What Size Portable AC Do I Actually Need?
Size by the manufacturer's SACC-based square-footage figure, not the advertised ASHRAE number — and assume a standard ~8-foot ceiling. If your room runs taller, gets direct sun, has large windows, or opens into another room, size up one tier from what the SACC coverage figure suggests.
The one thing to know: Room size guidance for portable ACs should be built on the SACC-based square-footage figure the maker discloses (not the ASHRAE-based number, which will overstate coverage), and it assumes a standard ~8-foot ceiling with normal insulation and shading. Higher ceilings, large windows, direct sun, an open floor plan, or a kitchen in the same space all reduce real coverage below the advertised SACC figure — when in doubt, size up one tier rather than down.
The sizing mistake almost everyone makes
Buyers compare their room's square footage against the big number on the box — the ASHRAE-rated BTU figure — and end up undersized. That number doesn't reflect the heat leaking back in through the exhaust hose or the outside air infiltrating to replace what's vented out. The manufacturer's own SACC-based coverage claim (where disclosed) already builds those losses in, which is why it's the more honest number to size against.
What changes the real coverage
- Ceiling height: coverage figures generally assume a standard ~8 ft ceiling; a taller room has more air volume per square foot, so real coverage drops.
- Sun exposure and window area: a west-facing room with large windows gains more heat than the room type these figures are typically modeled on.
- Hose configuration: single-hose units lose more real-world capacity to infiltration than dual-hose designs — see Single-Hose vs Dual-Hose.
- Open floor plans: a portable AC cooling an open-concept space is effectively cooling a bigger, harder-to-seal volume than the coverage figure assumes.
A practical rule of thumb
Compare your room's actual square footage against the SACC-based coverage figure in our SACC Reality Index, and if any of the factors above apply to your room, size up to the next model tier rather than down. It costs less to run a slightly larger unit at part-capacity than to run an undersized one at full tilt and still not reach your target temperature.
This page is general sizing guidance based on manufacturer-disclosed figures, not a room-specific load calculation.
Frequently Asked Questions
Size by the manufacturer's SACC-based square-footage figure, not the ASHRAE-based number — the SACC figure is designed to reflect real-world performance in a room with an exhaust hose installed. If your room has a ceiling higher than the standard ~8 feet, large windows, direct sun, or an open floor plan, size up one tier from what the SACC coverage figure suggests.
We found this ourselves comparing retailer listings — the same model number sometimes shows different coverage claims across different retailers, and the basis for a specific sq ft figure (ceiling height, insulation level, climate assumption) is rarely stated. Treat any single sq ft number as an estimate, not a guarantee.
You need to compare rooms against the SACC-based coverage, not the bigger ASHRAE-based BTU number. Comparing your room size to the advertised (ASHRAE) BTU figure will lead you to undersize, since that number doesn't reflect duct and infiltration losses.
Most manufacturer coverage claims we reviewed do not state a ceiling-height assumption explicitly, but the sizing methodology behind SACC-based guidance (and AHAM room-sizing conventions generally) assumes a standard ~8-foot ceiling. A taller ceiling means more total air volume to cool for the same floor area, so real coverage will be lower than the stated figure.