What size mini split does a 300 sq ft bedroom in Saint Charles, MO need?
300 ft² bedroom in Saint Charles, MO
9,000BTU/hr
7,148 BTU/hr of load at 23.8 BTU/ft² — the figure for 1,939 cooling degree days
| Head size to buy | 9,000BTU/hr |
|---|---|
| Calculated load300 ft² × 23.8 BTU/ft² | 7,148BTU/hr |
| BTU per square foot hereinterpolated from Saint Charles's 1,939 cooling degree days | 23.8BTU/ft² |
| What the 20 BTU/ft² rule givesthis room needs 19% more than the rule says | 6,000BTU/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 | 5°F |
| Summer day–night swinga moderately humid climate | 18°F |
| Days at or above 90°F | 48days/yr |
| Oversize from the size stepa big step — read the note | 26% |
Notes
- The "20 BTU per square foot" rule is a Midwest number. Saint Charles runs 1,939 cooling degree days, which works out to 23.8 BTU/ft² — so for this bedroom the rule is 19% 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.
- 9,000 BTU/hr is 26% above the load, and the gap between head sizes is wide — the next size down is 6,000. 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.
- Saint Charles is moderately humid — summer nights stay within 18°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. Saint Charles 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. Lambert-St Louis International Airport, 39 km from downtown Saint Charles, 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 bedroom in Saint Charles, MO needs a head of about 9,000 BTU per hour. The calculated load is 7,148 BTU/hr.
Where that number comes from
Saint Charles, MO runs 1,939 cooling degree days a year, which puts the room at 23.8 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 6,000 BTU/hr.
The winter half of the question
Saint Charles, MO 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.
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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.