What size mini split does a 800 sq ft bedroom in Tacoma, WA need?

City

Room area (sq ft)

Room

800 ft² bedroom in Tacoma, WA

12,000BTU/hr

9,453 BTU/hr of load at 11.8 BTU/ft² — the figure for 208 cooling degree days

Head size to buy	12,000BTU/hr
Calculated load800 ft² × 11.8 BTU/ft²	9,453BTU/hr
BTU per square foot hereinterpolated from Tacoma's 208 cooling degree days	11.8BTU/ft²
What the 20 BTU/ft² rule givesthe rule overshoots by 41% here	16,000BTU/hr
Summer design temperature	86°F
Winter design temperaturea mini split heats too — this is the number that decides whether it can carry the room alone	19°F
Summer day–night swinga fairly dry climate	24°F
Days at or above 90°F	6days/yr
Oversize from the size stepa big step — read the note	27%

Notes

The "20 BTU per square foot" rule is a Midwest number. Tacoma runs 208 cooling degree days, which works out to 11.8 BTU/ft² — so for this bedroom the rule overshoots by 41%. 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.
12,000 BTU/hr is 27% above the load, and the gap between head sizes is wide — the next size down is 9,000. 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.
Tacoma is dry — nights fall 24°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. Tacoma designs to 19°F. A cold-climate rated head will hold most of its capacity there, but a standard one will not — ask for the capacity at 19°F in writing.
Where these numbers come from. Tacoma / McChord Air Force Base, 12 km from downtown Tacoma, 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 800 square foot bedroom in Tacoma, WA needs a head of about 12,000 BTU per hour. The calculated load is 9,453 BTU/hr.

Where that number comes from

Tacoma, WA runs 208 cooling degree days a year, which puts the room at 11.8 BTU per square foot rather than the 20 BTU/ft² the generic charts use. Applied to this room the rule gives 16,000 BTU/hr.

The winter half of the question

Tacoma, WA designs to 19°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.