What size mini split does a 300 sq ft living room in Montgomery, AL need?
300 ft² living room in Montgomery, AL
9,000BTU/hr
8,469 BTU/hr of load at 25.7 BTU/ft² — the figure for 2,728 cooling degree days
| Head size to buy | 9,000BTU/hr |
|---|---|
| Calculated load300 ft² × 25.7 BTU/ft² × 1.1 for a living room | 8,469BTU/hr |
| BTU per square foot hereinterpolated from Montgomery's 2,728 cooling degree days | 25.7BTU/ft² |
| What the 20 BTU/ft² rule givesthis room needs 28% more than the rule says | 6,600BTU/hr |
| Summer design temperature | 97°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | 24°F |
| Summer day–night swinga fairly dry climate | 21°F |
| Days at or above 90°F | 91days/yr |
| Oversize from the size stepwithin the normal step | 6% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Montgomery runs 2,728 cooling degree days, which works out to 25.7 BTU/ft² — so for this living room the rule is 28% short. A single national figure has to be wrong somewhere, and this is where.
- Living room: more people, more glazing, more electronics. The 1.1× factor accounts for that.
- 9,000 BTU/hr sits close to the load, which is where a mini split works best. Inverter equipment is happiest running long and slow at part load; it is the short, hard cycles of an oversized head that leave a room cold and damp.
- Montgomery is fairly dry — summer nights stay within 21°F of the day's high, the signature of humid air. A meaningful share of this head's work is removing moisture, and it only removes moisture while it is running. This is the climate where an oversized head produces a room that reads cold and feels clammy.
- Do not forget the winter side. Montgomery designs to 24°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 24°F in writing.
- Where these numbers come from. Montgomery Dannelly Field, 23 km from downtown Montgomery, daily records 2015–2024. Design temperatures are calibrated against published ASHRAE values at eight airports and land within about 2°F of them.
The short answer
A 300 square foot living room in Montgomery, AL needs a head of about 9,000 BTU per hour. The calculated load is 8,469 BTU/hr.
Where that number comes from
Montgomery, AL runs 2,728 cooling degree days a year, which puts the room at 25.7 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 6,600 BTU/hr.
The winter half of the question
Montgomery, AL designs to 24°F. A mini split is a heat pump, and its heating capacity falls as the outdoor temperature drops — the nameplate is measured at 47°F, not at the temperature you actually need it.
Sizing a single room is not sizing a house
There is no other room to absorb a mistake. An oversized head short-cycles, never runs long enough to dehumidify, and leaves the room cold and damp.
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Read more
- Sizing a Head for a Garage or WorkshopThe standard rule of thumb assumes a room inside a house. A garage is not one, and applying it gives a number that is too small.
- The Room Over the Garage — Why It Is Always Wrong and What Fixes ItEvery house with a room over the garage has the same complaint about it, and the reasons are structural rather than mechanical.
- Where the Head Goes Matters as Much as What Size It IsA wall-mounted head throws air in one direction, and the room is conditioned in that direction.
- Ductless or Central — What Each One Is Actually Good AtThe comparison is usually framed as efficiency. The real differences are distribution, control and what the house already has.