🧳What size portable air conditioner does this room need?

✓ Tested & verified Updated: Aug 8, 2026 How we calculate this

Portable AC size calculator

Room length (ft)

Room width (ft)

Ceiling height (ft)

Sun on this room

Insulation

People in the room

5,000 BTU/h≈ 1.5 kW

The room works out at 4,680 BTU/h — 5,000 is the nearest size actually sold.

Floor area: 180 sq ft
In tons of refrigeration: 0.4
Working rate: 26 BTU/h per sq ft

This tool works in SACC — the rating on boxes sold in the US since 2017. An old ASHRAE figure is roughly SACC ÷ 0.7, so 10,000 SACC ≈ 14,000 ASHRAE.

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What size portable air conditioner do you need?

Allow about 30% more than you would for a window unit — roughly 26 BTU per hour per square foot. A 15 × 12 ft room is 180 sq ft: a window unit would take 3,600 BTU/h, a portable wants about 4,700 and buys the 5,000.

The penalty is physical, not marketing. A portable stands inside the room it is cooling. Its condenser dumps heat into a hose that runs hot along the floor and radiates part of it straight back, and a single-hose model takes the air it needs for that condenser from the room, which pulls unconditioned outside air in through every gap in the building to replace it. Both effects work against the machine.

Since 2017, US labels have carried SACC — a seasonally adjusted capacity that measures the unit as installed, infiltration included. It is typically 60–70% of the old ASHRAE number, so a portable that once said 14,000 now says 10,000. Same machine, honest figure.

Portables are the one product on this site where the number printed on the box and the cooling that arrives in the room are different quantities. Since 2017 the US label has been honest about it, but shops, listings and older reviews still quote the old figure — often both, in the same listing, without saying which is which.

On the box (SACC)	Roughly, in old ASHRAE terms	Room it actually suits
6,000 BTU/h	~8,500 BTU/h	up to 230 sq ft
8,000 BTU/h	~11,500 BTU/h	up to 300 sq ft
10,000 BTU/h	~14,000 BTU/h	up to 385 sq ft
12,000 BTU/h	~17,000 BTU/h	up to 460 sq ft
14,000 BTU/h	~20,000 BTU/h	up to 540 sq ft

If a listing quotes a big BTU figure with no mention of SACC, assume it is the middle column. A “14,000 BTU” portable in that sense is a 10,000 machine, and it will disappoint anyone who sized their room against a window-unit chart.

Frequently asked questions

What is the difference between SACC and ASHRAE BTU?

ASHRAE measured the cooling produced at the evaporator, ignoring the hot exhaust hose and the air a single-hose unit pulls out of the room. SACC — Seasonally Adjusted Cooling Capacity, required on US labels since 2017 — measures the unit as it is actually used, including infiltration, and averages two test conditions. SACC is typically 60 to 70% of the ASHRAE figure for the same machine.

Is a dual-hose portable better?

Meaningfully, yes. A second hose feeds the condenser with outside air rather than room air, so the unit is not pumping conditioned air out of the house and pulling hot air in through the gaps. Dual-hose models score noticeably higher on the same SACC test, and the difference grows the hotter it gets outside.

Why does my portable AC not cool the room?

Usually three things at once: the unit is sized on an ASHRAE number rather than a SACC one, the exhaust hose is long, uninsulated and coiled so it radiates heat back into the room, and the window bracket leaks. Shorten and straighten the hose, insulate it, and seal the window kit properly before concluding you need a bigger machine.

Should I buy a portable or a window unit?

A window unit if the window can take one — it is cheaper, quieter, more efficient and its rating is honest. A portable is for windows that do not open the right way, rented flats where nothing may be fitted, casement windows and rooms with no exterior window at all. Buying a portable when a window unit would fit is paying more for less cooling.

Do portables need draining?

Most modern ones evaporate condensate out through the exhaust hose and only need draining in very humid weather or when used as a dehumidifier. There is always a drain plug for that case. A unit that fills its tank daily in ordinary weather is usually oversized for the room and cycling too much.

How much room does the hose need behind the unit?

Enough that the hose runs straight and short — a metre or so of gentle rise to the window is ideal. Every extra foot of hose is extra surface radiating heat back into the room, and every coil is worse than a bend. Some manufacturers void performance claims above a stated hose length for exactly this reason.

How we calculate this

Portable sizing starts from the same 20 BTU per hour per square foot base and applies a 30% allowance — an effective 26 BTU/h per square foot — for the single-hose exhaust and infiltration penalty, so the figure produced is comparable with the SACC rating that has appeared on US labels since the Department of Energy’s 2017 test procedure. Ceiling height, sun, insulation and occupancy corrections are the standard ones. The result rounds up to the capacities portables are sold in. Occupancy uses +600 BTU/h per person past two; kitchen loading is left out because a portable is rarely the right machine for a kitchen. The SACC-to-ASHRAE ratio quoted is the 60–70% range documented in the DOE test procedure.

Worked example

With Room length (ft) 15, Room width (ft) 12, Ceiling height (ft) 8, Sun on this room Average light, Insulation Average — typical home, People in the room 2, this page works out 5,000 BTU/h ≈ 1.5 kW. The room works out at 4,680 BTU/h — 5,000 is the nearest size actually sold.

Floor area180 sq ft
In tons of refrigeration0.4
Working rate26 BTU/h per sq ft

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