What size mini split does a 300 sq ft living room in Bellevue, WA need?
300 ft² living room in Bellevue, WA
6,000BTU/hr
4,828 BTU/hr of load at 14.6 BTU/ft² — the figure for 351 cooling degree days
| Head size to buy | 6,000BTU/hr |
|---|---|
| Calculated load300 ft² × 14.6 BTU/ft² × 1.1 for a living room | 4,828BTU/hr |
| BTU per square foot hereinterpolated from Bellevue's 351 cooling degree days | 14.6BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 27% here | 6,600BTU/hr |
| Summer design temperature | 86°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | 25°F |
| Summer day–night swinga fairly dry climate | 20°F |
| Days at or above 90°F | 6days/yr |
| Oversize from the size stepa big step — read the note | 24% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Bellevue runs 351 cooling degree days, which works out to 14.6 BTU/ft² — so for this living room the rule overshoots by 27%. 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.
- 6,000 BTU/hr is 24% above the load, and the gap between head sizes is wide — this is already the smallest head. In a single room that matters more than it would on a central system: there are no other rooms to absorb the excess capacity. If the room is well insulated and shaded, the smaller head is usually the better buy, and modern inverter compressors modulate down far better than they modulate up.
- Bellevue is fairly dry — summer nights stay within 20°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. Bellevue designs to 25°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 25°F in writing.
- Where these numbers come from. Seattle-Tacoma Airport, 20 km from downtown Bellevue, 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 Bellevue, WA needs a head of about 6,000 BTU per hour. The calculated load is 4,828 BTU/hr.
Where that number comes from
Bellevue, WA runs 351 cooling degree days a year, which puts the room at 14.6 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
Bellevue, WA designs to 25°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.