Choosing the right portable AC size is critical. Too small and it will never cool your room. Too large and it wastes energy and increases humidity. Use our BTU calculator to find the correct size for your space.
The standard starting point for sizing a portable AC:
So for common room sizes:
200 sq ft
~4,000 BTU
300 sq ft
~6,000 BTU
400 sq ft
~8,000 BTU
Important: This is only the baseline. Keep reading for the adjustments that matter.
| Room Size | Recommended BTU |
|---|---|
| 100–200 sq ft | 5,000–6,000 BTU |
| 200–300 sq ft | 6,000–8,000 BTU |
| 300–400 sq ft | 8,000–10,000 BTU |
| 400–500 sq ft | 10,000–12,000 BTU |
| 500+ sq ft | 12,000+ BTU |
These are ideal ranges for portable AC units. Always apply the adjustments below before making your final decision.
The basic formula is only a starting point. These five factors can significantly change your required BTU.
Each additional person beyond one occupant generates body heat:
Cooking appliances and heat-generating equipment add significant load:
Standard formula assumes 8 ft ceilings. Higher ceilings mean more air volume to cool:
Humid environments feel warmer and require more cooling power to maintain comfort:
See our best portable AC for humid climates guide.
Most brands list two values — and the difference is huge:
ASHRAE BTU
Higher number
Marketing figure. Measured under controlled lab conditions that don't reflect real-world use.
DOE BTU
Real-world number
Accounts for exhaust heat re-entering the room. This is what actually cools your space.
Example:
12,000 BTU (ASHRAE) ≈ 8,000–9,000 BTU (DOE)
Always focus on DOE BTU for accuracy. When in doubt, size up.
Solution: Upgrade to higher BTU unit
Solution: Match BTU to actual room size
Enter your room details to calculate the recommended BTU for your portable AC.
Room Area: 300 sq ft
Base BTU: 6,000 BTU
After adjustments: 6,000 BTU
Recommended BTU
6,000 BTU
Rounded up to the nearest common portable AC size. When in doubt, go one tier higher — oversizing is safer than undersizing.
Even with the correct BTU, hose type affects real-world performance:
Single Hose
Dual Hose
Learn more in our dual hose vs single hose guide.
Better to have a bit more power than too little. The efficiency cost of mild oversizing is minimal.
Going far above the recommended BTU causes short cycling and poor humidity removal. One tier up is fine; two tiers up is a problem.
Open floor plans and rooms that connect to hallways need more BTU than a sealed, insulated room of the same area.
Good insulation and sealing gaps around doors and windows can improve performance dramatically — often more than upgrading BTU.
Size: 220 sq ft
Base: 220 × 20 = 4,400 BTU
Adjusted for normal conditions: ~6,000 BTU
Recommendation: 6,000 BTU unit
See our best portable AC for small rooms.
Size: 450 sq ft
Base: 450 × 20 = 9,000 BTU
Sunny + humid adjustment: ~12,000 BTU
Recommendation: 12,000 BTU unit
See our best portable AC for large rooms.
Yes, for rooms up to approximately 300 sq ft (28 m²) under normal conditions. Add 10–20% if the room gets significant direct sunlight.
No. Oversizing beyond one tier causes short cycling, poor dehumidification, and wasted energy. Match BTU as closely as possible to your calculated need.
Yes. High humidity makes air feel warmer and requires more cooling power. Add 10–15% BTU in consistently humid climates.
ASHRAE BTU is measured under controlled conditions and inflates the number. DOE BTU reflects real-world performance and is always lower. Use DOE BTU when comparing units.
Yes. Higher ceilings mean more air volume. Add 10–20% to your base BTU calculation for ceilings above the standard 8 ft (2.4 m).
For most users, the right BTU comes down to a simple process:
Use our BTU calculator above to get your personalized recommendation, then browse our best portable AC picks for top-rated options at every size.
Top picks for rooms up to 300 sq ft — compared on BTU, noise, and efficiency.
High-BTU portable ACs for spaces 400 sq ft and above.
How hose type affects real-world cooling performance and efficiency.
Calculate the running cost of your portable AC based on BTU and usage hours.
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