How much paint do I need? It is the most-Googled DIY question, and most answers online are wrong in subtle ways that cost you either money (buying too much) or an extra trip to the store (buying too little). The math itself is simple. The trick is knowing the four numbers that go into it — wall area, openings, coats, and coverage — and adjusting for the realities of your specific room. This is the complete guide, with a lookup table for the most common room sizes at the end.
The short answer
A standard 12 ft × 12 ft bedroom with 8 ft ceilings needs about 2 gallons of paint for two coats on the walls. Add another gallon if you are painting the ceiling. Buy 3 gallons total and you are covered, with a little left over for touch-ups.
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The four numbers you need
1. Wall area
Walls are the perimeter of the room times the ceiling height. A 12 ft × 10 ft room has a perimeter of 2 × (12 + 10) = 44 feet. With 8 ft ceilings, that is 44 × 8 = 352 square feet of wall.
For non-rectangular rooms (L-shapes, alcoves), break the room into rectangles and add the perimeters. Sloped ceilings are trickier — for a typical attic with a ceiling that meets the wall at 5 ft and peaks at 8 ft, average the heights and use that in the perimeter calculation.
2. Openings to subtract
A standard interior door is about 21 sq ft (3 × 7). A standard window is about 15 sq ft (3 × 5). Subtract one door area and one window area for a typical bedroom. Use actual measurements for picture windows or French doors.
3. Number of coats
Almost every paint job needs 2 coats. One coat looks streaky in most light. Three coats is for: going from a dark color to a light color (or vice versa), painting on fresh unprimed drywall, or using a cheap paint that just refuses to cover.
4. Coverage per gallon
Quality interior latex covers about 350 sq ft per gallon on smooth, primed drywall. Adjust for your surface:
- Smooth primed drywall: 350 sq ft / gal
- Smooth drywall, no primer (1st coat): 300 sq ft / gal
- Textured drywall or popcorn: 250 sq ft / gal
- Bare wood: 200 sq ft / gal
- Brick or stucco: 150 sq ft / gal
Premium paints (Sherwin-Williams Emerald, Benjamin Moore Aura) legitimately cover more — sometimes one coat over a similar color. Cheap paints often need a third coat.
The formula
Put the four numbers together:
gallons = (wall area − openings) × coats ÷ coverage
For our 12 × 10 ft bedroom: 352 sq ft of wall, minus 21 (door) minus 15 (window) = 316 sq ft. Two coats: 316 × 2 = 632 sq ft of paint coverage needed. Divided by 350 = 1.8 gallons. Round up to 2 gallons.
Room-by-room guide
| Walls only (2 coats) | Add for ceiling | |
|---|---|---|
| Small bathroom (5 × 8 ft) | 1 gallon | + 1 quart |
| Small bedroom (10 × 10 ft) | 1.5 – 2 gallons | + 1 gallon |
| Standard bedroom (12 × 12 ft) | 2 gallons | + 1 gallon |
| Large bedroom (14 × 14 ft) | 2 – 3 gallons | + 1 gallon |
| Small living room (16 × 12 ft) | 2 – 3 gallons | + 1 gallon |
| Large living room (20 × 15 ft) | 3 – 4 gallons | + 1 gallon |
| Kitchen (12 × 14 ft, with cabinets) | 1 – 2 gallons | + 1 gallon |
| Hallway (4 × 20 ft) | 1 – 2 gallons | + 1 quart |
| Two-car garage (20 × 20 × 10 ft) | 4 – 5 gallons | + 2 gallons |
Add roughly 1 gallon if you are painting the ceiling. Add 0.5-1 quart for trim (door frames, baseboards).
Three common mistakes
Mistake 1: Forgetting the second coat
The most common cause of "I ran out of paint" is doing the math for one coat. Always assume two coats unless you are painting over an identical color in good condition.
Mistake 2: Trusting "covers 400 sq ft" on the can
That number is the maximum under ideal conditions. In a real room, with edges to cut in, mistakes to fix, and texture eating more paint than expected, plan on 350 sq ft per gallon. Premium paints sometimes hit 400; budget paints often do 250-300.
Mistake 3: Buying too many quarts
Quarts cost roughly half a gallon but contain a quarter of the paint. They make sense only when you are between sizes — like needing 1.1 gallons for a small room. Buy one gallon and one quart. Buying two quarts when you needed a gallon costs more and gives you less paint.
Primer counts
If you are using primer (always over fresh drywall, water stains, or dramatic color changes), it counts as one of your coats for the math. Primer typically covers about the same as paint (350 sq ft per gallon). Self-priming paints can skip the dedicated primer step but you still need two coats of the self-priming paint.
Two-coat rule for dark colors
How much to over-buy
Buy 10% extra. Touch-ups, wall damage two years later, a kid's chair scraping the paint — having a sealed quart of the exact color is worth far more than the $15 it costs. Write the date and the room on the can lid when you store it.
Common questions
How much paint for a whole house?
A typical 2,000 sq ft (~186 m²) house with 9 ft ceilings needs about 15-20 gallons for the interior (walls only, 2 coats), plus another 8-10 gallons for ceilings. Exteriors are different — figure on 10-15 gallons for a typical two-story house.
What about ceiling paint?
Ceiling area = length × width. A 12 × 12 ceiling is 144 sq ft. Most ceiling paints cover the same 350 sq ft per gallon, but most are flat finish so one coat often looks fine if you are painting white over white.
Does the paint sheen matter?
Coverage is roughly the same across flat, eggshell, satin, and semi-gloss. Glossy finishes show more imperfections so you may need more careful prep, but the per-gallon coverage does not change much.
Can I return unopened paint?
Yes, at all major US retailers (Home Depot, Lowe's, Sherwin Williams). You typically have 30-60 days with receipt. Custom tinted paints sometimes cannot be returned — ask at the counter. This is exactly why "buy one extra gallon" is the right strategy.
How long does paint last in the can?
Unopened: 2-10 years depending on type. Opened: 1-2 years if sealed and stored cool. Test on cardboard before using stored paint on a wall.