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This BTU calculator tells you what size air conditioner you need for a room. It uses the ENERGY STAR room air conditioner sizing chart and adjusts it for sun exposure, ceiling height, the number of people and whether the room is a kitchen, then suggests a standard unit size.
How to Use the BTU Calculator
- Enter the room length and width. The calculator converts these to square feet for the sizing chart.
- Enter the ceiling height. Rooms with ceilings above 8 ft (2.4 m) have more air to cool.
- Choose the sun exposure: very sunny rooms need more capacity and heavily shaded rooms need less.
- Enter the number of people who regularly use the room.
- Tick the kitchen option if the unit will cool a kitchen.
How the BTU Calculation Works
The starting point is the ENERGY STAR chart, which matches room area to cooling capacity. The calculator then applies these standard adjustments:
- Very sunny room: add 10%.
- Heavily shaded room: subtract 10%.
- More than two people regularly in the room: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
- Ceilings above 8 ft: increase the capacity in proportion to the extra height.
1 kW = 3,412 BTU per hour; 1 ton of cooling = 12,000 BTU per hour
Air Conditioner Size by Room Area
| Room area | Cooling capacity (BTU/h) |
|---|---|
| 100–150 sq ft (9–14 m²) | 5,000 |
| 150–250 sq ft (14–23 m²) | 6,000 |
| 250–300 sq ft (23–28 m²) | 7,000 |
| 300–350 sq ft (28–33 m²) | 8,000 |
| 350–400 sq ft (33–37 m²) | 9,000 |
| 400–450 sq ft (37–42 m²) | 10,000 |
| 450–550 sq ft (42–51 m²) | 12,000 |
| 550–700 sq ft (51–65 m²) | 14,000 |
| 700–1,000 sq ft (65–93 m²) | 18,000 |
| 1,000–1,200 sq ft (93–111 m²) | 21,000 |
| 1,200–1,400 sq ft (111–130 m²) | 23,000 |
| 1,400–1,500 sq ft (130–139 m²) | 24,000 |
| 1,500–2,000 sq ft (139–186 m²) | 30,000 |
| 2,000–2,500 sq ft (186–232 m²) | 34,000 |
Worked Examples
- A 15 × 12 ft bedroom (180 sq ft) with normal sun and two people needs 6,000 BTU (about 1.8 kW).
- A 20 × 20 ft sunny kitchen-living area (400 sq ft) with 10 ft ceilings and four people needs about 17,600 BTU, so an 18,000 BTU unit.
Metric example: a 5 × 4 m living room is 20 m² (215 sq ft), which falls in the 6,000 BTU (about 1.8 kW) band.
Why the Right Size Matters
An undersized air conditioner runs constantly and never reaches the set temperature. An oversized unit cools the room too quickly and switches off before it has removed enough moisture, leaving the room cold and clammy and wasting energy through frequent cycling. Matching the size to the room gives the best comfort and efficiency.
Tips
- Rooms with poor insulation, many windows or upstairs locations may need more capacity than the chart suggests.
- For whole-house central air or heat pumps, ask an HVAC contractor for a Manual J load calculation.
- Electrical work for larger units should be done by a licensed electrician.
- Thinking about running the AC on solar power? See our solar panel calculator.
Frequently Asked Questions
How many BTU do I need per square foot?
A rough rule is about 20 BTU per square foot of living space, which is close to the ENERGY STAR chart for medium-sized rooms. Smaller rooms need a bit more per square foot and larger spaces a bit less.
What size AC do I need for a 12×12 room?
A 12 × 12 ft room is 144 sq ft, which needs about 5,000 BTU in normal conditions. A sunny room or more occupants may push it to 6,000 BTU.
How many BTU is a 1-ton air conditioner?
One ton of cooling equals 12,000 BTU per hour, or about 3.5 kW.
How do I convert BTU to kW?
Multiply BTU per hour by 0.000293, or divide by 3,412. For example, 12,000 BTU is about 3.5 kW of cooling capacity.
Is a bigger air conditioner better?
No. An oversized unit short-cycles, removes less humidity and uses more energy. Choose the size that matches the room and its conditions.
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