Multiply the area you are painting by the number of coats, then divide by what one gallon covers — that is the paint order. A 12x12 ft room with 8 ft ceilings is 48 ft of wall perimeter × 8 ft = 384 sq ft of wall, or 348 sq ft after one door (21 sq ft) and one window (15 sq ft), so two coats is 696 sq ft of painting. At the common 350 sq ft per gallon that is 2 gallons; at 400 sq ft per gallon it is 1.74 gallons, still 2 on the shelf. One gallon of paint covers 350–400 sq ft per coat on smooth interior walls, a quart covers about 100 sq ft, and a gallon of primer covers 200–300 sq ft.
The formula, in three steps
- Get the wall area. Perimeter × ceiling height. A 12x12 room is 2 × 12 + 2 × 12 = 48 ft around, times 8 ft high = 384 sq ft. An L-shaped room is two rectangles added together.
- Subtract the openings. Measure each door and window and deduct the area. If you skip the measuring, the standard sizes are 21 sq ft for a door and 15 sq ft for a window, which is what the paint calculator applies by default.
- Divide by coverage and multiply by coats. Gallons = net area × coats ÷ coverage per gallon. 348 × 2 ÷ 350 = 1.99, so 2 gallons.
| What you need | Formula |
|---|---|
| Wall area | (2 × length + 2 × width) × ceiling height |
| Net area to paint | wall area − (21 × doors) − (15 × windows) |
| Gallons | net area × coats ÷ 350–400 |
| Quarts instead of gallons | 1 gallon = 4 quarts; a quart covers ~100 sq ft |
| Ceiling area | length × width |
The square footage calculator handles the rooms that are not rectangles — a bay, a stairwell, a triangular gable.
How much one gallon actually covers
| Surface | Coverage per gallon, per coat | Where the number comes from |
|---|---|---|
| Smooth primed interior wall | 350–400 sq ft | Sherwin-Williams; This Old House (via Lowe’s) |
| Manufacturer calculators, in general | 350 sq ft (Benjamin Moore, Behr) to 400 sq ft (Sherwin-Williams, PPG) | Bob Vila |
| Rough, textured, unpainted or porous surface | about 250 sq ft | Bob Vila |
| Primer | 200–300 sq ft | Sherwin-Williams; This Old House |
| A quart of paint | about 100 sq ft | Sherwin-Williams; Bob Vila; This Old House |
Two gallons cover up to 800 sq ft in a single coat, which is why a two-coat job is always quoted in whole gallons plus the odd quart. The spread from 250 to 400 is not brand marketing noise: it is the surface. A gallon spread on primed drywall goes nearly twice as far as the same gallon soaked into bare stucco, T1-11 siding or new wood.
Gallons by room size
8 ft ceilings, walls only, one door and one window (36 sq ft) subtracted, two coats:
| Room | Net wall area | Two coats | At 350 sq ft/gal | At 400 sq ft/gal |
|---|---|---|---|---|
| 10 × 10 ft | 284 sq ft | 568 sq ft | 2 gal (1.62) | 2 gal (1.42) |
| 12 × 12 ft | 348 sq ft | 696 sq ft | 2 gal (1.99) | 2 gal (1.74) |
| 12 × 14 ft | 380 sq ft | 760 sq ft | 3 gal (2.17) | 2 gal (1.90) |
| 14 × 16 ft | 444 sq ft | 888 sq ft | 3 gal (2.54) | 3 gal (2.22) |
| 20 × 20 ft | 604 sq ft | 1,208 sq ft | 4 gal (3.45) | 4 gal (3.02) |
The parenthetical figure is the raw quotient before rounding — useful when you are deciding between one big can and a can plus a quart. Two patterns hold across the table: a small room almost always lands on 2 gallons, and only the 12×14 room changes its answer depending on which coverage figure you believe, which is a good argument for buying that one as 2 gallons plus a quart.
Worked example: the 12x12 room, line by line
| Step | Walls | Walls + ceiling |
|---|---|---|
| Area | 384 sq ft | 384 + 144 = 528 sq ft |
| Minus openings (1 door, 1 window) | 348 sq ft | 492 sq ft |
| Two coats | 696 sq ft | 984 sq ft |
| Gallons at 350 sq ft/gal | 1.99 → 2 gal | 2.81 → 3 gal |
| Gallons at 400 sq ft/gal | 1.74 → 2 gal | 2.46 → 3 gal |
One coat of that room is 348 sq ft, which is a hair under a single gallon at 350 sq ft per gallon — so a same-color repaint genuinely runs on one gallon, while a color change or a room with patched walls needs the second coat and the second gallon.
If the walls are new drywall or the patches are fresh, budget primer as well: 348 sq ft ÷ 200–300 sq ft per gallon is 1.2 to 1.4 gallons, so one gallon plus a quart at the 300 end, or two gallons if the surface is porous.
Ceilings, trim and doors
- Ceiling. A 12x12 ft ceiling is 144 sq ft; two coats is 288 sq ft, which is 0.82 gallons at 350 sq ft per gallon. In quarts that is 2.9 cans. Since a quart covers about 100 sq ft and two quarts usually cost about what a gallon costs, buy the gallon and keep the leftover for touch-ups. Ceilings are commonly painted in one coat rather than two.
- Baseboard and casing. A 12x12 room has 48 ft of baseboard. At 3.25 in tall that is 48 × 3.25 ÷ 12 = 13 sq ft per coat, and roughly double once you add door and window casing — under 30 sq ft for the whole room, so a single quart covers both coats.
- Doors. A standard 3 ft × 6 ft 8 in slab is 20 sq ft per side, about 42 sq ft for both faces and the edges. Two coats is 84 sq ft, which is one quart.
- Cabinets. Cabinet jobs are measured run by run: a typical kitchen works out near 156 sq ft of visible surface, doubled to 312 sq ft for two coats — a little over 3 quarts, so a gallon is the sensible buy.
Rounding up is the entire waste allowance
Tile and drywall get a 10% cut allowance; paint does not need one, because it is a liquid that stores for years and can be topped up in quarts. Round the quotient up to the next container size and treat what is left over as your touch-up stock. A job that computes to 3.08 gallons is 3 gallons plus one quart, not 4 gallons. When the number lands just above a whole gallon — 1.16, 2.17, 3.45 — the question is whether the job fits in a quart, and a quart buys about 100 sq ft of coverage per coat.
Exterior and rough surfaces
Take a single-story 40 × 28 ft house with 10 ft walls: perimeter 136 ft × 10 ft = 1,360 sq ft of siding, minus 8 windows (120 sq ft) and 2 doors (42 sq ft) = 1,198 sq ft. Two coats is 2,396 sq ft:
| Coverage assumption | Gallons for 2,396 sq ft | When it applies |
|---|---|---|
| 250 sq ft/gal | 10 gal | Rough, weathered, unpainted or highly porous siding |
| 300 sq ft/gal | 8 gal | Mildly textured or previously painted wood |
| 350 sq ft/gal | 7 gal | Painted siding in good condition |
| 400 sq ft/gal | 6 gal | Smooth primed siding |
That 6-to-10 gallon spread on the same house is the single biggest estimating risk on exteriors, and it is decided by the surface, not the brand. Deduct doors and windows as you did inside, measure gables and dormers as their own triangles, and add trim separately rather than folding it into the siding figure.
Quick checklist
- Measure each surface: perimeter × height for walls, length × width for ceilings.
- Add the areas of separate surfaces, subtract doors (21 sq ft) and windows (15 sq ft).
- Multiply by the number of coats — 2 for a color change, 1 for a same-color refresh.
- Divide by 350–400 sq ft per gallon for smooth interior walls, 250 for rough or unpainted surfaces.
- Round up to gallons and quarts, and keep the partial can for touch-ups.
- Add primer at 200–300 sq ft per gallon on bare drywall, patches, stains or dark-to-light changes.
If the same room is getting new walls before it gets paint, size them with the drywall calculator; if the paint is going on a deck rather than a wall, the deck calculator handles the boards, and the tile calculator covers the surfaces paint does not.