To estimate paint, find the wall area, subtract doors and windows, multiply by the number of coats, and divide by the coverage per gallon. A 12 × 14 ft room with 8-foot ceilings has 416 ft² of wall; minus one door and two windows (about 51 ft²) that leaves 365 ft². Two coats at 350 ft² per gallon need 365 × 2 ÷ 350 ≈ 2.1 gallons, so buy 2 gallons plus a quart.
The paint formula
The first part of the wall formula, 2 × (length + width), is the room’s perimeter. Multiplying by the wall height gives the area of all four walls at once.
Step-by-step room example
Room: 12 ft × 14 ft, 8 ft ceilings, one door and two windows.
- Perimeter: 2 × (12 + 14) = 52 ft.
- Wall area: 52 × 8 = 416 ft².
- Openings: door 3 × 7 = 21 ft²; two 3 × 5 windows = 30 ft². Total 51 ft².
- Paintable area: 416 − 51 = 365 ft².
- Paint: 365 × 2 coats ÷ 350 ft²/gal = 2.09 gallons.
2.09 gallons = 2 gallons + 0.09 gallon
0.09 gallon × 4 quarts = 0.34 quart → buy 2 gallons and 1 quart
The paint calculator does this for any room and includes the ceiling as an option. If the room is not a simple rectangle, work out the floor plan with how to calculate square footage first.
Typical sizes to subtract
| Opening | Typical size | Area |
|---|---|---|
| Interior door | 3 ft × 6 ft 8 in | about 20 ft² |
| Exterior door | 3 ft × 7 ft | 21 ft² |
| Average window | 3 ft × 5 ft | 15 ft² |
| Sliding patio door | 6 ft × 6 ft 8 in | 40 ft² |
| Closet bifold (pair) | 5 ft × 6 ft 8 in | about 33 ft² |
Large windows and patio doors are worth subtracting. Small ones can be left in as a buffer.
Coverage by surface
| Surface | Coverage per gallon per coat |
|---|---|
| Smooth, primed drywall | 350–400 ft² |
| Previously painted walls | 350–400 ft² |
| New drywall (primer coat) | 250–300 ft² |
| Textured or popcorn ceiling | 250–300 ft² |
| Rough wood siding, stucco, brick | 150–300 ft² |
Porous and textured surfaces absorb more paint and have more surface area than their flat dimensions suggest. Use the rate on the can for smooth walls and the low end of the range for rough ones.
Ceilings, trim and doors
Ceilings. The ceiling area equals the floor area. A 12 × 14 ceiling is 168 ft², so one coat takes 168 ÷ 350 ≈ 0.48 gallon and two coats about 0.96. One gallon of ceiling paint covers it.
Trim. Baseboards, casings and crown molding take little paint. For a typical bedroom, a quart of semi-gloss usually covers all the trim with two coats.
Doors. Allow about 20 ft² per side for a flat slab door; paneled doors take more. A quart typically covers two or three doors, both sides, with two coats.
Primer: when and how much
Prime when painting new drywall or bare wood, covering stains, switching from a dark color to a light one, or painting over glossy surfaces. Primer covers about the same area per gallon as paint, sometimes slightly less on new drywall. For the 365 ft² example, one coat of primer needs about a gallon. Tinting the primer toward the finish color can save a topcoat.
Exterior example
A house with a 40 × 30 ft footprint, 10-ft walls and two triangular gable ends, each 30 ft wide and 8 ft tall at the peak.
Walls: 2 × (40 + 30) × 10 = 1,400 ft²
Gables: 2 × ½ × 30 × 8 = 240 ft²
Minus openings (about 200 ft²): 1,440 ft²
Two coats at 300 ft²/gal: 1,440 × 2 ÷ 300 = 9.6 → 10 gallons (two 5-gallon buckets)
The gable math comes from how to find the area of a triangle, and the triangle area calculator handles odd roof shapes.
Accent walls and two-color rooms
Estimate each color separately. In the 12 × 14 room, a single 14-ft accent wall is 14 × 8 = 112 ft², which at two coats needs 112 × 2 ÷ 350 ≈ 0.64 gallon, so a gallon (or three quarts) of the accent color. The other three walls are 416 − 112 = 304 ft² minus the openings on them. Deep, saturated colors often need a third coat or a gray-tinted primer to reach full color, so check the label before deciding on two coats.
Choosing a sheen
Sheen does not change how much paint you need, but it changes where you should use it. Flat or matte finishes hide surface flaws and suit ceilings and low-traffic rooms. Eggshell and satin are the usual choice for living areas and bedrooms because they wipe clean. Semi-gloss and gloss are harder and more washable, which is why they are standard for trim, doors, kitchens and bathrooms.
Buying tips
- Quarts vs gallons. A quart is a quarter gallon and covers about 90 to 100 ft². If you need more than about 2½ quarts, a gallon is usually cheaper.
- Mix cans together (called boxing) when using several gallons of a custom color, so slight batch differences do not show on the wall.
- Keep leftovers labeled with the room and date for touch-ups.
- Metric: 1 US gallon = 3.785 liters, and 350 ft² per gallon is about 8.6 m² per liter. The gallons to liters converter handles other amounts.
Related estimates
If you are papering instead of painting, the wallpaper calculator accounts for roll size and pattern repeat. For new walls, the drywall calculator counts sheets from the same wall area you measured here, and the square footage calculator combines multiple rooms.
Common mistakes
- Using floor area instead of wall area. A 168 ft² floor has 416 ft² of walls.
- Forgetting the second coat, which doubles the paint.
- Ignoring texture, which can cut coverage by a quarter or more.
- Buying exactly what the math says. Rollers, trays and the first coat on fresh drywall always use a little more than expected.
Frequently asked questions
How much paint do I need for a 12 × 14 room?
With 8-foot ceilings, the walls total 2 × (12 + 14) × 8 = 416 square feet. Subtracting one door and two windows (about 51 square feet) leaves 365. Two coats at 350 square feet per gallon need about 2.1 gallons, so buy 2 gallons and a quart.
How many square feet does a gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon per coat on smooth, primed surfaces. Rough, porous or textured surfaces can drop that to 250 to 300. The exact figure is printed on the can.
Do I need two coats of paint?
Usually. Two coats give even color and full durability, especially when changing colors or painting over patches. One coat can be enough when repainting the same color over a sound, clean surface, or when using a paint-and-primer product over a similar color.
How much paint do I need for a ceiling?
A ceiling's area is length × width. A 12 × 14 ft ceiling is 168 square feet, about half a gallon per coat at 350 square feet per gallon, so one gallon covers two coats.
Should I subtract doors and windows?
Yes, for an accurate estimate. A standard interior door is about 20 to 21 square feet and an average window about 15. In a room with only one or two small openings, many painters skip the subtraction and treat the difference as a margin.