How Much Paint Do I Need?

One gallon of interior paint covers 350–400 square feet per coat on a smooth wall — that single number answers most paint-buying questions. For exterior paint, plan on 250–300 square feet per gallon on rough or textured surfaces (smooth siding stretches toward 400), and primer covers 200–300 square feet per gallon. A standard 12×12 bedroom needs about two gallons for a proper two-coat job; a full 2,000-square-foot house interior runs roughly five gallons per coat. The expensive mistakes all come from the same three places: measuring wall area wrong, assuming one coat will be enough, and pricing textured walls at smooth-wall coverage. Here is the exact formula painters use — measure once, buy once, and stop guessing at the paint counter.

Paint coverage by surface

Coverage ratings are printed on every can, and they are always per-coat numbers — one coat on the stated surface type. Use the low end of each range in your math; a gallon that claims 400 square feet on a perfect test panel rarely delivers that on your real walls. When in doubt, budget at 350 for interior and 250 for exterior.

How many gallons per room (worked examples)

These are net wall areas — doors and windows subtracted — for standard 8-foot ceilings, using 350 square feet per gallon and rounding to whole gallons. Two coats is the standard for a durable finish; the one-coat numbers are for reference when you are refreshing the same color in the same sheen. Between coats, don't guess — check how long to wait between coats of paint so the second coat doesn't wreck the first.

A note on the house row, because two competing rules of thumb exist: the common version says a 2,000-square-foot house needs about 5 gallons per coat at 360–400 square feet per gallon — roughly 1,800–2,000 square feet of paintable wall. Other estimators use a larger convention (2,000 square feet of floor ≈ 5,000+ square feet of wall when you count every wall in every room). Both exist because layouts differ wildly — open plans, vaulted ceilings, and how many walls actually get painted. That is exactly why pros measure instead of trusting a rule. For the full breakdown, see our guide on how much paint a 2,000 sq ft house needs.

The formula: measure it like a pro

Five steps, and you will land within a quart of the right number. Grab a tape measure and a calculator:

• 1. Measure the walls. Add the four wall lengths (the perimeter) and multiply by the ceiling height. A 12×15 room with 8-foot ceilings: 2 × (12 + 15) = 54 feet of perimeter × 8 = 432 square feet of gross wall.
• 2. Subtract the openings. A standard door is about 21 square feet and a window about 15. That room has two doors and two windows: (2 × 21) + (2 × 15) = 72 square feet. 432 − 72 = 360 square feet of paintable wall.
• 3. Divide by coverage. Use 350 for interior walls to stay conservative: 360 ÷ 350 ≈ 1.03 gallons per coat.
• 4. Multiply by coats. Two is the standard for finish paint: about 2 gallons for the finish coats.
• 5. Add waste. Multiply by 1.10–1.15 for cut-ins, roller soak-up, and touch-ups: roughly 2.2 gallons. Buy two gallons — not three — and keep one quart back in the budget for touch-ups if needed. Stores generally will not take back tinted paint, so you buy to the estimate, not past it.

In-store shortcut when you did not bring the tape measure: length + width, times 2, times ceiling height, minus 70 for a typical room's doors and windows, divided by 350, times two coats. It lands within a quart of the careful version for most bedrooms and living rooms.

Ceilings, trim, and doors

Ceilings are their own calculation: width × length, divided by 350–400 square feet per gallon, always in flat finish — flat hides drywall imperfections and kills glare. A 12×12 ceiling is 144 square feet, so one gallon covers it for two full coats with room to spare. Never fold the ceiling into your wall gallons; it is a different paint, usually bought untinted, and sized on its own.

Trim — baseboards, window casings, door frames — gets semi-gloss and its own purchase. A quart covers about 100 square feet, and a typical room's trim runs to roughly one quart. Do not try to “borrow” trim paint from the wall gallons: different sheen, different color, and you will come up short on both.

Doors belong with the trim, not the walls. Count each side of a door at roughly 20 square feet and add it to the trim total — a room with three doors is about 120 square feet of door surface (both sides), which is another quart-plus of semi-gloss on top of the baseboards and casings.

When the math lies: the exceptions

The formula assumes smooth, previously painted drywall and a roller. Change any of those conditions and the coverage rating on the can stops applying:

Buying paint without wasting money

• Buy five-gallon buckets for whole-house, same-color jobs. The per-gallon price drops, and you “box” — mix — the buckets together so the tint is uniform across rooms. Nothing screams DIY like a visible tint shift between the living room and the hallway.
• Stores generally will not refund tinted paint. That is the whole game: buy 80–100% of your estimate, not 120%. A second trip for a touch-up quart costs twenty minutes; five leftover tinted gallons cost real money and become shelf clutter.
• Be conservative on color, confident on the basics. Untinted paint like primer and stock-white ceiling paint is the low-risk part of the purchase — the tinted wall color is where you hold to the estimate and come back for touch-ups.
• Keep one labeled quart per room and color. Write the room, paint name, color code, and date on the lid. Future touch-ups — the scuff behind the door, the nail holes from the picture frame — take ten minutes when the exact paint is on the shelf and an hour of guesswork when it is not.

Costly mistakes to avoid

• Measuring floor space instead of wall space. A 2,000-square-foot house is not 2,000 square feet of wall — the conventions range from ~1,800 to 5,000+ square feet of wall depending on how you count. Measure the walls, not the floor.
• Counting primer as a finish coat. Primer seals; it does not hide like finish paint. Over new drywall or a color change, the ticket is primer plus two finish coats — primer is never coat number one of two.
• Planning for one coat. One coat refreshes the same color in the same sheen; almost everything else needs two. The second coat is where the color actually becomes the color on the swatch.
• Pricing textured walls at smooth-wall coverage. Rough surfaces can double the need. Budget at the rough rating or you are buying twice — and the second trip lands mid-wall with a drying edge that shows.
• Overbuying tinted paint. It cannot go back to the store. Buy 80–100% of the estimate, then run a touch-up trip. The store run is cheap; the five stranded gallons are not.
• Skipping the waste factor. Cut-ins, roller soak-up, touch-ups, and the last half-inch in the tray eat 10–15%. The math that ignores waste is the math that runs dry one wall from finished.

Run your own numbers in the paint calculator — punch in your room dimensions and it does the perimeter math for you — then turn the gallons into a quote if you are pricing the job for a client. Planning a specific project? Start with what it costs to paint a 12×12 room for the full material-and-labor picture, or how much paint a 2,000 sq ft house needs for the whole-house breakdown.