Air Conditioner BTU Calculator
Enter the room, answer four questions about it, and the calculator applies the ENERGY STAR room-air-conditioner sizing table with ENERGY STAR's own adjustments for sun, people, and kitchens. Two further adjustments (ceiling height and top-floor rooms) are common HVAC-calculator conventions and are labelled as such. If you're shopping for a portable AC, switch the unit type and you'll get two numbers, because portable ACs carry two ratings and the difference is large enough to buy the wrong unit.
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Data last updated: 2026-08-24
How this calculator works
ENERGY STAR publishes a table of cooling capacity by floor area — 5,000 BTU/hr for 100–150 sq ft, up through 34,000 for 2,000–2,500 — with four rules attached: cut 10 % for a heavily shaded room, add 10 % for a very sunny one, add 600 BTU for each person beyond two who regularly occupies the room, and add 4,000 BTU if the unit is in a kitchen. That table is the spine of the calculator. DOE's rougher rule of thumb is 20 BTU per square foot; the two agree up to about 700 sq ft and then the table flattens, which is why the calculator uses the table.
The table assumes ordinary ceilings. For taller rooms the calculator scales by height ÷ 8 ft — a volumetric convention most HVAC calculators use, not something ENERGY STAR publishes — and it adds 10 % for a top-floor room under an unconditioned attic, likewise a convention. Both are shown in the result so you can ignore them if you'd rather.
DOE (SACC) vs ASHRAE: the portable-AC problem
A window unit has one BTU number. A portable unit has two, and they can differ by 40 %.
The older figure is the ASHRAE rating (ASHRAE 128 / AHAM PAC-1), measured with the unit cooling a room under ideal conditions — the same basis as a window AC. The newer figure is the DOE rating, formally Seasonally Adjusted Cooling Capacity (SACC), from a test procedure DOE finalized in 2016 (81 FR 35242; 10 CFR 430 Appendix CC). The DOE test does two things the ASHRAE test doesn't: it subtracts the heat the exhaust hose radiates back into the room, and it subtracts the heat carried in by the outdoor air that gets pulled into the room to replace what a single-hose unit blows out the window. Then it weights the result 80/20 across 83 °F and 95 °F outdoor conditions. The unit didn't get weaker; the test stopped ignoring where the heat goes.
The gap depends on the design. Sourced examples: a Whynter ARC-14S dual-hose is 14,000 ASHRAE / 9,500 SACC (32 % lower); a single-hose BLACK+DECKER 14,000 ASHRAE unit is 10,000 SACC in one version and 7,700 in another (29–45 % lower); a Midea 14,000 ASHRAE single-hose lists 10,000 SACC. Retail listings now show both; the DOE figure is the one the federal standard regulates.
How to shop with the result. The calculator's BTU figure is a room load, comparable to the window-AC table. For a portable, match it to the DOE/SACC rating, not the ASHRAE one. The calculator also shows an approximate ASHRAE equivalent (result ÷ 0.65) so you can recognize the box — treat that as a rough guide, and lean toward dual-hose units if the room is on the edge.
Reading the result
- BTU needed — after the table and adjustments, rounded to the nearest 500.
- Suggested unit size — the nearest common retail size (5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 15,000 / 18,000 / 24,000). Go up one size only if the room is at the top of its band and very sunny; DOE notes an oversized unit "will cool the room too quickly without dehumidifying it properly," which is the clammy-room complaint.
- Estimated running watts — BTU ÷ a typical EER (10 for window, 8.5 for portable on the ASHRAE basis, 12 for a mini-split). It's a planning number; the nameplate on a specific model is the real one. The link hands it to the electricity cost calculator with your state's rate.
Things the calculator can't know
- Insulation and windows. The ENERGY STAR table assumes a typical room. A 1920s sunroom with single-pane glass on three sides needs more than the table says; a new build with low-E windows needs less.
- Open floor plans. A unit in a room that opens to a hallway or a loft is cooling the volume it can reach, not the room you measured. Measure the connected space or close the door.
- Climate. The table doesn't vary by region. In Phoenix, take the "very sunny" adjustment even for a room that isn't; in Seattle, a shaded room really is a shaded room.
- Existing units that don't cool. If you already own the right size and the room is still warm, the problem is the unit, not the number — see window AC not cooling.
Embed this calculator
The badge under the tool links here and can be dropped into any page. Please cite the methodology page if you use the numbers.
Parts and supplies mentioned on this page
For measuring, mounting, and sealing.
- Laser distance measureCheck price on Amazon
- Window AC support bracket (no-drill)Check price on Amazon
- Portable AC window seal kitCheck price on Amazon
Frequently asked questions
How many BTU do I need per square foot?
DOE's rule of thumb is about 20 BTU per square foot. ENERGY STAR's table is more precise: 5,000 BTU for 100–150 sq ft, 8,000 for 300–350, 12,000 for 450–550, 18,000 for 700–1,000, 24,000 for 1,400–1,500 — then adjust for sun (±10 %), people (+600 each beyond two), and kitchens (+4,000).
How many BTU for a 500 sq ft room?
ENERGY STAR's band for 450–550 sq ft is 12,000 BTU. Add 10 % if the room is very sunny (13,200 → a 14,000 unit), subtract 10 % if it's heavily shaded (10,800 → a 10,000 or 12,000).
Why does my portable AC have two BTU ratings?
Portable ACs now carry an ASHRAE rating (the older, ideal-conditions test) and a DOE/SACC rating (a 2016 federal test that subtracts hose heat and infiltration). The DOE number is typically 25–45 % lower and is the one to compare against a room's needs.
Is a bigger air conditioner better?
No. An oversized unit hits the set point fast, shuts off before it has dehumidified, and leaves the room cool but damp (DOE). Size to the table and go up one size only for a sunny room at the top of its band.
How many watts does a 12,000 BTU window AC use?
Roughly 1,000–1,200 W running — 12,000 BTU ÷ an EER of 10–12. Check the nameplate; efficient units with a CEER above 12 run under 1,000 W.
Does ceiling height change the BTU I need?
ENERGY STAR's table is stated in floor area only. Most HVAC calculators scale by height (this one uses height ÷ 8 ft) because a 10-foot ceiling means 25 % more air to cool. The calculator shows the adjustment separately so you can see its effect.