Field Guide
How Much Paint Do I Need? Walls, Ceilings & Primer
Paint is sold by the gallon, and every can tells you how many square feet a gallon covers. So the job is: find the area you're painting, multiply by the number of coats, and divide by the coverage on the can.
1. How to Calculate Paint (The Math Explained)
Here's the exact process this calculator uses, with a 12 × 12 ft bedroom, 8 ft walls, one door and two windows as the example.
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1
Find the wall area
Add up the length of every wall (the room's perimeter) and multiply by the wall height.
Perimeter = 12 + 12 + 12 + 12 = 48 ft Wall area = 48 ft × 8 ft = 384 sq ft -
2
Subtract doors and windows
Use about 21 sq ft for a door and 15 sq ft for a window.
Openings = 1 × 21 + 2 × 15 = 51 sq ft To paint = 384 − 51 = 333 sq ft -
3
Multiply by coats and divide by coverage
Two coats is standard. Most cans cover about 350 sq ft per gallon.
Walls = 333 × 2 ÷ 350 = 1.9 → 2 gallons Ceiling = 144 × 2 ÷ 350 = 0.8 → 1 gallon
2. Coverage by Surface
The coverage on the can assumes a smooth, sealed surface. Rough, porous and unprimed surfaces soak up more paint, so set the coverage lower in the calculator.
| Surface | Coverage per gallon | Notes |
|---|---|---|
| Smooth, previously painted walls | 350–400 sq ft | The figure most cans list. |
| New drywall (after primer) | 300–350 sq ft | Prime first; bare paper and joint compound absorb paint unevenly. |
| Textured walls and ceilings | 250–300 sq ft | Orange peel, knockdown and popcorn textures have more surface. |
| Masonry, stucco and brick | 200–250 sq ft | Use a thick-nap roller and a masonry primer. |
3. Painting Tips
Box your paint. Pour all the cans of one color into a bucket and mix them, so every wall is the same shade.
Cut in, then roll. Brush the corners and edges first, then roll while the edges are still wet to avoid lap marks.
Keep a little extra. A quart of each color is handy for touch-ups later; label it with the room and the date.
Pro tip: don't forget the trim
Doors, baseboards and window trim usually get a semi-gloss or gloss paint. A quart covers the trim of a typical bedroom; a gallon is enough for several rooms.
4. Frequently Asked Questions
How much paint do I need for a 12×12 room?
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. Take off one door and two windows (51 sq ft) and you have 333 sq ft. Two coats is 666 sq ft, or 1.9 gallons at 350 sq ft per gallon: buy 2 gallons for the walls. The ceiling (144 sq ft, two coats) needs 1 more gallon.
How many square feet does a gallon of paint cover?
Most interior paints cover about 350 to 400 sq ft per gallon per coat on smooth, previously painted walls. Expect less, around 250 to 300 sq ft, on textured walls, bare drywall and masonry. The exact figure is printed on the can.
Do I need two coats of paint?
Usually yes. Two coats give even color and sheen, hide roller marks, and last longer. One coat can be enough when you repaint the same color over a clean wall in good condition. Big color changes, especially dark to light, may need primer plus two coats.
When do I need primer?
Prime new drywall and joint compound, patched areas, stains, glossy surfaces and bare wood, and use it when covering a dark color with a light one. A paint-and-primer product can replace a separate primer on walls that are already painted and in good shape.
How much paint do I need for a ceiling?
Multiply the room's length by its width for the ceiling area, multiply by the coats, and divide by the coverage. A 12 × 12 ft ceiling is 144 sq ft; two coats at 350 sq ft per gallon is 0.8 gallon, so 1 gallon. Ceiling paint is usually a flat white sold separately from wall paint.
Should I buy a 5-gallon bucket or 1-gallon cans?
A 5-gallon bucket usually costs less per gallon than five single gallons, and one batch keeps the color consistent across a large area. For small rooms, 1-gallon cans are easier to carry and store. If you buy several gallons of one color, mix them together in a bucket (called "boxing") so every wall matches.
Do I subtract doors and windows when calculating paint?
Yes, for walls. A standard door takes off about 21 sq ft and a window about 15 sq ft. For a room with only one or two openings, leaving them in gives you a little extra paint for touch-ups.