How Much Paint Do I Need? Planning One Room or a Whole House

The second trip to the paint store is where most painting budgets go wrong: the new can is a slightly different batch, the colour does not quite match and the afternoon is gone. Working out the quantity properly takes ten minutes and this guide shows how, for a single room and for a whole house.

Step 1: measure the walls

Measure the length of each wall with a tape and add them up to get the perimeter, then multiply by the wall height. A 12 by 14 foot room with 8 foot walls is (12 + 14 + 12 + 14) x 8 = 416 square feet. For a room with a bay window or alcove, measure each flat section; for a stairwell, use the longest height.

Subtract the openings. A standard door is about 20 square feet, a window about 15, a sliding patio door around 40. Our 416 square foot room with one door and two windows has 366 square feet to paint.

Step 2: decide the coats

• Same colour, same sheen, sound wall: one coat of a quality paint-and-primer may do. Check in daylight before you commit.
• Any colour change: two coats. This is the normal case.
• Dark to light, or strong reds and yellows: two coats over a tinted primer, or three coats.

Step 3: check the coverage on the can

Most interior paints state 350 to 400 square feet per gallon. That is for smooth, previously painted walls. Use a lower figure for texture, bare drywall or masonry:

So for our room: 366 square feet x 2 coats / 350 = 2.1 gallons. Buy two gallons and a quart, which also leaves paint for touch-ups.

Step 4: ceiling, trim and primer

Ceiling: length times width. The 12 x 14 room is 168 square feet, so one coat of flat ceiling paint is half a gallon; buy two quarts, or one gallon if it needs two coats.

Trim: add up the lengths of baseboard, door and window casing and crown. Most trim is 3 to 6 inches wide, so 60 linear feet of 4 inch trim is 20 square feet per coat. Trim paint is semi-gloss, needs two coats and wastes more on the brush; a quart does a typical bedroom, two quarts a living room.

Primer: one coat at normal coverage, only where it is needed: bare drywall, patches, glossy surfaces after sanding, stains, and big colour changes.

Planning a whole house

For several rooms, make a table. Measure each room once, decide the colours, and total the gallons per colour rather than per room; the store can box (pour together) the gallons of one colour so the batch is consistent across rooms.

A 5 gallon bucket is usually cheaper per gallon than five singles and guarantees one batch.

The shopping list people forget

• Roller frame, two or three covers (3/8 inch nap for smooth walls, 1/2 inch for texture) and a tray with liners.
• A 2 to 2.5 inch angled brush for cutting in, and a 1.5 inch brush for trim.
• Painter's tape, drop cloths, a putty knife and spackle for nail holes, sandpaper.
• An extension pole for ceilings. It saves the ladder and your back.
• A stir stick and a can opener; ask at the counter, they are usually free.

How long it takes

A single room is a weekend for a careful first-timer: half a day of prep (moving furniture, filling holes, taping, cutting in), a day of two coats with drying time, and an hour of cleanup. A crew of two professionals does the same room in a day, including trim, and a whole house in a week. If you are painting more than two rooms, exterior siding or any high stairwell, the time saved and the quality of the finish usually justify a quote.