How Much Paint Do I Need for a Room?
Paint gallons equal paintable wall area times coats divided by coverage. Wall area is roughly perimeter × height minus doors and windows. Many interior paints cover about 350–400 square feet per gallon per coat as a rule of thumb—confirm the can and use the paint calculator for your room.
How room paint quantity is calculated
For a rectangular room:
Wall area = 2 × (length + width) × height.
Paintable area ≈ wall area − door areas − window areas.
Gallons per coat ≈ paintable area ÷ coverage (sq ft per gallon).
Total gallons ≈ gallons per coat × coats.
YardTally’s paint calculator uses that layout with default door and window sizes (about 20 and 15 sq ft each unless you override) and a default coverage of 350 sq ft per gallon—within the range many consumer labels publish. Behr’s how-to guidance, for example, states that one gallon of several BEHR Ultra or Premium Plus interior products covers about 250 to 400 sq ft per coat, with textured or drastic color changes needing more. Sherwin-Williams and other manufacturers publish similar label ranges; the can in your hand controls the buy.
Ceilings, trim, and doors are separate takeoffs. This guide focuses on walls. Closets, hall returns, and accent walls should be measured individually so you do not under-order a specialty color.
Worked example: 12 × 14 ft room, 8 ft ceilings
Perimeter contribution: 2 × (12 + 14) = 52 ft. Wall area = 52 × 8 = 416 sq ft.
Subtract one door (20 sq ft) and two windows (2 × 15 = 30 sq ft): paintable = 416 − 50 = 366 sq ft.
At 350 sq ft per gallon: 366 ÷ 350 ≈ 1.05 gallons for one coat. Two coats ≈ 2.09 gallons—plan on buying three gallons so tint lots match and touch-up remains, or two gallons plus a quart if the store can match. These figures match the paint calculator for the same inputs.
If the label’s practical coverage on your textured plaster is closer to 300 sq ft per gallon, two coats need (366 × 2) ÷ 300 ≈ 2.44 gallons—round up accordingly.
Coverage planning table
| Coverage (sq ft/gal) | 1 coat | 2 coats |
|---|---|---|
| 250 | 1.46 | 2.93 |
| 300 | 1.22 | 2.44 |
| 350 | 1.05 | 2.09 |
| 400 | 0.92 | 1.83 |
Dark-to-light color changes, unrepaired drywall, and bare wood often need primer plus two finish coats. “Paint-and-primer” marketing does not erase surface preparation requirements printed on the label.
Ceilings, trim, and accent walls
Ceiling paint is usually a separate product and a separate area: length × width, then apply the ceiling can’s coverage and coat count. Coffered or highly textured ceilings drink more paint than smooth drywall. Trim and doors are often measured in linear feet times an average width (for example, baseboard length × about 0.5 ft if the trim is roughly six inches tall)—or simply budget a quart or gallon of enamel based on experience once walls are ordered.
Accent walls use the single-wall formula: wall width × height, minus openings on that wall only. Ordering a deep accent color in a tiny test size first is wise; coverage on saturated colors can sit toward the low end of the label range. If you change from a dark existing color to a pale one, plan on primer or an extra finish coat even when the wall area math looks modest.
Open-concept spaces should be split into paintable planes. A long great-room may share paint color with a hall; measure each run of wall so you do not forget stairwells, pony walls, or the back of a freestanding fireplace surround.
Step-by-step
- Measure length, width, and ceiling height in feet.
- Count doors and windows; measure unusual openings if they differ from typical sizes.
- Compute paintable wall area (or use the paint calculator).
- Read coverage and recommended coats on the product data sheet or can.
- Divide and multiply for total gallons; round up to whole containers of the same tint batch.
- Add primer quantity separately when the label calls for it.
- Estimate trim linear feet later if you are brushing a contrasting enamel.
Surface prep that changes how much paint you need
Patched drywall, bare paper facing, glossy enamel, and nicotine-stained walls all affect how far a gallon goes. Sanding gloss, washing grease, and priming repairs usually improves finish uniformity and can reduce the urge to apply a third finish coat “just to hide.” Skipping prep rarely saves paint—it usually wastes it on uneven absorption and flashing.
Roller nap thickness matters on texture: deeper texture holds more paint in the first pass. When in doubt, use the lower coverage number from the manufacturer’s range for the first coat estimate, then adjust after you see how the wall looks when dry.
Tips
- Buy all wall paint for a continuous color at once; dye lots shift between batches.
- Use the lower end of the coverage range on rough textures and new drywall.
- Cut in edges first, maintain a wet edge when rolling, and respect recoat times.
- Ventilate; follow VOC and cleanup directions on the label.
- Keep a quart of leftover for future touch-ups, labeled with room and date.
- Box multiple gallons together in a clean pail so slight tint differences blend.
Common mistakes
- Ordering from floor area (length × width) instead of wall area.
- Forgetting a second coat in the gallon math.
- Ignoring doors/windows on small rooms (overbuys) or skipping them on large open plans with many openings (underbuys).
- Assuming 400 sq ft/gal on a porous unprimed surface.
- Mixing leftover sheens (flat vs eggshell) on the same wall.
Sheen, application method, and waste
Sheen and tools influence how far paint goes. Flat finishes hide wall flaws but scuff more easily; satin and eggshell wipe clean in kitchens and hallways. A roller with a thicker nap holds more paint and suits textured walls, but it applies more paint per square foot than a thin nap on smooth drywall. Airless sprayers can cover fast but typically waste more paint to overspray and cleanup than rolling does, so plan extra if you spray.
Cutting in around trim and corners adds to usage, and so does paint left in trays, rollers, and brushes. A small buffer of around 10 percent is a common planning allowance. When a room has a dramatic color change, tinted primer can reduce the number of finish coats needed.
Sources
- BEHR – How much interior paint to buy (250–400 sq ft/gal guidance)
- Sherwin-Williams – Product labels & coverage (verify on can)
FAQ
How much paint do I need for a 12×12 room?
With 8 ft ceilings, wall area = 2 × (12+12) × 8 = 384 sq ft before openings. Subtract doors/windows, divide by label coverage, then multiply by coats. Run the paint calculator for exact gallons.
How many square feet does a gallon of paint cover?
Many interior latex products list about 250–400 sq ft per gallon per coat. Smooth, sealed walls land toward the high end; check your can.
Do I include the ceiling?
Not in the basic four-wall estimate. Ceiling area ≈ length × width; calculate gallons separately if you are painting it.
How much do doors and windows subtract?
YardTally defaults to about 20 sq ft per door and 15 sq ft per window—adjust if your units differ.
Should I buy extra?
Rounding up a bit helps with touch-up and sheen matching. Store leftovers sealed and frost-free.
Related calculators
- Paint Calculator — Room paint in gallons and liters with openings deducted.