How this paint calculator works
Paint is an area problem with two multipliers — coats and coverage — and two deductions that most people forget:
gallons = area × coats ÷ coverage, rounded up · 350 ft²/gal typical
Worked example: how much paint for a 12 × 12 room
Walls run 2 × 24 × 8 = 384 ft²; a door and two windows deduct 51, leaving 333 ft². Two coats is 666 ft² of spreading — at 350 ft² per gallon that's 1.9, so you buy 2 gallons (about $70). Tick the ceiling and it becomes 3.
How many gallons of paint by room size?
| Room | Wall area | Gallons |
|---|---|---|
| 10 × 10 × 8 | 269 ft² | 2 |
| 12 × 12 × 8 | 333 ft² | 2 |
| 14 × 16 × 8 | 429 ft² | 3 |
| 16 × 20 × 9 | 597 ft² | 4 |
Paint FAQ
How much paint do I need for a 12 × 12 room?
Two gallons for two coats on the walls: 384 ft² of wall minus a door (21 ft²) and two windows (30 ft²) leaves 333 ft², times two coats at 350 ft² per gallon = 1.9 gallons. Painting the ceiling too takes it to three gallons.
How much does a gallon of paint cover?
About 350 ft² per coat on smooth, previously painted walls — the number printed on most cans is 350–400. Textured, porous or bare surfaces drink more; coverage of 250–300 is realistic there, and the coverage field accepts whatever your can says.
Do I need two coats of paint?
Almost always yes for a color change, and usually for a same-color refresh — one coat covers unevenly as it flashes against patched spots. Quality paint-and-primer products can one-coat over similar colors; drastic changes may need a tinted primer plus two.
When do I need primer?
Bare drywall, patched areas, stains, glossy surfaces, and going light over dark. The primer checkbox adds one coat of coverage at the same rate — cheap insurance compared to a third coat of finish paint.
How much does it cost to paint a room?
The default room is about $70 in paint at $35 a gallon, plus roughly $20–40 in sundries (tape, roller covers, patching). Hiring it out, interior painting typically runs $2–6 per square foot of floor area in 2026 — labor is most of the bill.
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