HVAC BTU Room Sizing Calculator
Size a window or room air conditioner with the ENERGY STAR chart: cooling capacity in BTU per hour from the room's floor area, adjusted for sun, extra occupants and kitchen use.
Last updated: October 2026
Recommended Cooling Capacity
6,000 BTU/hr
Formula below · 2 sources (energystar.gov, e-education.psu.edu) · Updated Oct 2026
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About this calculator
This calculator follows the ENERGY STAR sizing guidance for room air conditioners. First the floor area (length × width) is matched to the capacity chart: for example 150 up to 250 sq ft calls for 6,000 BTU/hr, 350 up to 400 sq ft for 9,000 and 450 up to 550 sq ft for 12,000. The full chart is in the table below.
Adjustments
ENERGY STAR then adjusts the chart value for the room: reduce capacity by 10% if the room is heavily shaded, increase it by 10% if it is very sunny, add 600 BTU/hr for each person beyond two who regularly uses the room, and add 4,000 BTU/hr if the unit cools a kitchen. Rooms under 100 sq ft are given the smallest chart size, 5,000 BTU/hr.
Why not bigger
An oversized unit cools the air quickly and shuts off before it has removed much moisture, leaving the room cold and clammy, and the frequent cycling wastes energy. Pick the standard size closest to the result. Rooms with very high ceilings, large west-facing windows or poor insulation may need more, and whole-house or central systems should be sized with a full load calculation such as ACCA Manual J.
Units
Capacity is in BTU per hour. One ton of cooling is 12,000 BTU/hr, so an 18,000 BTU/hr unit is a 1.5-ton unit.
| Area to be cooled (sq ft) | Capacity (BTU/hr) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
How to use
Enter the room's length and width in feet, choose the sun exposure, the number of people who regularly use the room and whether it is a kitchen. The default 16 ft × 14 ft room is 224 sq ft, which the chart puts at 6,000 BTU/hr. Make it very sunny with four regular occupants and it becomes 6,000 × 1.10 + 2 × 600 = 7,800 BTU/hr, so an 8,000 BTU/hr unit fits.
Frequently asked questions
How many BTUs do I need for a 12x12 room?
A 12 × 12 room is 144 sq ft, which the ENERGY STAR chart puts at 5,000 BTU/hr (100 up to 150 sq ft). Add 10% if it is very sunny or take 10% off if it is heavily shaded.
How many square feet does a 10,000 BTU air conditioner cool?
On the ENERGY STAR chart, 10,000 BTU/hr matches 400 up to 450 sq ft, before adjustments for sun, people and kitchen use.
Is a bigger air conditioner better?
No. An oversized room unit short-cycles: it lowers the temperature fast but runs too briefly to pull moisture out of the air, so the room feels damp, and it uses more energy. Choose the size closest to the calculated capacity.
Does this work for central air or a whole house?
No. The chart is for room air conditioners up to 2,500 sq ft. Central systems and heat pumps should be sized with a Manual J load calculation, which accounts for insulation, windows, climate and duct losses.
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Sources & references
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