Perimeter times height, minus the openings. That part is easy. The number that decides how much paint you buy is the coverage rate, and it varies by a factor of two depending on what you are painting.
Add up the length of every wall in the room to get the perimeter, then multiply by the ceiling height.
A room 12 by 15 feet has a perimeter of 54 feet. At an 8 foot ceiling that is 432 square feet of wall before openings.
Measure the ceiling height rather than assuming it. Eight feet is common in newer US construction but nine and ten foot ceilings are not unusual and older properties vary considerably. It takes five seconds with a laser measure pointed at the ceiling and it scales every subsequent number.
A standard interior door is roughly 20 square feet. A typical window is 12 to 15.
Whether to subtract them is a judgement rather than a rule. Professional decorators often do not on a single room, because the saving is smaller than the amount of paint you want left over for touch-ups, and because cutting in around an opening uses more paint per square foot than rolling a flat wall.
On a large job, or a room with a lot of glazing, subtract them. A room with three large windows and two doors can easily have 80 square feet of openings, which is a fifth of a small room's wall area and a full gallon of paint.
| Surface | Coverage per gallon, one coat |
|---|---|
| Smooth drywall, previously painted | 350 to 400 sq ft |
| Lightly textured walls | 300 to 350 sq ft |
| Heavy texture, bare drywall, stucco | 200 to 300 sq ft |
| Primer | around 300 sq ft |
Look at the spread in that table. Smooth previously painted drywall gives you 350 to 400 square feet per gallon. Bare drywall or heavy texture gives you 200 to 300. That is nearly double the paint for the same wall.
Texture is the reason. A textured surface has considerably more actual area than its measured dimensions suggest, and every peak and hollow takes paint. Bare drywall is worse again because the paper and the compound absorb rather than just coat.
The figure printed on the tin is the manufacturer's best case, which means smooth, sealed, and applied by someone efficient. Plan on the lower end of the range for your surface type rather than the number on the label.
One coat is rarely enough. Two is the working assumption for almost any repaint, and you need more when going light over dark, covering a strong colour, or painting onto new drywall.
New drywall needs primer first, and primer covers around 300 square feet per gallon, so it is a separate calculation at its own rate rather than an afterthought.
So the full sum is: wall area, divided by the realistic coverage rate for your surface, multiplied by the number of coats, plus primer separately if needed.
For our 432 square foot room with openings subtracted, say 380 square feet, on lightly textured previously painted walls at 320 square feet per gallon: 380 divided by 320 is 1.19 gallons per coat, so 2.4 gallons for two coats. Buy three, because the alternative is running out mid-wall.
Tinted paint is mixed to order and batches vary slightly. A wall finished from a second batch can show a visible difference in the right light, and the fix is repainting the whole wall. Buying an extra can costs less than that, and leftover paint is genuinely useful for touch-ups.
Ceiling area is the floor area, so measure it as length times width rather than trying to derive it from the walls. Ceiling paint is usually a different product and covers at its own rate.
Trim is measured linearly rather than by area: total the run of baseboard, architrave, window casing and any picture rail. Trim paint covers considerably less per gallon than wall paint because of the surface profile and because you are brushing rather than rolling. Measuring trim runs is covered here.
Keeping the three as separate line items stops the classic error of buying enough total paint but the wrong split between them. The general wall measuring method is here, and wallpaper works on entirely different arithmetic.
For the overview, our main laser measure guide covers the picks and the reasoning in one place.