What size mini split does a 600 sq ft bedroom in West Valley City, UT need?

City

Room area (sq ft)

Room

600 ft² bedroom in West Valley City, UT

15,000BTU/hr

13,737 BTU/hr of load at 22.9 BTU/ft² — the figure for 1,631 cooling degree days

Head size to buy	15,000BTU/hr
Calculated load600 ft² × 22.9 BTU/ft²	13,737BTU/hr
BTU per square foot hereinterpolated from West Valley City's 1,631 cooling degree days	22.9BTU/ft²
What the 20 BTU/ft² rule givesthis room needs 14% more than the rule says	12,000BTU/hr
Summer design temperature	100°F
Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone	13°F
Summer day–night swinga dry climate	25°F
Days at or above 90°F	67days/yr
Oversize from the size stepwithin the normal step	9%

Notes

The "20 BTU per square foot" rule is a Midwest number. West Valley City runs 1,631 cooling degree days, which works out to 22.9 BTU/ft² — so for this bedroom the rule is 14% short. A single national figure has to be wrong somewhere, and this is where.
Bedroom: the room where oversizing hurts most — quiet, low load, long runtimes wanted.
15,000 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.
West Valley City is dry — nights fall 25°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. West Valley City designs to 13°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 13°F in writing.
Where these numbers come from. Salt Lake City International Airport, 11 km from downtown West Valley City, 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 600 square foot bedroom in West Valley City, UT needs a head of about 15,000 BTU per hour. The calculated load is 13,737 BTU/hr.

Where that number comes from

West Valley City, UT runs 1,631 cooling degree days a year, which puts the room at 22.9 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 12,000 BTU/hr.

The winter half of the question

West Valley City, UT designs to 13°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

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.