What size mini split does a 300 sq ft living room in Cheyenne, WY need?
300 ft² living room in Cheyenne, WY
6,000BTU/hr
5,347 BTU/hr of load at 16.2 BTU/ft² — the figure for 470 cooling degree days
| Head size to buy | 6,000BTU/hr |
|---|---|
| Calculated load300 ft² × 16.2 BTU/ft² × 1.1 for a living room | 5,347BTU/hr |
| BTU per square foot hereinterpolated from Cheyenne's 470 cooling degree days | 16.2BTU/ft² |
| What the 20 BTU/ft² rule givesthe rule overshoots by 19% here | 6,600BTU/hr |
| Summer design temperature | 91°F |
| Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone | -5°F |
| Summer day–night swinga dry climate | 29°F |
| Days at or above 90°F | 16days/yr |
| Oversize from the size stepwithin the normal step | 12% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Cheyenne runs 470 cooling degree days, which works out to 16.2 BTU/ft² — so for this living room the rule overshoots by 19%. 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 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.
- Cheyenne is dry — nights fall 29°F below the afternoon high, which only happens when there is little water in the air. Latent load is small, so nearly all of the head's capacity goes into temperature. A slightly undersized head is forgiving here: it simply runs longer.
- Do not forget the winter side. Cheyenne designs to -5°F. That is cold enough that a standard head will have lost a large part of its rated capacity by the time it is needed most — check the manufacturer's capacity table at -5°F, not the nameplate, and look at hyper-heat rated equipment.
- Where these numbers come from. Cheyenne Airport, 2 km from downtown Cheyenne, 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 Cheyenne, WY needs a head of about 6,000 BTU per hour. The calculated load is 5,347 BTU/hr.
Where that number comes from
Cheyenne, WY runs 470 cooling degree days a year, which puts the room at 16.2 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
Cheyenne, WY designs to -5°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.
Nearby sizes
- What size mini split does a 300 sq ft living room in Saint Charles, MO need?
- What size mini split does a 300 sq ft living room in Ames, IA need?
- What size mini split does a 300 sq ft living room in Ellicott City, MD need?
- What size mini split does a 300 sq ft living room in Shawnee, KS need?
- What size mini split does a 200 sq ft living room in Cheyenne, WY need?
- What size mini split does a 400 sq ft living room in Cheyenne, WY need?
- What size mini split does a 300 sq ft bedroom in Cheyenne, WY need?
Read more
- Mini Split Blowing Cold Air in Heat Mode — Defrost, UsuallyEvery air-source heat pump does this. Knowing what a defrost cycle looks like is the difference between waiting ten minutes and calling somebody out.
- Mini Split Leaking Water Indoors — Four Causes, One CommonA cooling head makes a couple of litres a day. It leaves through a small pipe with no pump behind it, and that pipe is where nearly every leak starts.
- Multi-Zone Mini Split Not Cooling Every Room — The Minimum Output ProblemThe system is not underpowered. It is oversized for the one room that is calling, and multi-zone units handle that far worse than single-zone ones.