Buying too little paint means a second trip to the store and mismatched batches; buying too much wastes money. Enter your room’s dimensions to find out exactly how many gallons you need.
How it works
- Wall area = perimeter × height = 2 × (length + width) × height.
- Subtract about 20 sq ft per door and 15 sq ft per window.
- Add the ceiling (length × width) if you’re painting it.
- Multiply by the number of coats.
- Divide by the paint’s coverage – about 350–400 sq ft per gallon for most interior paints.
Example
A 12 × 10 ft bedroom with 8 ft walls, one door and two windows has about 302 sq ft of wall. Two coats = 604 sq ft ÷ 350 = 1.73 gallons – buy one gallon and three quarts, or simply two gallons.
Paint needed by room size (8 ft walls, 2 coats, walls only)
| Room | Approx. paint |
|---|---|
| 10 × 10 ft (small bedroom) | ≈ 1.5 gallons |
| 12 × 12 ft | ≈ 1.9 gallons |
| 12 × 15 ft (living room) | ≈ 2.2 gallons |
| 15 × 20 ft | ≈ 3 gallons |
(Assumes one door and two windows.)
When to buy extra
- Textured or porous walls (brick, new plaster) soak up more paint – add 10–20%.
- Big colour changes (dark to light) may need a primer coat plus two top coats.
- Keep a little left over for touch-ups and label it with the room and date.
Metric version: litres of paint
Outside the US, paint is sold in litres and coverage is quoted per square metre – usually 10–12 m² per litre per coat for smooth interior walls. For a 4 m × 3 m room with 2.5 m walls, one door and two windows:
- Wall area: 2 × (4 + 3) × 2.5 = 35 m², minus about 1.9 m² for the door and 2.8 m² for windows ≈ 30.3 m².
- Two coats: 60.6 m² ÷ 11 m²/L ≈ 5.5 litres – buy a 5 L tin plus a 1 L tin, or 2 × 2.5 L plus a 1 L.
Enter metric measurements converted to feet in the calculator, or use the figures above as a quick guide.
How much primer do you need?
Primer covers a little less than paint – roughly 200–300 sq ft per gallon. Use it when painting new drywall or plaster, covering stains, switching from dark to light colours, or painting over glossy surfaces. One coat is usually enough.
Paint sheen: where to use what
| Finish | Best for |
|---|---|
| Flat / matte | Ceilings, low-traffic rooms, hiding wall imperfections |
| Eggshell | Living rooms, bedrooms, hallways |
| Satin | Kitchens, bathrooms, kids’ rooms – easy to wipe |
| Semi-gloss | Trim, doors, cabinets |
| Gloss | Furniture, railings, high-wear surfaces |
Estimating cost
Multiply gallons by the price per gallon, then add supplies: rollers, brushes, tray, painter’s tape, drop cloths and filler. For a typical bedroom, supplies add $30–60 if you’re starting from scratch. Professional painters in the US commonly charge by square foot or by room; getting two or three quotes is the best way to compare.
Painting tips that save paint
- Fill holes and sand rough patches first – uneven surfaces soak up extra paint.
- Use a quality roller cover with the right nap: shorter for smooth walls, longer for textured ones.
- “Box” your paint: mix all cans in one bucket so the colour is consistent across walls.
- Paint the ceiling first, then walls, then trim.
Real-life scenarios
- Kid’s bedroom refresh: An 11 × 10 ft room with 8 ft walls, one door, one window and two coats needs about 1.7 gallons – buy two.
- Open-plan living area: 20 × 16 ft with 9 ft walls, two doors and four windows comes to about 3.1 gallons for two coats.
- Accent wall: One 12 ft wall, 8 ft high, is 96 sq ft. Two coats need about 2.2 quarts, so buy three quarts – or a gallon if you want leftovers for touch-ups.
Related calculators
- Square footage calculator – room area
- Tile calculator – floors and backsplashes
- Concrete calculator – outdoor projects
FAQs
How much paint for a 12×12 room?
With 8 ft walls, one door and two windows, about 1.9 gallons for two coats – so buy 2 gallons.
How much paint for a ceiling?
A 12 × 12 ft ceiling is 144 sq ft; two coats need about 0.8 gallon, so one gallon is enough.
How much does a gallon of paint cover?
Usually 350–400 sq ft per coat on smooth, primed walls. Check the label – some premium paints claim up to 400+.
Do I need two coats?
Almost always for an even finish, especially with colour changes. “Paint and primer in one” products can sometimes cover in one coat over a similar colour.
Laying a new floor too? Try the tile calculator.