BTU Calculator
Find the right air conditioner or heater size in BTU for a room based on square footage, sun exposure and occupants.
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How to Calculate
Baseline: about 20 BTU per square foot of living space (ENERGY STAR guideline).
Add 10% for very sunny rooms, subtract 10% for shaded rooms, add 600 BTU per person beyond two, and 4,000 BTU for kitchens.
Oversized units short-cycle and leave the room clammy; undersized units run constantly.
See our methodology for how every calculator on this site is built.
Worked Example: A Sunny Living Room
The room is 18 ft by 20 ft, with big west-facing windows, and four people usually use it in the evening.
- Area: 18 × 20 = 360 ft²
- Base load: 360 × 20 = 7,200 BTU
- Sun adjustment: 7,200 × 1.10 = 7,920 BTU
- Extra people: (4 − 2) × 600 = 1,200 BTU
- Total: 7,920 + 1,200 = 9,120 BTU
- Rounded: about 9,000 BTU (0.76 tons)
Window units are sold in steps such as 8,000, 10,000 and 12,000 BTU. Here a 10,000 BTU unit is the practical pick. Going up to 12,000 would cool the room quickly but tends to leave it damp.
For a 12 × 14 kitchen with average sun and two people: 168 × 20 = 3,360, plus 4,000 for the kitchen = 7,360 BTU, which rounds to 7,500. The stove and fridge heat matter more than the room size.
ENERGY STAR Room Size Chart
| Room area | Cooling capacity |
|---|---|
| 100–150 ft² | 5,000 BTU |
| 150–250 ft² | 6,000 BTU |
| 250–300 ft² | 7,000 BTU |
| 300–350 ft² | 8,000 BTU |
| 350–400 ft² | 9,000 BTU |
| 400–450 ft² | 10,000 BTU |
| 450–550 ft² | 12,000 BTU |
| 550–700 ft² | 14,000 BTU |
| 700–1,000 ft² | 18,000 BTU |
The chart assumes 8 ft ceilings and typical insulation. Apply the same sun, occupant and kitchen adjustments to it.
What Else Changes the Load
Insulation and windows
An upstairs room under an uninsulated attic or a room with old single-pane windows can need 20% or more above the chart. A well-insulated basement room may need less.
Climate
The 20 BTU per ft² rule is a middle-of-the-road number. In the hot, humid South or desert Southwest, rooms with heavy sun exposure may need more. In the Pacific Northwest, the chart is often generous.
Open floor plans
If the room opens to a kitchen or hallway with no door, include that space. Cool air drifts out of the area you measured and into the rest of the house.
Choosing the Unit
- Window units are the most efficient per dollar. Check the window type: casement and sliding windows need special models.
- Portable units sit on the floor and vent through a hose. Single-hose models pull room air out through the window, which draws hot outside air in elsewhere. Their real-world output is often lower than the box number suggests; look at the SACC rating, not just the older ASHRAE rating.
- Ductless mini-splits cost more up front but are quiet, efficient, and can also heat. Sizing works the same way, though a contractor should run a proper load calculation (Manual J) for whole-house systems.
- Heating: a typical 1,500 watt space heater puts out about 5,100 BTU per hour, enough to supplement a room of roughly 150 ft² in moderate weather.
Rough Cost
Window air conditioners typically cost around $150 to $250 for 5,000 to 6,000 BTU models and roughly $350 to $600 for 10,000 to 12,000 BTU. Portable units cost more for the same rating. A single-zone mini-split installed often runs a few thousand dollars.
Sources
Common questions
- What size air conditioner do I need for a 300 sq ft room?
- What size AC unit do I need for 500 square feet?
Frequently Asked Questions
How many BTU for a 12×12 room?
About 3,000 BTU by the formula; the smallest common window AC is 5,000 BTU, which is fine.
How many BTU is one ton?
One ton of cooling equals 12,000 BTU per hour.
Does ceiling height matter?
Yes — for ceilings above 8 ft, increase capacity by about 10% per extra foot.
Is a bigger air conditioner better?
No. An oversized unit cools the air fast and shuts off before it pulls moisture out, so the room feels damp and cold.
Can one window unit cool two rooms?
Only if they are open to each other with a wide doorway. Behind a regular door, the second room will stay warm.