BTU Calculator
Calculate how many BTUs of air conditioning a room needs from its size, climate, sun exposure and occupants to size a window unit or mini-split.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Cooling load
- 4,500 BTU per hour
- 25 BTU per sq ft plus adjustments
- Air conditioner size
- 0.4 tons
- 1 ton of cooling = 12,000 BTU per hour
- Window or portable unit
- 5,000 BTU
- Smallest common size that meets the load
- Room area
- 180 sq ft
- 8 ft ceiling
BTU formula
Base load = Area (sq ft) x BTU per sq ft (20 mild, 25 moderate, 30 hot, 35 very hot)
Adjusted load = Base load x sun factor (0.9 shaded, 1.1 sunny) x Ceiling height / 8
Total BTU = Adjusted load + 600 x (people - 2) + 4,000 if kitchen
Tons = Total BTU / 12,000Worked example
A 20 x 18 ft living room with 10 ft ceilings in a hot climate, very sunny, with 4 people and an open kitchen: 20 x 18 = 360 sq ft. 360 x 30 = 10,800 BTU. Times 1.1 for sun and 10 / 8 for ceiling height: 10,800 x 1.1 x 1.25 = 14,850 BTU. Add 2 x 600 = 1,200 for the extra people and 4,000 for the kitchen: 20,050 BTU per hour.
That is about 1.7 tons of cooling. The smallest common window unit that covers it is 24,000 BTU, but a room this size is usually better served by an 18,000 to 24,000 BTU mini-split.
Quick reference
| Room size | Area | BTU per hour | Window unit |
|---|---|---|---|
| 10 x 10 ft | 100 sq ft | 2,500 | 5,000 BTU |
| 10 x 12 ft | 120 sq ft | 3,000 | 5,000 BTU |
| 12 x 12 ft | 144 sq ft | 3,600 | 5,000 BTU |
| 12 x 15 ft | 180 sq ft | 4,500 | 5,000 BTU |
| 15 x 20 ft | 300 sq ft | 7,500 | 8,000 BTU |
| 20 x 20 ft | 400 sq ft | 10,000 | 10,000 BTU |
| 20 x 25 ft | 500 sq ft | 12,500 | 14,000 BTU |
| 25 x 30 ft | 750 sq ft | 18,750 | 24,000 BTU |
How to calculate BTUs for a room
Start with the floor area in square feet and multiply by a climate factor: about 20 BTU per square foot in a mild northern climate, 25 in most of the country, and 30 to 35 in the hot South and Southwest. Then adjust for the things that add heat: a very sunny room needs 10 percent more, tall ceilings hold more air, each extra person adds about 600 BTU, and a kitchen adds about 4,000 BTU for the range and refrigerator.
These are the Energy Star rules of thumb for sizing room air conditioners. They work well for window units, portable units and single-room mini-splits. Whole-house systems need a proper Manual J load calculation that accounts for insulation, windows, orientation and ductwork, which an HVAC contractor can run.
Picking the right size
Round up to the next common window unit size, but not further. A unit that is too big short-cycles: it hits the thermostat setting before it has run long enough to wring humidity out of the air, so the room feels cold and damp. A unit that is slightly small runs longer but keeps the room drier and more comfortable.
Window units are sold from 5,000 to about 25,000 BTU. Above 12,000 BTU most units need a dedicated circuit, and above 15,000 BTU many require 240 V. Mini-splits cover 9,000 to 36,000 BTU per zone and are quieter and far more efficient for large rooms or additions.
Key facts
- One ton of air conditioning removes 12,000 BTU per hour, the heat needed to melt one ton of ice in 24 hours.
- Energy Star's rule of thumb is about 20 BTU per square foot for an average room, before adjustments.
- Energy Star suggests reducing capacity 10% for a heavily shaded room and increasing it 10% for a very sunny one.
- Add about 600 BTU for each person beyond two who regularly occupies the room, and 4,000 BTU for a kitchen.
- An oversized air conditioner cools the air quickly but shuts off before it removes humidity, leaving the room clammy.
Frequently asked questions
How many BTUs do I need for a 12 x 12 room?
A 144 sq ft room needs about 3,600 BTU per hour in a moderate climate, so the smallest 5,000 BTU window unit is plenty. In a very sunny room in a hot climate it rises to about 4,750 BTU.
How many square feet does a 12,000 BTU air conditioner cool?
About 450 to 550 square feet in a moderate climate, or 350 to 400 square feet in a hot, sunny location.
How many BTUs is a ton of cooling?
12,000 BTU per hour. A 2 ton central system delivers 24,000 BTU per hour, and a 3 ton system 36,000.
Is it better to oversize an air conditioner?
No. An oversized unit cycles on and off too quickly to dehumidify, wastes energy and wears out faster. Size it to the load and round up only to the next available size.
Sources and references
- Room Air Conditioners: sizing chart and adjustments (ENERGY STAR)
- Purchasing Energy-Efficient Room Air Conditioners (U.S. Department of Energy)
- Manual J Residential Load Calculation (ACCA)
Next steps for this project
The usual order for a insulation & hvac project. See all insulation & hvac calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.