Understanding your result
The headline is the ASHRAE BTU figure to look for on the box, chosen from the sizes that exist (8,000, 10,000, 12,000, 13,000 and 14,000), with the real cooling the room needs in the sub-line. If the room needs more real cooling than the largest portable unit delivers, the headline says so instead of naming a size, and the notes point to the alternatives.
The table separates the three steps: the chart value for the room, the adjustments for sun, ceiling, people and a kitchen, and the conversion from real cooling (SACC) to the ASHRAE label through the hose factor. The row “common size that covers it” shows both figures for the recommended unit, so when you are in the shop comparing a unit’s two printed ratings you know which one has to be at least what.
For a room that has a suitable window, compare with the window AC size calculator: a window unit of the same real cooling will be smaller, cheaper and quieter. For a room that is humid as well as hot, the dew point and humidity calculator shows whether a portable unit, which dehumidifies less than a window unit, will leave it feeling sticky.
How we calculate this
The room’s need is worked out exactly as for a window unit, from the ENERGY STAR room air conditioner chart with its sun, occupant and kitchen adjustments and a correction for ceilings above 8 ft; that need is the real cooling the unit must deliver, which is what SACC measures. The hose factors are typical ratios of SACC to ASHRAE rating seen across the models tested under the 2017 DOE procedure: single-hose units deliver about half to 60% of their ASHRAE figure, dual-hose units about 60 to 70%. The chart itself is on the BTU per square foot table.
In metric mode the inputs are converted before the lookup and the cooling figures are shown in kilowatts, while the label sizes stay in BTU/hr because that is how US units are sold; the unit conversions page has the factors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Room cooling need | ENERGY STAR room AC chart with its adjustments | US EPA / DOE ENERGY STAR sizing guidance |
| Ceiling | +10% per foot above 8 ft | Air volume; the chart assumes 8 ft |
| SACC as a share of ASHRAE rating | 55% single hose, 65% dual hose | Typical ratios on units tested under the DOE 2017 portable air conditioner test procedure (10 CFR 430) |
| Common sizes | 8,000 to 14,000 BTU/hr ASHRAE | Portable units on the US market |
Assumptions last reviewed October 7, 2026.
The hose factors are averages; a particular model’s SACC is printed on its label and in the DOE compliance database and should be used in place of the estimate once you are choosing between specific units. The calculator does not account for a hose that is extended, kinked or uninsulated, which lowers delivered cooling further, nor for how well the window kit seals. The guide to window and portable AC sizing explains how to read both figures on the label.
Two worked examples
A 250 sq ft bedroom, single hose
A 250 sq ft bedroom with 8 ft ceilings, average sun, two people, no kitchen, and a single-hose unit.
- Chart value: 6,000 BTU/hr; no adjustments apply
- Real cooling needed (SACC): 6,000 BTU/hr
- ASHRAE label to look for: 6,000 ÷ 0.55 = 10,909 BTU/hr
- Unit: 12,000 BTU/hr ASHRAE, delivering about 6,600 BTU/hr SACC
The unit’s box will say 12,000; its real cooling is roughly what an 6,000 BTU window unit would give. A dual-hose unit of 10,000 BTU/hr ASHRAE would deliver about 6,500 SACC and do the same job while drawing less outdoor air into the room.
A sunny 400 sq ft living room, dual hose
A 400 sq ft living room with large west windows, 8 ft ceilings, two people, dual-hose unit.
- Chart value: 9,000 BTU/hr; sun × 1.1 = 9,900 BTU/hr
- Real cooling needed (SACC): 9,900 BTU/hr
- ASHRAE label to look for: 9,900 ÷ 0.65 = 15,231 BTU/hr
- Result: beyond a single portable unit; the largest, 14,000 BTU/hr ASHRAE, delivers about 9,100 SACC
The room sits just past the edge of what one portable unit can do. Options are a 14,000 BTU dual-hose unit and accepting that it will fall behind on the hottest afternoons, two smaller units, a 10,000 BTU window unit, or a 12k mini-split head, which would also be the quietest and cheapest to run.
Where to find your inputs
Floor area and ceiling. Length × width of the room; measure the ceiling if the house is older or the room is a loft, because anything above 8 ft adds to the load.
Sun exposure. West- and south-facing glass without shade is “very sunny”; north-facing or shaded by trees or a porch is “heavily shaded”.
Hose type. Count the hoses on the unit you are considering. One hose means the unit uses room air to cool its condenser and exhausts it; two means it draws condenser air from outside.
SACC on a specific unit. Printed on the EnergyGuide label and the box, usually in smaller type under the ASHRAE figure, and listed in the manufacturer’s specifications as “DOE” or “SACC” BTU.
Common mistakes
- Sizing by the ASHRAE number. The big number on the box is 40 to 50% more than the cooling you get. Size by SACC, which this calculator does.
- Comparing a portable with a window unit by BTU. A 12,000 BTU portable delivers what an 6,000 to 8,000 BTU window unit delivers.
- Extending the hose. A longer hose radiates more heat into the room and cuts airflow; keep it short, straight and insulated.
- Leaving the window kit loose. Gaps around the kit let hot air straight in; seal them with foam tape.
- Running a single-hose unit in a very hot climate. The hotter it is outside, the hotter the air it pulls in to replace the exhaust. Dual hose matters most where it is hottest.
- Ignoring the condensate. Most units evaporate condensate out through the exhaust, but in humid rooms the tank fills and the unit stops; check for a drain hose option if the room is damp.
Questions people ask
- What is SACC on a portable air conditioner?
- Seasonally adjusted cooling capacity, the US Department of Energy test rating introduced in 2017. It measures the net cooling a portable unit delivers to the room after subtracting the heat that leaks from its hot exhaust duct and the warm outdoor air that a single-hose unit pulls in through gaps around doors and windows to replace the air it exhausts. A unit labelled 14,000 BTU/hr ASHRAE typically has a SACC of 8,000 to 10,000 BTU/hr. SACC is the figure to size by.
- What size portable AC do I need for a 250 square foot room?
- A 250 sq ft room needs about 6,000 BTU/hr of real cooling by the ENERGY STAR chart. A single-hose portable unit delivers roughly 55% of its ASHRAE rating as SACC, so look for an ASHRAE label of about 11,000 BTU/hr, which in practice means a 12,000 BTU/hr unit with a SACC near 6,500. A dual-hose unit of 10,000 BTU/hr ASHRAE does the same job.
- Is a dual-hose portable AC better than single-hose?
- For cooling, yes. A single-hose unit blows room air out through its exhaust hose, which lowers the pressure in the room and draws hot outdoor air in through every gap; a dual-hose unit takes its condenser air from outside through the second hose and sends it back out, so the room air stays in the room. Dual-hose units typically deliver 65% of their ASHRAE rating as SACC against 55% for single-hose, and they do better still on very hot days.
- Why does my 12,000 BTU portable AC not cool like a 12,000 BTU window unit?
- Because it is not one. A window unit's condenser hangs outside, so the heat it removes goes outside. A portable unit's condenser is inside the room, and only the hot air leaving through the hose carries heat away; the hose itself radiates some of it back, and the air the room loses is replaced by warm air from outside. A 12,000 BTU ASHRAE portable delivers 6,000 to 8,000 BTU of real cooling, which is what an 8,000 BTU window unit delivers.
- How big a room can a portable air conditioner cool?
- The largest common portable units are rated about 14,000 BTU/hr ASHRAE, which is 8,000 to 10,000 BTU/hr SACC, enough for a room of about 350 to 450 sq ft with average sun, less for a sunny room or a kitchen. Above that, two units, a window unit or a mini-split are the realistic options; a single portable will run continuously and still fall behind on hot afternoons.
- Does the hose length matter?
- Yes, in two ways. A longer hose has more surface area to radiate heat back into the room, so keep it as short and straight as the window kit allows and insulate it if the kit does not include a sleeve. Extending the hose beyond the supplied length also increases the back pressure on the fan and reduces airflow. Most manufacturers void the warranty if the hose is extended.