To work out how much paint you need, add up the wall area (perimeter × height), subtract about 21 ft² per door and 15 ft² per window, multiply by the number of coats and divide by the coverage, usually 350 ft² per gallon. A 12 × 14 ft room with 8 ft walls needs about 2.1 gallons for two coats.
That formula gets you within a quart on most rooms. The details that change the answer are the surface (new drywall soaks up more than painted walls), the color change, whether you are doing the ceiling and trim too, and how the cans are sized. Our paint calculator runs the whole thing for you, but it helps to know what is behind the number so you can sanity-check it in the store.
Step 1: Measure the wall area
For a normal rectangular room, you do not need to measure each wall separately. Measure the length and width of the floor, add them, double the result to get the perimeter, then multiply by the ceiling height.
Wall area = 2 × (length + width) × height
A 12 × 14 ft bedroom with 8 ft ceilings: 2 × (12 + 14) × 8 = 2 × 26 × 8 = 416 ft². In metric, a 4 × 3.5 m room with 2.5 m ceilings has 2 × 7.5 × 2.5 = 37.5 m² of wall.
Measure to the nearest half foot; paint estimates do not need inch-level accuracy. If the room has an alcove, a sloped ceiling or a chimney breast, measure each wall as its own rectangle (or triangle, for a gable end) and add them. The wall square footage calculator handles walls of different sizes, and our guide on how to calculate square footage covers L-shapes, triangles and other awkward areas.
Step 2: Subtract doors and windows
You will not paint the glass or the door slab (unless you are, in which case count it separately), so take them off. Standard allowances:
- Door: about 21 ft² (3 × 7 ft), or 2 m².
- Window: about 15 ft² (3 × 5 ft), or 1.5 m².
- Large openings such as patio doors, wide archways or picture windows: measure them.
The bedroom has one door and two windows: 416 − 21 − 30 = 365 ft² of paintable wall.
Some painters skip this step and treat the extra as their waste allowance. That works for a room with one door, but in a sunroom full of windows you would end up with a gallon too many.
Step 3: Check the coverage on the can
Coverage is how many square feet one gallon covers in a single coat. Most interior wall paints claim 350 to 400 ft² per gallon (roughly 8.5 to 10 m² per litre) on smooth, primed surfaces. Real-world coverage depends on what is underneath.
| Surface | Typical coverage per gallon | Per litre |
|---|---|---|
| Smooth, previously painted drywall | 350–400 ft² | 8.5–10 m² |
| Light texture (orange peel, knockdown) | 300–350 ft² | 7.5–8.5 m² |
| Heavy texture or popcorn ceiling | 250–300 ft² | 6–7.5 m² |
| Primer on new drywall | 200–300 ft² | 5–7.5 m² |
| Bare wood or plaster | 250–350 ft² | 6–8.5 m² |
| Brick, block or stucco | 150–250 ft² | 3.5–6 m² |
When in doubt, plan on 350 ft² per gallon for walls and 300 for anything textured or porous. Thick, premium paints sometimes cover less area per gallon but hide better, so you may get away with fewer coats.
Step 4: How much paint do I need for two coats?
Two coats are the default for a new color. One coat often looks fine wet and patchy dry, especially with deep colors or when going light over dark. If you are repainting the same color in good condition, one coat may do.
Gallons = paintable area × coats ÷ coverage
Worked example: the 12 × 14 ft bedroom
- Wall area: 2 × (12 + 14) × 8 = 416 ft².
- Subtract one door and two windows: 416 − 21 − 30 = 365 ft².
- Two coats: 365 × 2 = 730 ft².
- Divide by coverage: 730 ÷ 350 = 2.09 gallons.
- Buy: two gallons and a quart (2.25 gal), or three gallons if you want plenty for touch-ups.
The same room in metric: 37.5 m² − 2 m² (door) − 3 m² (two windows) = 32.5 m². Two coats at 10 m² per litre: 32.5 × 2 ÷ 10 = 6.5 litres, so a 5 L tin plus a 2.5 L tin. If you need to switch between gallons and litres, the litres to gallons converter is quicker than remembering 3.785.
Quick reference for common room sizes
This table assumes 8 ft walls, one door, two windows, two coats and 350 ft² per gallon.
| Room (ft) | Paintable wall area | Wall paint, 2 coats | What to buy | Ceiling paint, 2 coats |
|---|---|---|---|---|
| 10 × 10 | 269 ft² | 1.54 gal | 1 gal + 2 qt | 0.57 gal |
| 10 × 12 | 301 ft² | 1.72 gal | 2 gal | 0.69 gal |
| 12 × 12 | 333 ft² | 1.90 gal | 2 gal | 0.82 gal |
| 12 × 14 | 365 ft² | 2.09 gal | 2 gal + 1 qt | 0.96 gal |
| 14 × 16 | 429 ft² | 2.45 gal | 2 gal + 2 qt | 1.28 gal |
| 16 × 20 | 525 ft² | 3.00 gal | 3 gal + 1 qt | 1.83 gal |
For 9 ft ceilings, multiply the wall figures by 1.125 before subtracting doors and windows; for 10 ft, by 1.25.
Primer: when you need it and how much
Primer seals the surface so the topcoat goes on evenly. You need a separate primer coat on new or patched drywall, bare wood, stains (water marks, smoke, marker), glossy surfaces and big color changes such as covering dark red with pale grey. On previously painted walls in decent shape, a self-priming “paint and primer” product is usually enough.
Primer is one coat, but bare drywall drinks it. For the bedroom above with brand-new drywall: 365 ÷ 300 = 1.22 gallons, so buy a gallon and a quart, or two gallons if the walls have a lot of fresh mud. A tinted primer close to your final color can save you a topcoat on deep shades. If you are hanging new board first, the drywall calculator will tell you how many sheets to buy.
Ceilings, trim and doors
Ceilings
The ceiling area is simply length × width: 12 × 14 = 168 ft². Ceiling paint is usually flat white and goes on thicker, so two coats at 350 ft² per gallon gives 168 × 2 ÷ 350 = 0.96 gallons. One gallon covers it, just. Textured or popcorn ceilings need about 25% more. The square footage calculator works out ceilings in odd-shaped rooms.
Trim and baseboards
Trim is measured in running feet, not area. The bedroom has 52 ft of perimeter, minus about 3 ft for the door, so 49 ft of baseboard. At 4 in tall that is only about 16 ft² per coat. Add door and window casings and you are still well under 100 ft². One quart of semi-gloss covers the trim in a typical room, two coats included.
Doors
A standard interior door is about 20 ft² per side. Painting both sides with two coats is 80 ft², so a quart covers two or three doors.
Buying paint: cans, quarts and waste
- Quarts cost almost half as much as a gallon, so two quarts rarely make sense; buy the gallon instead.
- Five-gallon buckets usually cost less per gallon than five single gallons once a job needs four or more. A whole-house interior or an exterior often does.
- Box the paint. If you buy several gallons of one custom color, pour them into one large bucket and mix. Small differences between cans then disappear.
- Keep extra for touch-ups. Having a quart left over is worth more than you think when the couch scuffs a wall next year.
Rounding up from 2.09 to 2.25 gallons is about 8% extra; going to 3 gallons is about 44%. Our percentage calculator will show the overage on any order, and for exterior projects such as decks the deck stain calculator uses the same area-times-coats logic.
Common mistakes
- Using floor area instead of wall area. A 12 × 14 floor is 168 ft²; the walls are 416 ft².
- Planning one coat for a color change. Most color changes need two coats, plus primer for dark-to-light.
- Trusting the best-case coverage. 400 ft² per gallon is on smooth, sealed walls with a good roller. Textured or patched walls get much less.
- Forgetting cut-in areas and closets. A walk-in closet can add 150 ft² of wall.
- Ignoring old paint in older homes. If the house was built before 1978, the existing paint may contain lead. The EPA’s lead guidance explains how to test and how to sand or scrape safely.
- Painting in a closed room. Even low-VOC paint off-gasses. Keep windows open and a fan running; the EPA’s indoor air quality pages cover what to look for on labels.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, the walls are about 333 ft². Two coats at 350 ft² per gallon need 1.9 gallons, so buy two gallons.
How many square feet does a gallon of paint cover?
About 350 to 400 ft² per coat on smooth, painted walls, and 250 to 300 ft² on textured, porous or new surfaces. In metric, roughly 8.5 to 10 m² per litre.
Is one gallon of paint enough for one room?
Only for small rooms or a single coat. A 10 × 10 ft room with 8 ft walls already needs about 1.5 gallons for two coats.
Do I need two coats of paint?
For a new color, almost always. One coat can work when you are refreshing the same color over a sound, clean surface.
How much paint for an accent wall?
A 12 ft wall 8 ft high is 96 ft². Two coats need 192 ft² of coverage, so one gallon is enough with plenty to spare, and a quart is too little.
Measure the walls, subtract the openings, multiply by coats, divide by coverage and round up to the nearest quart. That one calculation saves a second trip to the store halfway through the second coat, which is when the store has inevitably run out of your custom tint. For other home projects that start with the same tape-measure routine, see our guide to how much concrete you need.