How Much Paint Do I Need for a Room? Coats, Primer and Cans
The short answer
Most bedrooms need 2 to 3 gallons of wall paint for two coats, and most living rooms need 3 to 4. To get your own number, find the wall area, subtract doors and windows, multiply by the number of coats and divide by 350 to 400 square feet per gallon, the coverage Sherwin-Williams quotes for a gallon of paint.
The two things that change the answer most are not the room size. They are how many coats you really need and whether you prime first. This guide covers both, then works through a living room where the color is going from dark to light.
Step 1: Measure the wall area
Paint covers area, so start there:
- Walls: add the length and width of the room, double it, then multiply by the wall height. That is the perimeter times the height.
- Doors and windows: take off about 21 square feet for each standard door (3 × 7 ft) and 15 for each average window (3 × 5 ft). Measure any big openings, like a sliding door or a picture window, instead of guessing.
- Ceiling: length times width. Keep it separate, because ceiling paint is usually a different product.
If the room is L-shaped or has a bay, break it into rectangles. The square footage calculator adds up odd shapes for you, and the floor area it gives you is also your ceiling area.
Step 2: Decide on one coat or two
Benjamin Moore’s own paint calculator assumes two coats, and that is the safe default for walls. Plan on two coats when:
- You are changing the color, even slightly.
- The wall has patches, touch-ups or flashing (shiny or dull spots).
- You are painting over bare or freshly primed surfaces.
A single coat can be enough only when you are refreshing the same color on a clean, sound wall in good light. Even then, check the can: some paints list a lower coverage than the usual range, and the label is the number to trust.
Step 3: Decide whether you need primer
Benjamin Moore’s FAQ says most previously painted, undamaged walls do not need priming. Between the FAQ and its drywall primer data sheet (which allows spot-priming previously painted surfaces), these are the cases where primer earns its place:
| Situation | Prime? | Why |
|---|---|---|
| New, unpainted drywall | Yes, always | Drywall is porous and soaks up the first coat unevenly |
| Water, smoke or tannin stains | Yes, with a stain-blocking primer | Stains bleed through ordinary paint |
| Big color change (dark to light or light to dark) | Often | A tinted primer hides the old color so the topcoats do not have to |
| Patched or skim-coated spots | Spot-prime | Fresh compound absorbs paint differently from the old wall |
| Same or similar color on a sound wall | Usually no | Two topcoats are enough |
Benjamin Moore also says its premium paints can handle most color changes in two coats with no primer, and its drywall primer data sheet recommends tinting the primer toward the finish color for the best hiding. So “dark to light” is a judgment call: primer plus two coats, or a high-hiding paint and possibly a third coat.
Primer covers less ground than paint. Sherwin-Williams puts a gallon of primer at about 200 to 300 square feet, against 350 to 400 for paint. One coat of primer is the norm.
Worked example: a 15 × 18 ft living room going from navy to off-white
The room has 9 ft walls, two doors and three windows. The plan is to paint the walls and the ceiling.
Wall area. The perimeter is 2 × (15 + 18) = 66 ft. Times 9 ft of height is 594 square feet. Take off 2 × 21 = 42 for the doors and 3 × 15 = 45 for the windows: 507 square feet of wall.
Option A: prime, then two coats.
- Primer: 507 ÷ 300 = 1.69 gallons at the high end of coverage, 507 ÷ 200 = 2.54 at the low end. Buy 2 gallons if the primer is tinted and the old color is fairly even, 3 to be safe.
- Paint: 507 × 2 coats = 1,014 square feet. 1,014 ÷ 400 = 2.54 gallons and 1,014 ÷ 350 = 2.90. Either way, 3 gallons.
Option B: skip primer, three coats of paint.
- 507 × 3 = 1,521 square feet. 1,521 ÷ 400 = 3.80 gallons and 1,521 ÷ 350 = 4.35. That is 4 to 5 gallons.
Ceiling. 15 × 18 = 270 square feet. Two coats is 540 square feet, which is 1.35 to 1.54 gallons: buy 2 gallons of ceiling paint.
The two options use a similar number of cans. The difference is that tinted primer is built to hide the old color, while a third topcoat is a hope that it will. If the navy is glossy, primer also gives the new paint something to grip.
Gallons, pails or quarts?
Paint is sold in quarts, gallons and 5-gallon pails. The pail is usually cheaper per gallon, but only if you use most of it.
Take Option B above at 5 gallons. With example prices of $45 a gallon and $190 for a 5-gallon pail (example prices only; check your store), five single gallons cost $225 and the pail costs $190, so the pail wins. At 3 gallons, three singles ($135) beat the pail by a wide margin. The tipping point depends on your store’s prices, which is why the paint calculator asks for both and picks the cheapest mix of pails and gallons for you.
Quarts make sense for trim, doors and accent walls. In a typical bedroom the baseboard and casing come to well under a gallon for two coats.
Paint needed for common rooms
These figures assume 8 ft walls, one door, one window, two coats, and coverage of 350 to 400 square feet per gallon. Round up to whole gallons when you buy.
| Room | Wall area | Wall paint, 2 coats | Ceiling area | Ceiling paint, 2 coats |
|---|---|---|---|---|
| 10 × 10 ft | 284 sq ft | 1.4–1.6 gal | 100 sq ft | 0.5–0.6 gal |
| 10 × 12 ft | 316 sq ft | 1.6–1.8 gal | 120 sq ft | 0.6–0.7 gal |
| 12 × 12 ft | 348 sq ft | 1.7–2.0 gal | 144 sq ft | 0.7–0.8 gal |
| 12 × 14 ft | 380 sq ft | 1.9–2.2 gal | 168 sq ft | 0.8–1.0 gal |
| 14 × 16 ft | 444 sq ft | 2.2–2.5 gal | 224 sq ft | 1.1–1.3 gal |
| 15 × 18 ft | 492 sq ft | 2.5–2.8 gal | 270 sq ft | 1.4–1.5 gal |
| 16 × 20 ft | 540 sq ft | 2.7–3.1 gal | 320 sq ft | 1.6–1.8 gal |
Taller walls scale in a straight line: 9 ft walls need about an eighth more paint than 8 ft walls, and 10 ft walls about a quarter more.
Rough and textured surfaces
Coverage figures are for smooth, sealed walls. Sherwin-Williams notes that wall texture changes how much you need. Knockdown and orange-peel walls have more surface than their flat measurements suggest, and popcorn ceilings soak up paint.
A simple adjustment: lower the coverage you plan with. Using 250 to 300 square feet per gallon instead of 350 to 400 for a heavily textured surface adds roughly a third more paint. You can type your own coverage into the paint calculator.
Common mistakes
- Measuring floor area instead of wall area. A 12 × 14 room has 168 square feet of floor but 380 square feet of wall after one door and one window. Buying by floor area leaves you short by more than half.
- Forgetting the second coat. A gallon that “covers 400 square feet” covers 200 square feet of wall with two coats.
- Counting primer coverage as paint coverage. Primer goes about a third less far per gallon.
- Buying cans of one color on different days. Small shade differences show on a flat wall. Buy the whole amount at once and mix the cans together in a bucket (called boxing) before you start.
- Skipping spot-primer on patches. Fresh joint compound shows through as a dull patch, which then needs another coat anyway.
- Ignoring the label. Specialty and one-coat products can list coverage outside the usual range. The can wins over any rule of thumb.
When to call a pro
Homes built before 1978. Paint in older homes may contain lead. The EPA’s Renovation, Repair and Painting Rule requires anyone paid to disturb painted surfaces in homes built before 1978 to be certified and to follow lead-safe practices. The rule generally does not apply to homeowners working in their own homes, unless they rent the home out, run a child care center there, or fix and flip homes. The EPA still recommends lead-safe practices for do-it-yourselfers. If you plan to sand, scrape or remove old paint in a pre-1978 home, test first or hire a Lead-Safe Certified firm.
Other cases worth a pro: stairwells and two-story walls that need scaffolding, ceilings with water stains that keep coming back (that is a leak, not a paint problem), and peeling paint that lifts in sheets, which usually points to moisture or a poor bond underneath.
Quick checklist before the store
- Wall area for each room, minus openings.
- Number of coats: two, or three for a big color change without primer.
- Primer: yes for new drywall, stains and patches; tinted for big color changes.
- Coverage: 350 to 400 for paint, 200 to 300 for primer, lower for texture.
- Divide, round up, then compare a pail against single gallons.
The paint calculator does steps 1 to 5 for several rooms at once and keeps walls, ceilings, trim and primer apart, so the shopping list matches what is on the shelf.