How to Estimate Paint for a Room Makeover
Buying paint is where a lot of projects go sideways — you either run out two-thirds through the second coat, or you end up with three leftover gallons in the garage forever. Here's the simple way to get it right.
The formula
Paint coverage is sold in square feet per gallon. Most interior wall paints cover about 350–400 sq ft per gallon per coat. You'll almost always want two coats. So:
Where wall area = perimeter of the room × ceiling height, minus roughly 20 sq ft per standard door and 15 sq ft per average window.
Step by step
- Measure the perimeter. Add up the length of every wall. A 12×14 ft room = (12+14) × 2 = 52 linear ft.
- Multiply by ceiling height. 52 ft × 8 ft = 416 sq ft of gross wall area.
- Subtract openings. One door (−20) and two windows (−30) → 416 − 50 = 366 sq ft of actual paintable wall.
- Apply the formula. (366 ÷ 375) × 2 coats ≈ 1.95 gallons. Round up: buy 2 gallons.
Worked examples
| Room | Paintable wall area | Paint (2 coats) |
|---|---|---|
| Small bathroom (5×8, 8 ft) | ~180 sq ft | 1 gallon |
| Bedroom (12×12, 8 ft) | ~330 sq ft | 2 gallons |
| Living room (15×20, 9 ft) | ~580 sq ft | 3 gallons |
| Open kitchen/dining (14×24, 9 ft) | ~640 sq ft | 4 gallons (incl. accent) |
Don't forget primer
You need primer in three situations: bare drywall or patched areas, a big color change (especially dark-to-light), or painting over glossy or stained surfaces. Primer covers a little less than paint — figure 300 sq ft per gallon for one coat. If your walls are already painted a similar color and in good shape, a quality paint-and-primer-in-one is usually fine for one coat of primer's worth of hiding.
Buy a little extra on purpose
Always round up to the next gallon, and keep the leftover for touch-ups — wall dings happen, and matching a color later is a hassle even with the formula written on the lid. Write the room name and date on the can with a marker before you store it.
A few things that change your number
- Deep or saturated colors (reds, deep blues, true black) often need three coats — plan accordingly.
- Textured walls (orange peel, knockdown) drink more paint; drop coverage to ~300 sq ft/gal.
- Sprayer vs roller: spraying uses noticeably more paint than rolling, though it's faster on large areas.
Skip the math entirely
BuildIt AI estimates materials — including paint — straight from a photo and your room dimensions, then builds a shoppable list. Redesign first, then buy exactly what you need.
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