How much paint for a 24 × 10 ft room?

Paint needed

3.71gallons

508 ft² of primed drywall in 3 coats — buy a 5 gal bucket

Paint	3.71gal
What to buycheaper per gallon from about 3.6 gal up	1 × 5 gal bucket
Wall area544 ft² less 1 door and 1 window	508ft²
Spread ratethe surface the label figure is measured on	375ft²/gal
First coat	1.35gal
Each later coat14.999999999999991% further — the surface is sealed now	1.18gal
Doubling the first coat would say0.35 gal more than you need	4.06gal
Masking tape99 ft of edge	260 yd rolls

Notes

Spread rate is a property of the surface, not the paint. Primed drywall takes about 375 ft² per gallon — the surface the label figure is measured on. The "400 square feet" on the tin is measured on smooth primed drywall, which is the best case and rarely the case.
The second coat is cheaper than the first. Once the surface is sealed, paint spreads about 14.999999999999991% further. 3 coats here is 3.71 gal, not 4.06 — doubling the first coat over-buys by 0.35 gal, which at retail is real money left in the garage.
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 3.71 gal of wall paint typically takes under a quart for the trim.
Cutting in is where the time goes, not rolling. 99 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.

What to buy

3.71 gallons — 1 × 5 gal.

508 ft² of wall after openings, at 375 ft² per gallon on primed drywall.

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. primed drywall takes about 375, 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. 3 coats here comes to 3.71 gallons — not 4.06, which is what doubling the first coat would say.

That difference is 0.35 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 3.71 gallons of wall paint typically takes under a quart of trim enamel.

Cutting in is the job

99 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.

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