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
The figures the calculator starts from. They are typical label figures, not a standard: every product prints its own spread rate on the can, and that is the one to type in.
| Paint | Coverage / gal | Notes |
|---|---|---|
| Interior wall paint | 350 sq ft | The calculator's starting figure for interior walls and ceilings |
| Exterior paint | 300 sq ft | The starting figure for siding; rough or porous surfaces take more |
| Primer | 200–300 sq ft | New drywall, bare wood, patches and stains soak up more |
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 2 doors and 2 windows at typical sizes (20 and 15 sq ft each), 350 sq ft per gallon and 2 coats, rounded up to the next half gallon. Wall area is the 8-ft figure after taking off those openings.
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. The time a single coat saves is rarely worth streaky, uneven walls.
Interior vs exterior paint
Interior paint
- Calculator starts at 350 sq ft per gallon (typical)
- Lower VOC (indoor air quality)
- Recoat time is on the can
- Formulated for spots and mildew resistance
Exterior paint
- Calculator starts at 300 sq ft per gallon (typical)
- UV-resistant, weatherproof
- Weather affects drying; follow the can
- Don't use interior paint outside: it isn't made for sun and rain
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
Trim is worked out by area like walls. A quart is a quarter of a gallon, so at 350 sq ft per gallon it spreads about 88 sq ft per coat. Both faces of a 36 × 80 in door are 40 sq ft. For baseboards and casings, multiply the run in feet by the board's height in feet. Semi-gloss stands up to knocks and wiping.
Frequently asked questions
How do I calculate how much paint I need?
Find the wall area (the distance round the room times the wall height), take off the doors and windows, multiply by the number of coats, then divide by what a gallon covers. This calculator starts at 350 sq ft a gallon, a common label figure; use the one on your can. So a 12 by 14 foot room with 8-foot walls, one door and two windows has 366 sq ft to paint, twice over is 732, and 732 ÷ 350 is 2.09 gallons. Buy 2.5.
How much does a gallon of paint cover?
It depends on the product and the surface, which is why the spread rate printed on the can is the figure to use. As a starting point this calculator uses 350 sq ft per coat for interior paint and 300 for exterior, typical label figures rather than a standard. Rough texture, bare or porous surfaces and dark colours use more paint, so lower the coverage figure for those. Primer usually covers less than finish paint.
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?
With 8-foot walls the four walls come to 384 sq ft, or 314 once two doors and two windows are taken off at typical sizes. Two coats is 628 sq ft, which at 350 sq ft a gallon is 1.8 gallons: buy 2.0. With 9-foot walls it is 2.1 gallons, so 2.5. Painting the ceiling too adds 288 sq ft over two coats and brings the room to 3.0 gallons.
Should I subtract doors and windows?
Yes for full-size openings. The calculator takes off 20 sq ft a door (a 36 × 80 in door) and 15 sq ft a window (3 × 5 ft), typical sizes rather than a standard, so measure yours if they are unusual. Small windows hardly change the total. Erring on the side of more paint is safer, since a second batch may not quite match.
How much primer do I need?
Work it out like paint, one coat, at the primer's own spread rate, which is usually lower than finish paint; 200–300 sq ft a gallon is a typical label range. For the 366 sq ft of wall in the 12 by 14 example, one coat of primer at that range is 1.2 to 1.8 gallons. Prime new drywall, patches, bare wood and stains; a sound, previously painted wall in a similar colour often doesn't need it.
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?
A ceiling is simply length × width: 168 sq ft for a 12 by 14 room. At 350 sq ft a gallon one coat is 0.48 gallons and two coats 0.96, so a gallon does it. Two coats on a 10 by 10 ceiling is 0.57 gallons (buy 1.0), and on a 20 by 20 ceiling 2.29 (buy 2.5). Ceiling paint is usually flat, which hides flaws overhead.
How much exterior paint do I need?
Use the Exterior tab: perimeter × wall height × stories, plus an allowance for gable ends (15% here, a rough rule of thumb), less doors and windows. The default house, 40 by 30 feet, two stories of 9-foot walls with gables, 3 doors and 12 windows, comes to 2,658 sq ft. Two coats at 300 sq ft a gallon is 17.7 gallons, so 18.0. Trim and touch-ups are extra, and for tall houses get quotes, since ladders and scaffolding change the job.
How long does a gallon of paint last?
Longer unopened than opened, and it keeps best sealed tightly, somewhere cool that doesn't freeze. The maker's label or website gives its own shelf life. Paint that has gone off smells sour or rotten, stays lumpy after stirring or has mould on top; don't use it, because it won't cover evenly. Decanting a part-used can into a smaller container cuts the air it sits with.