Sometimes yes and sometimes it makes no difference. The useful way to decide is to look at what the deduction does to the gallon count, not at the square footage alone.
What a deduction is worth
A standard door is about 21 sq ft (3 × 7 ft) and a standard window about 15 sq ft (3 × 5 ft), the sizes a Sherwin-Williams training example deducts and the ones the planner assumes. Two windows and a door take off 51 sq ft, about an eighth of a typical room’s walls.
In a 12 × 14 ft room with 8 ft walls and two coats, that decides between two sizes of purchase:
| Openings | Calculated | Buy |
|---|---|---|
| Subtracted (1 door, 2 windows) | 1.95 gal | 2 gallons |
| Ignored | 2.22 gal | 3 gallons |
Here it is worth deducting: the tighter order is 2 gallons, and ignoring the openings pushes the list to 3 gallons.
When it makes no difference
In a larger room the same openings are a smaller share of the walls. A 16 × 20 ft room with 9 ft walls needs 4 gallons with the openings subtracted and 4 gallons without. When both calculations land on the same number of gallons, the extra deduction cost you nothing but also saved nothing.
A practical way to decide
Rules of thumb that hold up
- Subtract large openings: patio doors, picture windows, wide garage or closet openings
- Skip a few small windows if the calculated amount sits comfortably inside a gallon
- Keep a small margin if your paint is on a textured wall or needs more coats, since coverage will be lower than the label
- If the calculation lands close to a whole gallon either way, buy the larger quantity
Trim, doors themselves and window frames are not part of the wall area. If you are painting those, buy for them separately.