Buying paint comes down to one number: how much wall you actually have to cover. Here's the simple method — including the doors, windows and coats most people forget — with a worked example.

Step 1: find the wall area

For a rectangular room, the wall area is 2 × (length + width) × ceiling height. For a 12 × 12 room with 8-foot ceilings that's 2 × 24 × 8 = 384 square feet before you take anything out.

Step 2: subtract doors and windows

You don't paint the openings, so subtract them. Use about 21 square feet per door and 15 square feet per window. One of each drops our room to 384 − 36 = 348 square feet.

Step 3: multiply by coats, divide by coverage

A gallon covers roughly 350 square feet per coat on smooth walls. Two coats is standard, so:

348 × 2 coats = 696 square feet of coverage needed
696 ÷ 350 = 1.99 gallons → buy 2 gallons

Always round up to whole cans. Running out halfway up a wall leaves visible lap marks that are hard to hide.

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How far does a gallon really go?

Manufacturers quote 350–400 square feet per gallon, and that holds on smooth, primed drywall. New drywall, textured surfaces, and dramatic color changes (light over dark) all drink more paint — which is exactly why two coats is the sensible default.

One or two coats?

Two coats is standard for an even, durable finish. Go to three when you're painting a bold color, going light over dark, or covering fresh drywall that hasn't been primed.

Don't forget the ceiling

This method covers the walls. For the ceiling, calculate its area separately (length × width) and add it if you're painting it too.

Which finish goes where

The gallon count is only half the order. The other half is sheen, and picking the wrong one is the most common reason a freshly painted room looks worse than expected. Higher sheens are tougher and easier to wipe clean, but they also show every dent and patch in the wall.

Finish	Best for	Watch out for
Flat / matte	Ceilings, low-traffic bedrooms and living rooms	Marks easily; hard to wipe clean
Eggshell	Most living areas and hallways	Shows roller marks if touched up later
Satin	Kitchens, bathrooms, kids' rooms	Highlights wall flaws more than eggshell
Semi-gloss	Trim, doors, cabinets	Too shiny for whole walls; every bump shows
Gloss	Furniture, railings, front doors	Needs very smooth prep

The paint people forget: trim, doors and ceilings

Wall paint and trim paint are separate purchases, usually in different finishes. A few planning figures:

Trim: the baseboards, door casings and window casings in an average bedroom take about one quart of semi-gloss for two coats. A large living room with crown molding can take two.
Doors: one side of a standard interior door is about 20 ft², so both sides with two coats is roughly 80 ft² of coverage. That is close to what a quart covers, so plan on about one quart per door if you are painting both faces.
Ceilings: measure length × width. A 12 × 12 ceiling is 144 ft², a little under half a gallon per coat. Use a flat ceiling paint; it hides the imperfections that a raking light from a window exposes.

The order that saves the most time

Prep first: fill holes, sand the patches, wipe the walls, and tape what you cannot cut in by hand.
Ceiling before walls, so drips land on surfaces you have not painted yet.
Walls next: cut in the edges with a brush, then roll while the cut-in is still wet so the two blend.
Trim and doors last, because a clean line of semi-gloss over the wall color is easier than the reverse.

Wait for the recoat time on the can between coats. For most latex wall paints it is a few hours, but cool or humid rooms stretch it, and a second coat rolled over tacky paint drags and leaves marks.