How to calculate how much paint you need
Paint coverage is driven by wall area, not room floor size. You add up the area of the four walls, subtract the doors and windows you won't paint, then multiply by the number of coats and divide by how far a gallon stretches.
wall area = 2 × (length + width) × height
paintable = wall area − doors − windows
gallons = (paintable × coats) ÷ 350
Worked example
A 12 ft × 12 ft room with 8 ft walls has 2 × (12 + 12) × 8 = 384 sq ft of wall. Subtract one door (about 21 sq ft) and two windows (about 15 sq ft each) and you're left with 333 sq ft. Two coats is 666 sq ft of coverage; divide by 350 and you need just under 2 gallons — so you buy 2.
How much does a gallon of paint cover?
One gallon covers roughly 350 to 400 square feet in a single coat on a smooth, primed wall. Real-world coverage drops with texture, color changes, and porous surfaces:
| Surface | Coverage per gallon | Notes |
|---|
| Smooth, primed drywall | 350–400 sq ft | Best case |
| Lightly textured wall | 300–350 sq ft | Most rooms |
| Rough / knockdown texture | 250–300 sq ft | Buy extra |
| New, unprimed drywall | 200–250 sq ft | Prime first |
This calculator uses 350 sq ft per gallon as a practical middle-ground for a typical second coat. Dark-to-light color changes almost always need two coats regardless of the math.
Frequently asked questions
How many gallons of paint for a 12x12 room?
A standard 12 by 12 room with 8-foot walls needs about 2 gallons for two coats, after subtracting a door and a couple of windows. One coat would take about 1 gallon.
Do I need two coats of paint?
Most jobs look best with two coats, especially when changing colors or painting over a darker shade. One coat can work when repainting the same color on a well-primed wall.
Should I buy primer separately?
Yes, for new drywall, bare wood, patched areas, or big color changes. Primer is calculated the same way as paint but usually needs only one coat. Paint-and-primer-in-one products can skip it on already-painted walls.
How much does it cost to paint a room?
For paint alone, a typical bedroom runs about $60 to $100 using 2 gallons of mid-grade paint at $30 to $50 per gallon. Add primer, rollers, tape, and drop cloths for a full DIY total.