How to use it
- Enter the floor area in square feet and the ceiling height.
- Choose the insulation level and sun exposure.
- Enter how many people use the room and the watts of devices.
- Read the cooling load and the recommended unit size.
How the estimate works
The calculator adds up four sources of heat:
- Base heat gain through walls, windows and roof, from the room's volume and an insulation factor (good, average or poor).
- Sun exposure: north-facing and shaded rooms get 10% less, sunny south or west rooms with big windows 15% more, attic rooms 30% more.
- People: about 120 W (roughly 400 BTU/h) per person at rest.
- Devices and lights: TVs, computers, a fridge, lamps, in watts.
The total is shown in BTU/h, kW and tons (1 ton = 12,000 BTU/h). The tool then picks the smallest standard size (9,000, 12,000, 18,000 or 24,000 BTU/h, the common mini-split sizes) that covers the load, allowing a 5% margin because inverter units can run slightly above their nominal rating.
Worked example
A 300 sq ft bedroom-office with 8 ft ceilings, average insulation and average sun, two people and 300 W of devices:
- volume: about 2,400 cu ft;
- base heat gain and sun: about 6,500 BTU/h;
- two people: about 800 BTU/h;
- devices: about 1,000 BTU/h.
Total ≈ 8,300 BTU/h (0.7 ton), so a 9,000 BTU unit is recommended. The same room in an attic with poor insulation would need noticeably more.
Quick rule of thumb: BTU per square foot
A common shortcut is about 20 BTU per square foot for a typical room with 8 ft ceilings. ENERGY STAR's sizing guide for room air conditioners uses bands like these:
| Room area | Cooling capacity |
|---|---|
| 150–250 sq ft | 6,000 BTU/h |
| 250–300 sq ft | 7,000 BTU/h |
| 300–350 sq ft | 8,000 BTU/h |
| 350–400 sq ft | 9,000 BTU/h |
| 450–550 sq ft | 12,000 BTU/h |
| 700–1,000 sq ft | 18,000 BTU/h |
| 1,400–1,500 sq ft | 24,000 BTU/h |
The same guide suggests reducing capacity by 10% for a heavily shaded room, increasing it by 10% for a very sunny one, adding 600 BTU/h for each person beyond two, and adding 4,000 BTU/h for a kitchen. This calculator goes a step further by using the ceiling height, insulation and your actual devices.
Why size matters
- Too small: the unit runs nonstop on hot afternoons and the room never gets cool.
- Too big: it cools the air quickly and shuts off before it has removed much humidity, leaving the room cold and clammy, and it cycles on and off more, which wastes energy.
If you're between two sizes, a smaller inverter unit is often fine in a well-insulated room, since it can run continuously at varying speed. In a top-floor or west-facing room with lots of glass, pick the larger size.
Limits of a rule of thumb
This is a planning estimate, not a load calculation. For central air or a heat pump serving a whole house, an HVAC contractor should run an ACCA Manual J calculation, which accounts for window area and orientation, wall and roof construction, air leakage, duct losses and your local design temperatures. The estimate here is most useful for a single room, a window unit, a portable AC or a ductless mini-split head.
Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for a typical room with 8 ft ceilings. Sunny rooms, attics, high ceilings and kitchens need more.
What size AC do I need for a 12x12 room?
A 144 sq ft room usually needs about 5,000–6,000 BTU/h. Enter it in the calculator with your insulation and sun exposure for a better estimate.
How many square feet does a 12,000 BTU unit cool?
About 450–550 sq ft under average conditions, less in a sunny or poorly insulated space.
What is a ton of air conditioning?
One ton is 12,000 BTU per hour of cooling capacity. A 3-ton central system provides 36,000 BTU/h.
Is a bigger air conditioner better?
No. An oversized unit short-cycles and removes less humidity. Choose the size that matches the load.
Last updated: November 12, 2026