How to calculate paint needed
wall area = 2 × (length + width) × height
gallons = (net area × coats) ÷ coverage per gal
Example: 12×14 bedroom with 8ft ceilings, 1 door, 2 windows, 2 coats
- Perimeter: 2 × (12 + 14) = 52 ft
- Wall area: 52 × 8 = 416 sq ft
- Subtract door (20) + 2 windows (30) = 366 sq ft net
- For 2 coats: 366 × 2 = 732 sq ft
- Gallons: 732 ÷ 350 = 2.1 gallons → buy 2.5 gallons
Paint coverage by type
| Finish | Coverage / gal | Best for |
|---|---|---|
| Flat / Matte | 350–400 sq ft | Ceilings, low-traffic walls, hides imperfections |
| Eggshell | 350 sq ft | Living rooms, bedrooms — slight sheen, more washable |
| Satin | 350 sq ft | Kitchens, bathrooms, hallways — durable, easy to clean |
| Semi-gloss | 300–350 sq ft | Trim, doors, cabinets — wipes clean easily |
| High-gloss | 300 sq ft | Doors, cabinets, furniture — hardest, shiniest finish |
| Primer | 200–300 sq ft | Before paint on new/bare/stained surfaces |
| Exterior latex | 250–350 sq ft | Siding, stucco, wood exteriors |
| Exterior oil-based | 300–400 sq ft | Metal, harsh weather, historical applications |
Paint needed by room size (2 coats)
| Room | Wall area | 8 ft ceiling | 9 ft ceiling |
|---|---|---|---|
| 10×10 bedroom | 250 sq ft | 1.5 gal | 2 gal |
| 10×12 bedroom | 282 sq ft | 2 gal | 2 gal |
| 12×12 bedroom | 314 sq ft | 2 gal | 2.5 gal |
| 12×14 bedroom | 346 sq ft | 2 gal | 2.5 gal |
| 14×16 living | 410 sq ft | 2.5 gal | 3 gal |
| 16×20 great rm | 506 sq ft | 3 gal | 3.5 gal |
| 20×24 master | 634 sq ft | 4 gal | 4.5 gal |
Assumes 1 door + 2 windows, 350 sq ft per gallon coverage, 2 coats.
One coat vs two coats
Two coats is standard for most paint jobs. You can sometimes get away with one coat when:
- Painting the same color over itself (touch-up)
- Using premium paint-and-primer-in-one on a clean, primed surface
- Light color over light color
- Above a fresh primer coat (primer + 1 topcoat = 2 total coats of coverage)
Always plan for 2 coats unless you have strong reason to skip one. Saving 30 minutes of work isn't worth streaky, uneven walls.
Interior vs exterior paint
Interior paint
- 350 sq ft per gallon
- Lower VOC (indoor air quality)
- Dries in 1–2 hours
- Formulated for spots and mildew resistance
Exterior paint
- 300 sq ft per gallon
- UV-resistant, weatherproof
- Dries in 4–8 hours
- Never use interior paint outside — it will fade and peel in months
Paint finish guide
- Flat / matte — ceilings, low-traffic bedrooms. Hides imperfections best; hardest to clean.
- Eggshell — most bedrooms and living rooms. Slight sheen, somewhat washable.
- Satin — kitchens, bathrooms, hallways, kids' rooms. Good balance of sheen and cleanability.
- Semi-gloss — trim, doors, window frames. Wipes clean, resists fingerprints.
- High-gloss — cabinets, furniture, very high-moisture areas. Most durable, shows every imperfection.
Paint for trim and doors
Rule of thumb: 1 quart of paint covers about 1 standard interior door (both sides) or 150 linear feet of trim (baseboards, crown molding). A typical room has 60–80 linear feet of baseboards plus 1–2 doors — plan on 1 quart per 2 rooms for trim, plus 1 quart for every 4 doors. Use semi-gloss for durability.
Frequently asked questions
How do I calculate how much paint I need?
Calculate wall area: perimeter (2×(L+W)) × ceiling height, then subtract 20 sq ft per door and 15 sq ft per window. Divide by paint coverage (350 sq ft/gallon is typical) and multiply by the number of coats. For a 12×14 room with 8ft ceilings: perimeter 52 ft × 8 ft = 416 sq ft, minus 1 door (20) and 2 windows (30) = 366 sq ft. For 2 coats: 732 sq ft ÷ 350 = 2.1 gallons. Round up to 2.5 gallons, or 3 gallons for safety.
How much does a gallon of paint cover?
Standard interior paint covers 350–400 sq ft per gallon on smooth, primed surfaces. Textured walls and darker colors may need more paint — reduce coverage by 10–25%. Primer covers less: 200–300 sq ft. Exterior paint covers 250–350 sq ft depending on siding type. The gallon label shows 'coverage' — always assume the lower end for budgeting.
Do I need two coats of paint?
Usually yes, especially when: (1) changing colors (e.g., dark to light or vice versa), (2) painting over unprimed surfaces, (3) using flat paint on textured walls, (4) when the new color is lighter than the existing. Skip the second coat only when painting the same color over itself with premium paint and good coverage on the first coat. Paint-and-primer-in-one still typically needs 2 coats for best results.
How much paint for a 12x12 room?
A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area, or ~314 sq ft after subtracting typical doors and windows. For 2 coats: 628 sq ft ÷ 350 per gallon = 1.8 gallons. Round up to 2 gallons. For 9-foot ceilings, plan on 2.5 gallons. Add another half-gallon if you're also painting the ceiling (144 sq ft × 2 coats = 288 sq ft = ~1 gallon).
Should I subtract doors and windows?
Yes, but don't be too aggressive. Subtract 20 sq ft per standard door and 15 sq ft per window. Don't subtract small windows or decorative openings. If a room has many doors and windows, the deductions matter; for simple rooms they're minor. Better to have too much paint than run out in the middle of a job — touch-ups later never quite match.
How much primer do I need?
Primer covers 200–300 sq ft per gallon — less than topcoat paint. For a typical room needing 2 gallons of paint, plan on 1 gallon of primer for new/bare/stained surfaces. Skip primer only when repainting a similar color with a paint-and-primer combo product on a previously painted surface in good condition. Prime wood, drywall patches, and stain blockers always.
What is the best paint finish for a bedroom?
Eggshell is the most popular choice for bedrooms — low sheen (subtle glow), washable, and hides wall imperfections. Flat/matte is also fine for bedrooms (best at hiding imperfections) but less washable. Satin works if you have kids or pets. Avoid semi-gloss and gloss for bedroom walls — too shiny. For bedroom trim and doors, use semi-gloss.
How much paint do I need for a ceiling?
Ceiling area = length × width. For a 12×14 room: 168 sq ft. At 350 sq ft/gallon coverage: 168 ÷ 350 = 0.48 gallons per coat. Two coats = 0.96 gallons — buy 1 gallon with some to spare. Use ceiling paint (usually flat white) — it's formulated to minimize drips and hide imperfections. A 10×10 ceiling needs half a gallon; a 20×20 great room ceiling needs about 2.5 gallons.
How much exterior paint do I need?
Exterior paint coverage is about 250–350 sq ft per gallon. Calculate house perimeter × wall height, add 15% for gable ends, subtract doors and windows. A typical 2,000 sq ft 2-story house has about 2,400 sq ft of exterior wall area. For 2 coats: 4,800 sq ft ÷ 300 = 16 gallons. Buy 2 extra gallons for trim and touch-ups. Get professional quotes for anything over 2 stories — ladders and scaffolding matter.
How long does a gallon of paint last?
Unopened paint: 5–10 years stored in a cool, dark place. Once opened: 2–5 years if sealed tightly. Signs paint has gone bad: chunky texture, strong sour/rotten smell, won't mix back smooth after stirring, mold on surface. Never use visibly bad paint — it will streak, peel, and not cover properly. For partial gallons, transfer to a smaller container to reduce air exposure.