Measuring a room for paint
You need three numbers and a door count. Fifteen minutes with a tape measure beats any amount of squinting at the walls and guessing "two gallons, probably."
The estimator method
Wall area is perimeter times height. Measure the room's length and width at the floor, add them, double it: that's the perimeter. Times the ceiling height. A 14x11 bedroom under 8 ft ceilings: (14 + 11) × 2 = 50 ft of perimeter, times 8 is 400 sq ft of gross wall.
Then the deductions, and this is where estimating practice has already done the averaging for you: 20 sq ft per door, 15 per window. Our bedroom has an entry door, a closet door, and two windows: 400 − 40 − 30 = 330 sq ft of paintable wall. Two coats is 660 sq ft of spreading, which at 350 sq ft per gallon is 1.9 gallons. Buy two, or drop the numbers into the calculator and let it do the rounding.
When to skip the deductions
When the openings are small or few, skipping them just pads your margin. One door and one average window is 35 sq ft, a tenth of a small room's wall; leaving it in the total is how many pros build their waste allowance without thinking about it. Deduct when openings are big: sliding closet doors, a picture window, a wall that's half cased opening. And never deduct for furniture, radiators, or things you plan to cut in around; you still paint behind and above.
The awkward rooms
Stairwells: measure the wall's average height. A stairwell wall that starts at 8 ft and ends at 17 runs about 12.5 ft average; times its length, and be honest about the landing walls. Vaulted rooms: the gable-shaped end walls are a rectangle plus a triangle, and the triangle is width times rise divided by two. Board-and-batten or heavy trim walls: measure to the top of the paint, not the ceiling.
Write the numbers on painter's tape and stick it inside a cabinet. Future you, holding a chip card two years from now, will be grateful.