The coverage math
A gallon of interior paint covers roughly 350–400 sq ft per coat on smooth, previously painted drywall. The honest formula:
Gallons = (wall sq ft × number of coats) ÷ 350. Use 350, not the optimistic number on the can — texture, color changes, and roller loss all eat coverage. Round up to the nearest quart-or-gallon the store actually sells.
From our measuring guide: a 12′ × 10′ room with 9′ ceilings has about 396 sq ft of wall before openings — call it 360 after a door and window. Two coats is 720 sq ft ÷ 350 ≈ 2.1 gallons: buy two gallons and a quart, or three gallons if you want touch-up left over (you do).
Two coats is the default, not the upgrade
- Same-ish color refresh: occasionally one coat passes. Look at it in evening light before you decide.
- Any real color change: two coats, always. Deep or vivid colors over a contrasting base may need a tinted primer plus two.
- New drywall: one coat of drywall primer-sealer, then two finish coats. Skipping the primer means the finish coats soak in unevenly and you buy a third anyway — at finish-paint prices.
- Dark → light: stain-blocking primer first, or budget three coats and disappointment.
Sheen: decided by surface, not taste
| Surface | Sheen | Why |
|---|---|---|
| Ceilings | Flat | Hides every drywall sin; no glare. |
| Bedroom / living walls | Matte or eggshell | Soft look, hides roller marks. |
| Kitchen & bath walls | Eggshell or satin | Wipeable; stands up to humidity. |
| Trim, doors & casing | Semi-gloss | Durable, scrubbable, crisp edge line. |
| Cabinets | Semi-gloss or specialty enamel | Hardest-wearing surface in the house. |
The practical consequence: a single room is usually three different paint purchases — ceiling flat, wall eggshell, trim semi-gloss — each with its own quantity math. Trim paint is figured off linear feet (a gallon does roughly 350–400 linear ft of standard baseboard per coat), so the perimeter measurement you already took covers it.
Buying rules that save a second trip
- Buy all of a color at once and box it — intermix the cans in a bucket. Tint machines drift; two gallons mixed separately can differ visibly mid-wall.
- Keep a labeled quart of leftovers per color: name, brand, sheen, room, date. Future-you doing touch-ups will otherwise be color-matching off a chip of drywall at the paint counter.
- Record the exact color specs somewhere permanent — not the lid of a can that gets recycled. Three years from now, "the beige-ish one" is not a color. A per-room paint log (color, sheen, brand, where it was used) is one of the highest-value records a homeowner can keep.
- Don't forget the supporting cast: primer where needed, painter's tape, plastic, roller covers per color family, and a caulk gun for the pre-paint trim caulking that separates amateur jobs from crisp ones.