Paint for a 11 × 11 ft room with 9 ft ceilings on wood siding, 1 coats

Paint needed

1.2gallons

360 ft² of wood siding in 1 coat — buy 2 gal

Paint	1.2gal
What to buybuckets only pay from about 3.6 gal	2 × 1 gal
Wall area396 ft² less 1 door and 1 window	360ft²
Spread rategrain and end grain drink more than they look like they will	300ft²/gal
First coat	1.2gal
Each later coat14.999999999999991% further — the surface is sealed now	1.04gal
Masking tape75 ft of edge	260 yd rolls

Room length: 11 ft
Room width: 11 ft
Ceiling height: 9 ft
Surface: wood siding
Coats: 1
Doors: 1

What to buy

1.2 gallons — 2 × 1 gal.

360 ft² of wall after openings, at 300 ft² per gallon on wood siding.

Spread rate belongs to the surface, not the tin

The "400 square feet per gallon" printed on a can is measured on smooth primed drywall. wood siding takes about 300, and that difference is the single biggest reason people run out.

Rough and porous surfaces are worst: stucco can halve the figure on the label.

The second coat is cheaper than the first

Once the surface is sealed, paint spreads about 15% further. 1 coats here comes to 1.2 gallons — not 1.2, which is what doubling the first coat would say.

That difference is 0 gallons of paint that would sit in the garage until it went off.

Box it before you start

Pour every tin into one bucket and mix. Tins of the same color from different batches differ slightly, and a wall painted from two tins shows the join exactly where you stopped for lunch.

Trim does not scale with the room

Doors, trim and ceilings are linear-foot work and a completely separate calculation. A room needing 1.2 gallons of wall paint typically takes under a quart of trim enamel.

Cutting in is the job

75 ft of edge here — that is where the hours go, not the rolling. A good 2½″ angled sash brush beats tape on most edges; tape earns its place mainly against carpet and glass.

Notes

**Spread rate is a property of the surface, not the paint.** Wood siding takes about 300 ft² per gallon — grain and end grain drink more than they look like they will. The "400 square feet" on the tin is measured on smooth primed drywall, which is the best case and rarely the case.
One coat only works over a similar colour on a sealed surface. Anything else — new drywall, a colour change, a patch repair — needs two, and trying to make one heavy coat do the job of two gives you runs rather than coverage.
Buy all of it in one go and box it — pour every tin into one bucket and mix. Tins of the same colour from different batches differ slightly, and a wall painted from two tins shows the change where they met.
Trim, doors and ceilings do not scale with floor area — they scale with linear feet, and they are a separate calculation. A room that takes 1.2 gal of wall paint typically takes under a quart for the trim.
Cutting in is where the time goes, not rolling. 75 ft of edge here. A decent 2½″ angled sash brush and a steady hand beat tape on most edges, and tape is worth it mainly against carpet and glass.

Nearby sizes

Paint Coverage per Gallon by Surface — the Full Table, and Why the Label Is OptimisticEvery label says 400 square feet. Almost no real surface delivers it, and the ones that miss badly miss by half.
Cost to Paint a Room — Where the Money Goes, and Why Trim Costs More Than WallsPaint is about a tenth of the bill. Prep and trim are most of it, and trim does not scale with room size at all.
How Much Paint for a Room — Spread Rate Belongs to the Surface, Not the TinChoosing paint by the coverage claim on the label compares products on a number that describes a wall you do not have.
How Much Exterior Paint — Texture Decides It, Not BrandThe gap between smooth fibre cement and bare brick is nearly three to one. The gap between the cheapest and dearest paint is nothing like that.