How many BTUs do I need for a 12x12 room?

A 12x12 room is 144 sq ft, which falls in ENERGY STAR's 100–150 sq ft bracket — about 5,000 BTU as a baseline. If the room gets strong afternoon sun, add 10% (roughly 5,500 BTU); if more than two people regularly use it, add 600 BTU per extra person. For a portable air conditioner, buy on the SACC number, not the ASHRAE number, so a unit rated 5,000–6,000 BTU SACC (often labeled 8,000–10,000 BTU ASHRAE on the box) is the right size.

Is it better to oversize or undersize a portable air conditioner?

Undersize slightly rather than oversize. An oversized AC cools the air fast, hits the thermostat setpoint, and shuts off before it has run long enough to pull humidity out of the room — a problem called short-cycling that leaves the space feeling cold and clammy at the same time. A correctly sized or even slightly small unit runs longer cycles, dehumidifies properly, and is usually quieter and cheaper to run than an oversized one that's constantly starting and stopping.

Should I size a portable AC using the SACC number or the ASHRAE number?

Always size on SACC (Seasonally Adjusted Cooling Capacity). It's the U.S. Department of Energy's real-world rating — the number left over after accounting for heat that leaks back in through the exhaust hose — and every portable AC sold in the U.S. has carried it since a 2024 DOE rule change. ASHRAE is the older lab number and is always higher, sometimes by 30% or more on a single-hose unit. Match SACC to the ENERGY STAR square-footage table directly; don't add anything back for the hose.

How many BTUs per square foot does a garage or sunroom need?

More than a bedroom. The 20-BTU-per-square-foot baseline assumes a normally insulated room with average sun exposure. A garage, sunroom, or attic space is usually poorly insulated and often gets more direct sun, so both ENERGY STAR adjustments apply at once: add 10% for heavy sun, and mentally treat the space as one size bracket larger than its square footage suggests. See our best portable AC for a garage guide for picks built for that gap, or our sunroom AC picks for the glass-specific version of this correction.

Do ceiling height and open floor plans change the BTU math?

Yes. The ENERGY STAR table assumes a standard 8-foot ceiling and a closed room. A vaulted ceiling or an open-concept space has more air volume than its floor area implies, so a straight square-footage lookup will undersize the unit. As a rule of thumb, if your ceiling is 9–10 feet or the space opens into another room, size up one bracket on the chart — for example, treat a 350 sq ft open-plan living area like a 400–450 sq ft closed room.

What size portable AC do I need for a 500 sq ft apartment?

500 sq ft sits in ENERGY STAR's 450–550 sq ft bracket, which calls for about 12,000 BTU as a baseline — and because that's the real-world number you're sizing to, buy a portable rated 12,000 BTU SACC. Very few portables hit that on SACC alone; the closest is a dual-hose 14,000 ASHRAE unit like the Midea Duo , which retains about 12,000 SACC. A single-hose 14,000 ASHRAE unit typically only delivers 9,000–10,000 SACC — check our best portable air conditioner guide for the full room-size breakdown.

Portable AC BTU Guide 2026: What Size Air Conditioner Do You Need?

Quick Answer: Size a portable air conditioner using ENERGY STAR’s baseline of ~20 BTU per square foot (roughly 5,000 BTU for a 150 sq ft bedroom up to 14,000 BTU for a 700 sq ft open room), then adjust for sun (+10%), shade (−10%), kitchens (+4,000 BTU), and extra occupants (+600 BTU each). The one correction unique to portables: buy on the SACC number, not the ASHRAE number — SACC is the U.S. Department of Energy’s real-world rating, and it’s typically 25–35% lower than the inflated ASHRAE figure printed larger on the box. Match SACC directly to the square-footage chart below; don’t add anything back for hose losses, because SACC has already subtracted them.

Last updated 2026-07-28: added a below-grade worked example — a basement’s BTU math differs from a same-size room upstairs because the space starts cooler but the humidity target matters more, per EPA indoor air quality guidance.

Every portable AC box shows two BTU numbers, and buyers routinely size off the wrong one. Here’s the actual math ENERGY STAR and the DOE use, in a chart you can read in ten seconds, plus three worked examples so you can see it applied.

The BTU-per-square-foot chart

Room size	BTU needed (baseline)
100–150 sq ft	5,000 BTU
150–250 sq ft	6,000 BTU
250–300 sq ft	7,000 BTU
300–350 sq ft	8,000 BTU
350–400 sq ft	9,000 BTU
400–450 sq ft	10,000 BTU
450–550 sq ft	12,000 BTU
550–700 sq ft	14,000 BTU
700–1,000 sq ft	18,000 BTU

That table is ENERGY STAR’s own room-sizing guidance, built around the ~20-BTU-per-square-foot rule (a 147.5 sq ft room, in ENERGY STAR’s own example, needs 5,000 BTU). Four adjustments sit on top of the base number, straight from the DOE/ENERGY STAR sizing guide:

Heavy shade: subtract 10%.
Very sunny room (west- or south-facing windows, direct afternoon sun): add 10%.
Kitchen or space adjacent to one: add 4,000 BTU flat.
More than two regular occupants: add 600 BTU per additional person.

Round the final number down to the nearest thousand — that’s the capacity to shop for. Shopping for a window unit instead, where the box BTU rating is the real number (no SACC correction needed)? See our 5,000 BTU or 6,000 BTU window AC picks for the two smallest room sizes on this chart.

The correction that’s unique to portable ACs: use SACC, not ASHRAE

Window units have sold on one honest number for decades. Portables didn’t — until a 2024 DOE rule required every portable air conditioner sold in the U.S. to carry a second rating called SACC (Seasonally Adjusted Cooling Capacity), which accounts for the heat that leaks back into the room through the exhaust hose and hose connection. SACC is the real-world number. ASHRAE, still printed larger on most boxes, is the older lab test and is always higher — often by 25–35%, and sometimes worse on single-hose designs.

This isn’t a rounding error. Across the models we’ve tested and verified in our brand roundups, retained SACC as a share of ASHRAE ranges from the mid-60s to mid-80s percent depending entirely on hose design:

Model	ASHRAE	SACC	Retained
Midea Duo (dual-hose)	14,000 BTU	12,000 BTU	~86%
LG LP1419IVSM (single-hose)	14,000 BTU	10,000 BTU	~71%
Whynter ARC-14S (single-hose)	14,000 BTU	9,500 BTU	~68%
De'Longhi EL290HL (single-hose)	14,000 BTU	8,700 BTU	~62%

The takeaway: match SACC directly to the square-footage chart above. Don’t add anything back to compensate for the hose — SACC has already done that subtraction for you. A room that calls for 10,000 BTU needs a portable rated 10,000 BTU SACC, which might be labeled anywhere from 14,000 to 16,000 BTU ASHRAE on the front of the box depending on the model. Our dual-hose vs single-hose comparison covers why that gap exists.

Window units mostly sidestep this problem — there’s no hose to leak through, so the printed BTU rating is close to real-world delivered cooling — which is one reason window ACs remain the more efficient choice per BTU when a window is available. See our best window air conditioner guide.

Three worked examples

1. A 150 sq ft bedroom, no direct sun. Baseline from the chart: 6,000 BTU. No sun adjustment, no kitchen, two occupants — the baseline stands. Shop for a portable rated 6,000 BTU SACC, which typically shows up as an 8,000–10,000 BTU ASHRAE unit on the shelf. See our best portable AC for a bedroom picks — noise matters as much as capacity in a room you sleep in.

2. A 400 sq ft living room with west-facing windows. Baseline: 9,000 BTU (350–400 sq ft bracket). Add 10% for heavy afternoon sun: 9,900 BTU, round up to 10,000 BTU SACC. That’s exactly the LG LP1419IVSM’s rating, or a Midea U 12,000 window unit if you’d rather not deal with a hose at all — both covered in our LG air conditioner roundup and Midea air conditioner roundup. A dual-hose 12,000 BTU ASHRAE portable, like the Midea Duo 12K, gets close too, at 10,000 BTU SACC — see our best 12,000 BTU portable air conditioner comparison for how it stacks up against single-hose alternatives that only deliver 8,000 BTU SACC at the same ASHRAE rating.

3. A 700 sq ft garage used as a workshop. Baseline: 14,000 BTU (550–700 sq ft bracket). Garages run hotter and leakier than the ENERGY STAR baseline assumes — poor insulation, a big door, often direct sun on one wall — so treat it as the next bracket up: 18,000 BTU SACC territory, which is larger than almost any single portable AC on the market delivers. At this size, most buyers either run two mid-size units, choose a window unit if there’s a wall opening, or in dry climates only, an evaporative cooler. Our best portable AC for a garage guide and evaporative cooler guide cover both paths.

4. A 400 sq ft finished basement. Baseline from the chart: 9,000–10,000 BTU (350–450 sq ft bracket) — but a basement usually needs less than that baseline suggests, because below-grade space stays several degrees cooler than upstairs on its own. Size down slightly and lean on dehumidify-only mode instead of chasing the full BTU number: the EPA recommends keeping indoor relative humidity between 30 and 50 percent, and a basement’s cold concrete surfaces push effective humidity higher than a room-air reading shows, so hitting that target matters more than raw cooling capacity down there. Our best portable AC for a basement guide covers the units built around that trade-off.

Sizing quick-reference by use case

Space	Typical BTU (SACC) target	Where to shop
Small bedroom / home office (≤250 sq ft)	5,000–6,000	Small room picks
Large bedroom / mid-size living room (300–450 sq ft)	8,000–10,000	Portable AC pillar
Open living room / primary suite (450–700 sq ft)	12,000–14,000	Large room picks (dual-hose, 86% retention)
Garage, workshop	Size one bracket up	Garage picks
Sunroom, glass-heavy room	Size one bracket up, more if west-facing	Sunroom AC picks
Attic room or conversion	Size one bracket up, +10% sun — check the AC's rated operating temperature too	Attic AC picks
Basement / below-grade room	Size one bracket down; prioritize dehumidify mode	Basement picks
RV, camper, off-grid	Power draw > BTU is the constraint	RV & battery picks
Van, truck cab, hardwired DC build	Amp draw > BTU is the constraint	12V air conditioner picks
Cooling and heating from one unit	Match cooling BTU as above	Heat pump portables

The bottom line

Size a portable air conditioner the same way ENERGY STAR sizes a window unit — about 20 BTU per square foot, adjusted for sun, shade, kitchens, and occupancy — then apply the one portable-specific correction: shop on SACC, not ASHRAE. A room that needs 10,000 BTU needs a 10,000 BTU SACC portable, full stop, regardless of what larger number is printed on the front of the box. Get the room size right and the SACC number right, and you’ll skip both the short-cycling of an oversized unit and the all-day-running of an undersized one. Already seeing a unit click on and off every few minutes instead of running a full cycle? Our short cycling guide covers that specific symptom and the sizing fix behind it. If a unit you already own seems to be struggling despite correct sizing, our not-cooling troubleshooting guide covers the other six causes.

Ready to shop? Check portable air conditioners on Amazon — filter by SACC rating where it’s listed, not the bigger ASHRAE number on the title.

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