To calculate wall square footage, multiply the room's perimeter by the ceiling height, then subtract the area of doors and windows. That single formula gets you the paintable surface for any standard room.

If you'd rather skip the arithmetic, the paint calculator does this for you and converts the result straight into gallons. But the manual method is quick, and knowing it means you can double-check any estimate.

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The core formula

For a rectangular room, wall area comes down to two measurements:

Wall area = perimeter x ceiling height

where perimeter = 2 x (length + width).

So a room that's 12 feet by 15 feet has a perimeter of 2 x (12 + 15) = 54 feet. With 8-foot ceilings, that's 54 x 8 = 432 square feet of wall before you subtract any openings.

If you're painting the ceiling too, add its area separately: ceiling area = length x width. For our example room, that's 12 x 15 = 180 square feet on top of the walls.

Step by step

Here's the full process for any room:

  1. Measure the length and width of the room along the floor, in feet.
  2. Add them and double the result to get the perimeter: 2 x (length + width).
  3. Measure the ceiling height floor to ceiling.
  4. Multiply perimeter by height for gross wall area.
  5. Subtract openings: about 21 square feet per standard door and about 15 square feet per average window.
  6. Add the ceiling (length x width) only if you're painting it.

That's it. The result is your paintable square footage.

Subtracting doors and windows

Doors and windows don't get paint (the trim does, but you handle that separately), so subtract them from the wall total. Use these reliable averages:

  • Standard door: about 21 square feet
  • Average window: about 15 square feet

Continuing the example: our 432-square-foot room has one door and two windows. Subtract 21 for the door and 30 for the two windows (15 each), and you're left with 432 - 51 = 381 square feet of paintable wall.

Don't overthink unusual openings. For large picture windows or sliding doors, measure them directly (width x height) and subtract the actual figure. For ordinary doors and windows, the averages are close enough.

One judgment call worth making: if a room has only a single small window, subtracting its area barely changes your gallon count, and the leftover paint becomes touch-up stock. If a room is mostly glass, though, those subtractions matter a lot. The more openings you have, the more it pays to subtract them accurately rather than relying on the averages.

Turning square footage into paint

Once you have paintable square footage, the paint math is short:

  1. Multiply by your number of coats. Two coats is standard, so most rooms double the figure. See how many coats of paint you need to confirm.
  2. Divide by coverage. A gallon covers about 350-400 square feet per coat; use 350 to be safe.
  3. Round up to whole gallons.

For our 381-square-foot room at two coats: 381 x 2 = 762 square feet, divided by 350 is about 2.2 gallons, so buy three (or two gallons plus a quart for touch-ups). The full breakdown lives in our guide to paint coverage per gallon.

Handling tricky rooms

Real rooms aren't always tidy rectangles. A few adjustments:

  • L-shaped rooms: split the space into rectangles, measure each, and add the wall areas together.
  • Sloped or vaulted ceilings: measure the average height of each wall, or treat triangular gable ends as (base x height) / 2.
  • Closets and nooks: measure them as their own small rooms and add the totals.
  • Wainscoting or a chair rail: measure only the height you're actually painting, not the full wall.

For an open floor plan where rooms flow together, walk the perimeter of the whole connected space rather than treating each "room" separately. You're painting a continuous run of wall, so a continuous perimeter is the right measurement. Just remember to account for any interior partial walls or columns, which add surface on both sides.

A quick accuracy check

Two habits keep your numbers honest:

  • Measure twice. A foot off on the perimeter can swing your gallon count. Tape down your measurements and re-check before you buy. (Measure twice. Buy once.)
  • Round up, never down. It's better to have a half-gallon left for touch-ups than to run short mid-wall.

Before you start, mask your trim and edges with painter's tape so your measured wall area stays close to that, with clean lines and less overspill onto baseboards.

The summary: perimeter times height, minus doors and windows, plus the ceiling if you're painting it. Get that figure right and every step after it, coats, gallons, and budget, falls neatly into place.