Quick answer: Use about 20 BTU per square foot as a starting point. A 12 × 15 ft bedroom (180 sq ft) needs about 6,000 BTU, and a 500 sq ft living room about 12,000 BTU. Add 10% for sunny rooms, 600 BTU per extra person and 4,000 BTU for kitchens.
Buying the wrong size air conditioner is a common and expensive mistake. Too small and it runs all day without cooling the room; too big and it cycles on and off, leaving the air cold and damp. Here is how to size a window, portable or mini-split unit for a single room.
BTU Chart by Room Size
This chart is based on the ENERGY STAR room air conditioner sizing guide:
| Room area | BTU per hour |
|---|---|
| 100–150 sq ft | 5,000 |
| 150–250 sq ft | 6,000 |
| 250–300 sq ft | 7,000 |
| 300–350 sq ft | 8,000 |
| 350–400 sq ft | 9,000 |
| 400–450 sq ft | 10,000 |
| 450–550 sq ft | 12,000 |
| 550–700 sq ft | 14,000 |
| 700–1,000 sq ft | 18,000 |
| 1,000–1,200 sq ft | 21,000 |
Adjust for Your Room
- Very sunny room: add 10%.
- Heavily shaded room: subtract 10%.
- More than two people: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
- Ceilings above 8 ft: increase in proportion; a 10 ft ceiling needs about 25% more.
Worked Examples
- 12 × 12 ft bedroom (144 sq ft), normal sun, two people: 5,000 BTU.
- 15 × 20 ft living room (300 sq ft), very sunny, four people: 7,000 × 1.1 + 1,200 = 8,900 BTU, so a 9,000 or 10,000 BTU unit.
- 20 × 20 ft open kitchen-living area with 10 ft ceilings, sunny, four people: about 17,600 BTU, so an 18,000 BTU unit.
Skip the math: enter the room size, ceiling height, sun exposure and occupants for a recommended unit size in BTU, kW and tons. Open the free BTU Calculator →
BTU, kW and Tons
Air conditioner capacity is listed in different units around the world:
- 1 kW of cooling ≈ 3,412 BTU per hour
- 1 ton of cooling = 12,000 BTU per hour (about 3.5 kW)
- A 2.5 kW split system is roughly 8,500 BTU
Why Bigger Is Not Better
An oversized unit reaches the set temperature quickly and switches off before it has removed enough humidity. The room feels cold and clammy, and the frequent starts waste energy and wear the compressor. A correctly sized unit runs longer, steadier cycles and keeps humidity under control.
Whole-House Systems
For central air or a whole-home heat pump, rules of thumb are not enough. Ask an HVAC contractor for a Manual J load calculation, which accounts for insulation, windows, orientation, air leakage and local climate.
Want to offset the running cost with solar? See how many solar panels you need.
Frequently Asked Questions
What size AC do I need for a 12×12 room?
A 144 sq ft room needs about 5,000 BTU in normal conditions, or 6,000 BTU if it is very sunny or used by more people.
How many square feet does a 12,000 BTU AC cool?
About 450–550 sq ft in typical conditions, according to the ENERGY STAR sizing chart.
Is it bad to oversize an air conditioner?
Yes. Oversized units short-cycle, remove less humidity and use more energy. Choose the size that matches the room.
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