What size mini split does a 600 sq ft bedroom in Lenexa, KS need?

City

Room area (sq ft)

Room

600 ft² bedroom in Lenexa, KS

15,000BTU/hr

13,565 BTU/hr of load at 22.6 BTU/ft² — the figure for 1,546 cooling degree days

Head size to buy	15,000BTU/hr
Calculated load600 ft² × 22.6 BTU/ft²	13,565BTU/hr
BTU per square foot hereinterpolated from Lenexa's 1,546 cooling degree days	22.6BTU/ft²
What the 20 BTU/ft² rule givesthis room needs 13% more than the rule says	12,000BTU/hr
Summer design temperature	95°F
Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone	2°F
Summer day–night swinga fairly dry climate	21°F
Days at or above 90°F	39days/yr
Oversize from the size stepwithin the normal step	11%

Notes

The "20 BTU per square foot" rule is a Midwest number. Lenexa runs 1,546 cooling degree days, which works out to 22.6 BTU/ft² — so for this bedroom the rule is 13% 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.
Lenexa 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. Lenexa designs to 2°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 2°F, not the nameplate, and look at hyper-heat rated equipment.
Where these numbers come from. Olathe / Clare, 19 km from downtown Lenexa, 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 Lenexa, KS needs a head of about 15,000 BTU per hour. The calculated load is 13,565 BTU/hr.

Where that number comes from

Lenexa, KS runs 1,546 cooling degree days a year, which puts the room at 22.6 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

Lenexa, KS designs to 2°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

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.