What size mini split does a 200 sq ft bedroom in West Des Moines, IA need?

City

Room area (sq ft)

Room

200 ft² bedroom in West Des Moines, IA

6,000BTU/hr

4,313 BTU/hr of load at 21.6 BTU/ft² — the figure for 1,274 cooling degree days

Head size to buy	6,000BTU/hr
Calculated load200 ft² × 21.6 BTU/ft²	4,313BTU/hr
BTU per square foot hereinterpolated from West Des Moines's 1,274 cooling degree days	21.6BTU/ft²
What the 20 BTU/ft² rule givesthis room needs 8% more than the rule says	4,000BTU/hr
Summer design temperature	92°F
Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone	-6°F
Summer day–night swinga fairly dry climate	20°F
Days at or above 90°F	22days/yr
Oversize from the size stepa big step — read the note	39%

Notes

The "20 BTU per square foot" rule is a Midwest number. West Des Moines runs 1,274 cooling degree days, which works out to 21.6 BTU/ft² — so for this bedroom the rule is 8% 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.
6,000 BTU/hr is 39% 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.
West Des Moines 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. West Des Moines designs to -6°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 -6°F, not the nameplate, and look at hyper-heat rated equipment.
Where these numbers come from. Des Moines International, 7 km from downtown West Des Moines, 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 200 square foot bedroom in West Des Moines, IA needs a head of about 6,000 BTU per hour. The calculated load is 4,313 BTU/hr.

Where that number comes from

West Des Moines, IA runs 1,274 cooling degree days a year, which puts the room at 21.6 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 4,000 BTU/hr.

The winter half of the question

West Des Moines, IA designs to -6°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.