1,000 | 65.0 to 92.9 | 18,000 | 5.3 |
| 1,000 to 1,200 | 92.9 to 111.5 | 21,000 | 6.2 |
| 1,200 to 1,400 | 111.5 to 130.1 | 23,000 | 6.7 |
| 1,400 to 1,500 | 130.1 to 139.4 | 24,000 | 7.0 |
| 1,500 to 2,000 | 139.4 to 185.8 | 30,000 | 8.8 |
| 2,000 to 2,500 | 185.8 to 232.3 | 34,000 | 10.0 |
Each row runs from the first figure up to, but not including, the second. Rooms under 100 sq ft use the smallest size, 5,000 BTU/h.
Why the right size matters
Too small and the unit runs non-stop on hot afternoons without ever reaching the temperature you set.Too big is a problem too: an oversized unit cools the air quickly and then switches off, a pattern called short-cycling. It runs too briefly to pull much moisture out of the air, so the room feels cold and damp, and the frequent starts waste energy and add wear. That is why this calculator rounds only to the next common unit size, not to the biggest one you can afford.
Tips before you buy
- Check where the unit will go. Window units need the right window width and an outlet nearby; mini-splits need an outdoor spot and professional installation in most cases.
- Look at efficiency, not just BTU. An ENERGY STAR certified model uses less electricity for the same cooling.
- Treat open-plan spaces as one room. If a living room opens into a kitchen with no door, size for the combined area and include the kitchen adjustment.
- Reduce the heat load first. Blinds or curtains on sunny windows and LED lighting can let a smaller unit do the job.
- Whole house? For central air or multi-room systems, ask an HVAC contractor for a proper load calculation (Manual J in the US).
Frequently asked questions
What size air conditioner do I need for a 12x12 room?
A 12 ft × 12 ft room is 144 sq ft, which falls in the 100–150 sq ft row of the ENERGY STAR chart, so about 5,000 BTU/h is enough for a room with average sun, two people and an 8 ft ceiling. Go up a size if the room is very sunny or regularly has more people in it.
How many BTU do I need per square foot?
For mid-sized rooms the ENERGY STAR chart works out to roughly 20–25 BTU per square foot. For example, 10,000 BTU covers 400–450 sq ft. Small rooms need more per square foot, because even the smallest units start at about 5,000 BTU.
How many BTU is one ton of air conditioning?
One ton of cooling is 12,000 BTU per hour. In kilowatts, 1 kW of cooling is about 3,412 BTU/h, so a 12,000 BTU unit is roughly 3.5 kW.
Is it better to buy a bigger air conditioner?
No. ENERGY STAR warns that an oversized unit is less effective and wastes energy. It cools the air quickly and switches off before it has removed much moisture, so the room can feel cold and clammy. Choose the size that matches the room, then round up only to the next available unit size.
Does ceiling height change the BTU I need?
Yes. The ENERGY STAR chart assumes an 8 ft (about 2.4 m) ceiling. This calculator scales the estimate in proportion to the extra height above 8 ft, so a 10 ft ceiling adds 25%. That is an approximation; very tall or vaulted rooms are worth a professional assessment.
Can I use this to size central air or a whole-house system?
No. This calculator is for single-room units such as window, portable and single-zone mini-split air conditioners. Whole-house systems should be sized with a proper load calculation, such as ACCA Manual J in the US, which accounts for insulation, windows, ducts and climate.
How should I compare portable air conditioner sizes?
In the US, portable air conditioners now show a seasonally adjusted cooling capacity (SACC), which is usually lower than the older rating printed on many boxes. Compare units using the SACC figure so a portable unit is not undersized for your room.
Last reviewed November 5, 2026.