Paint gets ordered by the square foot of wall, not the square foot of floor. Multiply the room perimeter by the ceiling height, subtract the doors and windows, and you have the number that drives both the gallons and the labor.
A 14 by 12 room with 9-foot walls has 468 square feet of wall; take out 87 for two doors and three windows and you paint 381.
The example
Three rooms measured floor-plan style: length, width, wall height, and the total opening area to deduct.
| A | B | C | D | E | F | |
|---|---|---|---|---|---|---|
| 1 | Room | Length | Width | Height | Openings | Paintable |
| 2 | Master | 14 | 12 | 9 | 87 | 381 |
| 3 | Living | 20 | 16 | 10 | 120 | 600 |
| 4 | Hall | 24 | 5 | 9 | 40 | 482 |
The formula
The perimeter is twice the length plus width; height turns it into area; the openings come straight off the top:
How it works
Read it left to right:
B2+C2is half the perimeter; the2*in front makes it the full wall run around the room.*D2multiplies that run by the wall height to give gross wall area.-E2removes the openings — a 3 by 7 door is 21 square feet, a 3 by 5 window is 15.- Keep the openings in their own column so a change of one window does not force you to re-key the whole room.
Divide the paintable area by your paint's coverage — usually 350 to 400 square feet a gallon — and round up for the gallons to buy.
Try it: interactive demo
Enter the room size, wall height, and the total area of doors and windows.
Variations
Add a second coat
Two coats roughly doubles the coverage math, though the second goes on faster.
Gallons to buy
Round the paintable area up to whole gallons at 350 sq ft each.
Pitfalls & errors
Do not deduct trim, baseboard, and crown from the wall area — they are a separate line, usually priced by the linear foot, not by subtracting from the walls.
Only deduct openings bigger than a normal window. Painters generally paint right around outlets and switch plates, so those are not worth subtracting.
Practice workbook
Frequently asked questions
Should I paint the ceiling into this number?
What opening size is worth deducting?
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