How much paint for a room

Measure the walls

Add the lengths of all four walls to get the perimeter, and multiply by the wall height. A 12 × 14 ft room has a 52 ft perimeter, and at 8 ft that is 416 sq ft of wall before openings. Alcoves and bump-outs add wall: measure their extra sides and add them. For a room with a sloped ceiling, measure the tall and the short height and use the average.

Subtract the openings. A standard door is about 21 sq ft (3 × 7 ft) and an average window about 15 sq ft; the sheet uses those figures for each door and window you count. Don’t subtract for a small window, a closet door you are painting anyway, or trim; the rounding covers it and the roller wastes more. A patio door or a picture window counts as two.

Coats, coverage and primer

Coverage is printed on the can, usually 350–400 sq ft per gallon per coat on a smooth, previously painted wall. Textured or unprimed surfaces soak up more; use 300 if the wall is rough. Plan on two coats. One coat of even a good paint over a similar color shows roller marks and lap lines in raking light, and a second coat is cheaper than repainting. The sheet multiplies the area by the coats, divides by the coverage and rounds up to whole gallons.

Primer is a separate product for a separate purpose: it seals bare drywall and joint compound so the top coat doesn’t flash, blocks stains, and gives a color change a neutral start. Over new drywall, over patches, or going from dark to light, switch the primer coat on. Over a sound wall in a similar color, skip it. Self-priming paints cover most of the middle ground but still want two coats.

Ceiling and trim

The ceiling is a separate area, length × width, in flat ceiling paint, which hides roller marks better than wall paint. The sheet adds it when the ceiling switch is on. Trim, doors and windows are a different product again, a semi-gloss or satin enamel, and a different quantity: a quart does the trim of an ordinary room, a gallon does a whole floor of doors and baseboards. Buy it separately; the sheet counts wall and ceiling paint only.

Measuring and buying

1. Measure each wall’s length along the baseboard and the height from floor to ceiling.
2. Count the doors and windows on those walls; treat a double door or a picture window as two.
3. Type the sizes; set the ceiling and primer switches and the number of coats.
4. Set the coverage to the figure on the can you plan to buy, and the price to the shelf price for the sheen you want.
5. Read the gallons. If it lands just over a whole number, buy the extra gallon rather than a quart of a store-mixed color; the tint can differ between batches.
6. Buy the trim paint separately by the quart or gallon, and a primer if the sheet shows one.

Common mistakes

• One coat. It looks fine wet and patchy dry.
• Using the ceiling paint on the walls, or wall paint on the ceiling. They are formulated differently and it shows.
• Buying exactly the calculated gallons for a long job. Keep a pint of leftover for touch-ups; a fresh gallon later is a different batch.
• Skipping primer on new drywall. The joints and screw heads flash through two top coats.
• Measuring the floor instead of the walls. Paint goes on the perimeter times the height; the floor area only matters for the ceiling.