How to Estimate Paint for Any Room

3 August 2026 · 4 min read

Running out of paint halfway up the last wall is the worst way to spend a Saturday. The second trip costs an hour, and the replacement can is mixed from a different batch — close, but often close enough to see in daylight across a finished wall.

Estimating properly takes about five minutes with a tape measure. Here's the whole method.

Step 1: Measure the walls

Run a tape along the floor at the base of each wall and write down the four lengths. Don't average them, and don't measure diagonally across the room — you want the actual run of each wall, because that's what you'll be rolling.

Add the four numbers together to get the room's perimeter. A 12 by 14 foot room gives you 12 + 14 + 12 + 14, or 52 feet.

If the room isn't a plain rectangle, break it into rectangles. A closet, an alcove or an L-shaped extension each get measured as their own little room, then added to the total. Chimney breasts and bay windows are worth measuring separately too — they add more surface than people expect.

Step 2: Get the ceiling height

Most homes are 8 feet floor to ceiling. Newer builds are often 9 or 10, and older houses can be anything at all, so measure rather than assume. One misjudged foot on a 52-foot perimeter is 52 square feet of wall you didn't buy paint for.

Multiply the perimeter by the height and you have your gross wall area. For the example room: 52 × 8 = 416 square feet.

Vaulted or sloped ceilings need one extra step. Work out the rectangle up to the point where the slope starts, then treat the triangle above it separately — half the base times the height — and add the two together.

Step 3: Subtract doors and windows

You aren't painting the openings, so take them out. Standard sizes are close enough for an estimate:

• Interior door: about 20 square feet (3 feet by 6 feet 8 inches)
• Average window: about 15 square feet

One door and two windows removes 20 + 30 = 50 square feet, leaving 366 square feet of actual wall in our example room.

Two judgement calls here. Large picture windows, sliding doors and double doors are considerably bigger than the standard figures, so measure those properly rather than counting them as one opening. And a small transom or porthole window isn't worth subtracting at all — you'll spend more paint cutting in around it than you save.

Step 4: Decide on coats

Two coats is the default, and it's the right answer for most jobs. One coat only works when you're refreshing the same colour over a sound, clean wall. Three coats is for a genuinely difficult change — a strong red or deep navy going over pale, or a light colour going over something dark.

If you're covering a dark colour with a light one, prime first rather than adding coats. Primer blocks the old colour for far less money than a third gallon of finish paint, and it gives you a uniform base so the topcoat colour reads true.

Now the coverage figure. A gallon of interior wall paint covers roughly 350 square feet in one coat on smooth, previously painted drywall. That's the number most manufacturers print, and it's what the paint calculator uses. Porous surfaces drink far more — expect 250 to 300 square feet on new drywall or heavy texture, and as little as 150 on rough masonry.

The worked example

Always round up. You can't buy 2.09 cans, and the alternative to rounding up is the second trip you were trying to avoid. Buy an extra quart on top for touch-ups — scuffs, nail holes and the spot behind the door you'll find three weeks later.

One last thing worth knowing: this estimate covers walls only. Ceilings are usually painted in a dedicated flat ceiling paint rather than the wall colour, so work them out separately — length times width, divided by 350 per coat. Trim, doors and frames are a different paint again, and a quart usually handles an average room.