About Window AC BTU Calculator (ENERGY STAR Room Sizing)
The window AC BTU calculator sizes a room air conditioner from the published ENERGY STAR sizing table: the room area in square feet maps to a cooling capacity in BTU per hour, from 5,000 BTU/h at 100–150 ft² up to 34,000 BTU/h at 2,000–2,500 ft². Enter the area directly or as length × width, and the tool reads the table row for the displayed whole-square-foot area, so the printed area and the chosen row never disagree.
The base capacity is then adjusted per the same ENERGY STAR guidance, in the published order: reduce by 10% for a heavily shaded room or increase by 10% for a very sunny one, add 600 BTU/h for each person beyond two who regularly occupies the room, and add 4,000 BTU/h when the unit serves a kitchen. The adjusted figure is bracketed against the common retail unit sizes (5,000 through 24,000 BTU/h) so you can see the closest stock units above and below — with an honest note that rounding far up costs comfort: an oversized unit short-cycles and dehumidifies poorly.
How It Works
- Enter the room area in square feet directly, or switch to length × width and enter the room dimensions in feet — each mode keeps its own values.
- Pick the sun exposure: average, very sunny (+10% of the table capacity), or heavily shaded (−10%). This mirrors the two percentage rows of the ENERGY STAR guidance.
- Enter how many people regularly occupy the room (the table assumes two; each extra person adds 600 BTU/h) and tick the kitchen box if the unit will cool a kitchen (+4,000 BTU/h).
- The tool looks up the table row for the displayed whole-square-foot area (each published boundary starts the higher row: 150 ft² is the 6,000 row), applies the adjustments in the published order, and shows a line-by-line breakdown from the base capacity to the adjusted total.
- The adjusted BTU/h is bracketed against common retail unit sizes so you see the closest stock size at or below and at or above the target. Rooms under 100 ft² fall below the published table — the tool recommends the smallest common unit and warns it is likely oversized. Areas above 2,500 ft² are beyond a room unit entirely; run a full load calculation (hvac-load) instead.
Worked Example
A 20 × 17 ft living room is 340 ft², which displays as 340 and lands in the 300–350 ft² row of the ENERGY STAR table: 8,000 BTU/h base. With average sun exposure, two occupants, and no kitchen there are no adjustments, so the adjusted capacity is also 8,000 BTU/h — an exact hit on a standard retail size, so the closest stock unit above and below are both the 8,000 BTU/h unit. If the same room were very sunny the capacity would rise 10% of the base to 8,800 BTU/h, bracketed by the 8,000 and 10,000 BTU/h stock sizes.
Formulas
- Base capacity from the ENERGY STAR table
base = TABLE[row(A_displayed)]- Adjusted capacity (published order: sun, occupants, kitchen)
adjusted = base × f_sun + 600 × max(0, n − 2) + 4000 × kitchen- Closest standard sizes (retail ladder lookup)
below = max{ s ≤ adjusted }, above = min{ s ≥ adjusted }
Standards & References
- ENERGY STAR room air conditioner sizing table — energystar.gov "Room Air Conditioners" (verified 2026-08-05)
- ENERGY STAR "How To Choose the Right Sized Window AC" — published adjustments and order (verified 2026-08-05)
- Retail unit-size ladder — representative common stock sizes, not a published standard
Frequently Asked Questions
How many BTUs do I need per square foot?
The ENERGY STAR table works out to roughly 20–34 BTU/h per square foot, but it is not a flat rate — smaller rooms need proportionally more per square foot (5,000 BTU/h for 100–150 ft²) and large rooms less (34,000 BTU/h for 2,000–2,500 ft²). Use the table row for your room area rather than a per-square-foot rule of thumb, then apply the sun, occupancy, and kitchen adjustments.
Why is an oversized air conditioner a problem?
An air conditioner removes both heat and humidity, but a unit that is too large cools the air quickly and shuts off before it has run long enough to wring the moisture out. The result is a room that is cold but damp and clammy, plus more compressor start-stop cycles, which wastes energy and wears the unit. ENERGY STAR is explicit that bigger is not better — size to the room, and when in doubt between two sizes in a humid climate, favor the smaller one.
Why do sun exposure, occupants, and a kitchen change the size?
These are the published ENERGY STAR adjustments, and each reflects a real heat load: a very sunny room gains about 10% more solar heat (a heavily shaded one about 10% less), each person beyond two adds roughly 600 BTU/h of body heat, and cooking appliances in a kitchen add about 4,000 BTU/h. The percentages apply to the table capacity, and the occupant and kitchen additions stack on top of that.
What if my adjusted BTU number falls between standard unit sizes?
Window units come in steps (5,000, 6,000, 8,000, 10,000 BTU/h and so on), so the calculator shows the closest stock size on each side of your adjusted number. ENERGY STAR’s own tip is to round DOWN to the nearest thousand rather than up, because modest undersizing costs a little pull-down speed while oversizing costs dehumidification. In a very humid climate the size below is usually the better pick; in a hot, dry one the size above is reasonable.
Does this calculator work for portable air conditioners?
Not directly. Portable units are rated under a different convention (the DOE’s SACC — seasonally adjusted cooling capacity — accounts for the heat their exhaust duct leaks back into the room), so a portable’s headline ASHRAE BTU number overstates its delivered cooling compared with a window unit of the same rating. The room-load side of this tool still applies, but compare portables by their SACC rating, not the ASHRAE number.
Can I use a window unit to cool my whole house?
The ENERGY STAR room-unit table ends at 2,500 ft², and well before that point a single window unit struggles: it cools the room it sits in and leaks capacity through open doorways. Whole-house or multi-room cooling needs a real load calculation that accounts for insulation, windows, orientation, and infiltration — use the hvac-load tool for a Manual-J-style sensible/latent load and equipment tonnage instead of scaling up window units.