About Paint Calculator (Gallons of Paint & Primer)
The paint calculator estimates how many gallons of paint you need for a room. Enter the room length, width, and ceiling height — the wall area is the perimeter times the height — or type the wall square footage directly. Doors and windows are deducted at representative areas (21 ft² per door, 15 ft² per window, both editable), along with any custom exclusion such as a tiled backsplash, and an optional ceiling adds its own area.
Gallons come from the spread rate printed on the can: paintable area × coats ÷ spread rate, rounded up to whole gallons. The 350 ft²/gal default is a representative value — major manufacturers such as Sherwin-Williams and Behr publish 350–400 ft² per gallon for smooth, sealed surfaces — and textured, porous, or previously unpainted walls cover less, so always check your specific product. An optional primer line figures one coat at its own, lower rate.
How It Works
- Pick an entry mode: room dimensions (wall area = 2 × (length + width) × ceiling height, with the ceiling as length × width) or a direct wall-area entry with its own ceiling-area field.
- Enter the door and window counts. Each door deducts a representative 21 ft² and each window 15 ft² — edit the per-unit areas for oversized openings — and add a custom exclusion for tile, cabinetry, or wainscot that will not be painted.
- Set the number of coats (two is the standard finish) and the spread rate from your paint can; the 350 ft²/gal default is the conservative end of the published 350–400 range.
- Toggle primer on for bare drywall, patches, stain blocking, or a drastic color change — it is always a single coat at its own rate (default 275 ft²/gal).
- Read the whole-gallon paint and primer counts, the per-coat gallons, and the paintable-area breakdown; the chart compares the buy count at one, two, and three coats.
Worked Example
Take the 12 × 14 bedroom row from the table below. With 8 ft ceilings the perimeter is 2 × (12 + 14) = 52 ft, so the gross wall area is 52 × 8 = 416 ft². One door deducts 21 ft² and two windows deduct 30 ft², leaving 365 ft² of paintable wall. Two coats at 350 ft²/gal need 365 × 2 ÷ 350 = 2.09 gallons — buy 3 gallons, and the part-full third can becomes the touch-up reserve. Priming the same walls at 275 ft²/gal takes 365 ÷ 275 = 1.33 gallons — buy 2 gallons (or one gallon plus a quart if your line sells quarts). Painting the ceiling too adds 12 × 14 = 168 ft², bringing the paintable area to 533 ft² and the two-coat purchase to 4 gallons.
How much paint do I need for a room?
Whole-gallon buys for common repaint jobs, all at two coats and the tool’s default 350 ft²/gal spread rate, with primer as a single coat at 275 ft²/gal. Doors deduct the representative 21 ft² apiece and windows 15 ft²; ceilings are excluded throughout. The first five rows use room-dimension mode (wall area = perimeter × ceiling height); the apartment row enters its wall square footage directly.
| Job | Paintable area (ft²) | Paint (gal, 2 coats) | Primer (gal, 1 coat) |
|---|---|---|---|
| 10 × 12 bedroom, 8 ft ceilings, 1 door + 1 window | 316 | 2 | 2 |
| 12 × 14 bedroom, 8 ft ceilings, 1 door + 2 windows | 365 | 3 | 2 |
| 15 × 20 living room, 9 ft ceilings, 2 doors + 3 windows | 543 | 4 | 2 |
| 20 × 20 great room, 10 ft ceilings, 2 doors + 4 windows | 698 | 4 | 3 |
| 16 × 24 basement rec room, 8 ft ceilings, 1 door + 2 windows | 589 | 4 | 3 |
| Whole 1,200 ft² apartment — 3,200 ft² of wall, 8 doors + 10 windows | 2,882 | 17 | 11 |
Formulas
- Wall area (room mode)
A_walls = 2 × (L + W) × H- Paintable area
A_paint = A_walls − n_doors × A_door − n_windows × A_window − A_custom + A_ceiling- Gallons of paint
gal_paint = ceil( A_paint × coats ÷ spread_rate )- Gallons of primer (optional, always one coat)
gal_primer = ceil( A_paint ÷ primer_rate )
Standards & References
- Representative trade practice — spread rates vary by product; Sherwin-Williams and Behr publish 350–400 ft²/gal for smooth, sealed surfaces
- Representative opening deductions — 21 ft² per door (3 ft × 7 ft nominal) and 15 ft² per window, both editable
Frequently Asked Questions
How much area does a gallon of paint cover?
Major manufacturers such as Sherwin-Williams and Behr publish 350–400 ft² per gallon for one coat on a smooth, sealed surface, which is why the calculator defaults to a conservative 350. Real coverage varies with the product, the applicator, and the wall: heavy texture, bare drywall, masonry, or a first coat over patches can drop coverage well below 300 ft²/gal. Always use the spread rate printed on your specific can.
Why does the calculator default to two coats?
Two coats are the standard interior finish: the first coat seals and evens the surface while the second builds a uniform film, consistent color, and washable durability. One coat is usually enough only when repainting a similar color with a premium self-priming paint, and a third coat comes into play for drastic color changes — especially deep reds and yellows over light walls or light colors over dark ones.
When do I need primer?
Prime bare drywall or fresh joint compound, patched or skim-coated areas, stains that need blocking (water marks, smoke, tannin), glossy surfaces that need adhesion, and drastic color changes where a tinted primer saves a finish coat. Repainting a sound, similar-color wall usually does not need a separate primer, particularly with paint-and-primer-in-one products. The calculator figures primer as a single coat at its own, lower spread rate.
How much wall area do doors and windows really take up?
The trade convention deducts about 21 ft² per door — a nominal 3 ft × 7 ft slab — and about 15 ft² per average window. Both are editable in the calculator, so measure oversized openings such as double doors, sliders, or picture windows and enter their real areas. A custom exclusion field handles unpainted surfaces like a tiled backsplash, wainscot, or a run of cabinets.
Should I buy the exact gallons or round up? What about quarts?
The calculator rounds up to whole gallons because paint is sold that way and a part-full can from the same batch is invaluable for touch-ups. When the exact need is just over a whole gallon — say 2.1 gallons — many lines also sell quarts, so two gallons plus a quart can beat a third gallon on price. Avoid cutting it exactly to the estimate: running out mid-wall risks a visible lap mark from a new batch.
How many gallons of paint do I need for a bedroom?
Two coats on a typical bedroom is a 2–3 gallon purchase at the 350 ft²/gal default: a 10 × 12 room with 8 ft ceilings nets 316 ft² of wall after a door and a window and rounds up to 2 gallons, while a 12 × 14 room with a second window nets 365 ft² and needs 3 — it works out to 2.09 gallons, and paint sells whole. Painting the ceiling as well adds roughly a gallon more per room.
How much paint do I need for 1,000 square feet of wall?
At the representative 350 ft²/gal spread rate, 1,000 ft² takes 1,000 ÷ 350 = 2.86 gallons per coat — 3 whole gallons for one coat, and 1,000 × 2 ÷ 350 = 5.71 → 6 gallons for the standard two-coat finish. Textured or porous walls cover less: at the 250 ft²/gal end of the range the same two-coat job becomes 8 gallons, which is why the spread rate on your specific can matters more than any average.
How much paint does a whole apartment need?
Sum the wall areas room by room and enter the total in direct mode. The apartment row in the table above assumes 3,200 ft² of wall for a 1,200 ft² unit; deducting 8 doors and 10 windows at the representative areas leaves 2,882 ft², which at two coats and 350 ft²/gal rounds up to 17 gallons of paint — plus 11 gallons of primer if every surface is being primed. Buying in 5-gallon buckets usually beats 17 singles at that volume.
Does wall texture change how much paint I need?
Yes, substantially. Published spread rates assume smooth, sealed surfaces; orange peel and knockdown textures add real surface area and porosity, and heavy textures, stucco, or bare masonry can cut coverage by a quarter to a half. If your walls are textured or unusually porous, lower the spread rate toward the 250 ft²/gal end of the range — or prime first so the finish coats spread closer to their rated coverage.