To size a portable air conditioner, match the BTU number on the box to your room’s square footage. As a rough guide, about 5,000 to 8,000 BTU suits rooms up to around 350 square feet, and bigger rooms need more. Sun, high ceilings, kitchens and crowded rooms push you toward a larger unit.
The catch is that BTU numbers on the box can be misleading, so here’s how to read them and pick the right size.
What BTU means for cooling
BTU stands for British thermal unit, a measure of how much heat the air conditioner can remove from a room in an hour. A higher number means more cooling power. Since a portable unit has to keep up with heat leaking in from sun, walls and people, the right BTU depends on the room, not just its size.
Bigger isn’t always better, though. An oversized unit can cool the air quickly and shut off before it removes much humidity, leaving the room cold but clammy. A well-matched unit runs longer and dries the air better.
BTU by room size
These are common ballpark ranges for the older ASHRAE-style number printed on most boxes. Your manufacturer’s chart may differ slightly:
| Room size | Approximate BTU |
|---|---|
| Up to 250 sq ft | About 5,000 to 6,000 |
| 250 to 350 sq ft | About 7,000 to 8,000 |
| 350 to 450 sq ft | About 9,000 to 10,000 |
| 450 to 550 sq ft | About 12,000 |
| 550 to 700 sq ft | About 14,000 or more |
For a tiny bedroom or home office, check our picks for the best portable air conditioners for small spaces.
SACC vs ASHRAE: the rating that matters
You’ll often see two BTU numbers on the same product, and the difference is important. The older ASHRAE rating is a lab figure that doesn’t fully account for the heat a portable unit pulls back into the room. It tends to look bigger.
The newer SACC (Seasonally Adjusted Cooling Capacity) is a Department of Energy measure that’s lower, but more realistic. As a rule of thumb, SACC runs about 60 to 70 percent of the ASHRAE number, so a unit labeled 10,000 BTU may be around 6,000 to 7,000 SACC. Compare models by SACC when it’s listed. To use the chart above with a SACC number, divide it by 0.65 first. If only ASHRAE is shown, expect real cooling to be noticeably lower.
Adjust for sun, ceilings and heat sources
The charts above assume an average room. Adjust upward in these cases:
- Very sunny rooms: add roughly 10 percent to the capacity.
- Kitchens: add about 4,000 BTU for the heat from cooking and appliances.
- More than two people regularly: add a bit of capacity for each extra person.
- High ceilings or open layouts: a room with tall ceilings has more air to cool, so size up.
Shaded rooms, on the other hand, may get by with a little less. Always weigh your own conditions, since a top-floor room with big west-facing windows behaves very differently from a shaded basement room.
Apartments and other special cases
Renters have to think about windows, since the exhaust hose usually runs through a window kit. Measure your window and look at the kit’s range. Single-hose units can pull warm air into the room, while dual-hose models handle that better. Our roundup of portable air conditioners for apartments covers the setups that work well in rentals.
For open-plan living rooms or big bedrooms, you’ll need more power, and our list of portable air conditioners for large rooms shows the higher-capacity options. If a single unit still can’t keep up, two smaller ones in separate zones sometimes works better than one huge one.
Other things that affect real cooling
Even the perfect BTU won’t help if heat keeps getting in. For safety, plug the unit straight into a wall outlet, because extension cords and power strips can overheat. Close curtains on sunny windows, seal the gaps around the window kit and keep the exhaust hose short and straight. Empty the condensate tank or use a drain if your unit has one, and clean the filter regularly so airflow stays strong (our guide on how often to change air purifier and HVAC filters covers the general routine). Doors and windows left open work against you.
Frequently asked questions
How many BTU do I need for a 300 square foot room?
Roughly 7,000 to 8,000 BTU on the box, which often works out to about 4,500 to 5,500 SACC. Add a little more if the room is sunny or has a lot of heat sources.
Is SACC or ASHRAE more accurate?
SACC is generally more realistic, because it accounts for how portable units lose efficiency. Use it when comparing models if the number is available.
Can a portable AC cool a whole apartment?
Usually not. It’s best for one room or a small open area, since walls and doors block the cool air. For multiple rooms, you’d need more than one unit or a different type of system.
The bottom line
Getting the portable AC BTU size right means starting with your room’s square footage, checking the SACC rating and adjusting for sun, kitchens and ceiling height. Take a few minutes to measure and compare, and you’ll end up with a unit that cools well without wasting power.