The 350 rule — and why it lies a little
350 sq ft per gallon is a lab number: perfect application, smooth sealed surface, ideal humidity. Real walls drink differently. Rough or unprimed drywall, deep colors, and hot dry rooms that flash-dry the paint can push real coverage down to 250–300 sq ft per gallon. Cheap paint also thins coverage — solids content matters more than price per gallon. That's why the paint calculator sticks with 350 as its baseline and shows you the quart breakdown, because the difference between 1.4 and 1.6 gallons is the difference between one can and two.
Doors and windows: the honest deduction
A standard interior door is 3×7 ft — 21 sq ft — and a typical window is 3×5 ft, or 15 sq ft. Those are the exact figures the paint calculator deducts per opening, and they matter less than most people expect. Two doors and four windows in a room remove 102 sq ft — about a third of a gallon per coat. Cut-in loss at edges, paint left in the roller cover, and the dried-out tray all push real consumption back up, which is why so many painters treat openings as a wash. The calculator takes the deduction and adds nothing back, so treat it as a small honest credit, not a way to drop a gallon.
When one coat is actually enough
- Repainting the same or a very similar color over a clean, intact finish — the most common one-coat win.
- Paint-and-primer products over light colors — though “one coat” claims usually mean “one coat over primer,” and you're reading this because you don't have a primered wall.
- Never for new drywall, patch spots, dark-to-light transitions, reds/oranges/blues, or anything glossy-to-flat. These want primer + two coats, full stop.
Gallons by room size
Every figure below comes out of the same formula the calculator runs: perimeter × wall height, minus 21 sq ft per door and 15 sq ft per window, × coats, ÷ 350, rounded up to the nearest tenth of a gallon. Assumptions here are 8 ft walls, one door, and two windows.
| Room | Perimeter | Wall area | 1 coat | 2 coats | + ceiling |
|---|---|---|---|---|---|
| 10 × 10 | 40 ft | 269 sq ft | 0.8 gal | 1.6 gal | 2.2 gal |
| 12 × 12 | 48 ft | 333 sq ft | 1.0 gal | 2.0 gal | 2.8 gal |
| 12 × 16 | 56 ft | 397 sq ft | 1.2 gal | 2.3 gal | 3.4 gal |
| 14 × 14 | 56 ft | 397 sq ft | 1.2 gal | 2.3 gal | 3.4 gal |
| 16 × 16 | 64 ft | 461 sq ft | 1.4 gal | 2.7 gal | 4.1 gal |
| 20 × 20 | 80 ft | 589 sq ft | 1.7 gal | 3.4 gal | 5.7 gal |
The ceiling column is 2 coats on walls and ceiling. Two things worth noticing. First, 12×16 and 14×14 land on identical numbers: the calculator derives ceiling area from perimeter, and both rooms have the same 56 ft perimeter, so both get a 196 sq ft ceiling. Second, going from one coat to two is not a clean doubling — 1.2 gallons becomes 2.3, not 2.4, because rounding up to the nearest tenth twice is not the same as doubling a rounded number.
And when an answer lands on .8, look again before you buy: 2.8 gallons is 2 gal + 4 quarts, and 4 quarts is a gallon. Buy 3 and keep the leftover for touch-ups. The calculator prints that breakdown on purpose — paint counters sell quarts, and a tenth-of-a-gallon answer is not something you can carry out of the store.
Run your room, not a rule of thumb
Room dimensions, ceiling height, door/window count, and coat preference all shift the answer by a full gallon. The paint calculator takes all four and prints the exact quarts. That link opens on the 12 × 12 room from the chart — 48 ft of wall, one door, two windows, two coats, 2.0 gallons — so you can change one field at a time and see which one moves the answer before you drive to the store.