A gallon of paint covers roughly 350–400 square feet — but that number alone won't tell you how much to buy. Room height, the number of coats, doors, windows, and the porosity of your walls all affect the final quantity. Buy too little and you'll face a mid-project store run and potential color-match issues; buy too much and you're storing half-empty cans indefinitely. A five-minute calculation prevents both.
The standard coverage rate for interior latex paint is 350–400 sq ft per gallon on a smooth, previously painted surface. Use 350 if your walls are textured, bare drywall, or a dramatically different color from the new paint. Use 400 for smooth walls in good condition with a similar base color.
To find total paintable wall area for a rectangular room:
Then subtract for doors and windows. A standard interior door is roughly 20 sq ft; a standard window is 15 sq ft. If you're painting the ceiling, add length × width to your total. If you're painting trim, add a rough 10% to the wall calculation.
You're painting the walls and ceiling of a bedroom: 12 ft × 14 ft, 9-foot ceilings, one door, two windows. Two coats planned.
Step 1: Calculate total wall area.
(2 × 12 + 2 × 14) × 9 = (24 + 28) × 9 = 52 × 9 = 468 sq ft
Step 2: Subtract openings.
One door (20 sq ft) + two windows (2 × 15 = 30 sq ft) = 50 sq ft
468 − 50 = 418 sq ft of paintable wall
Step 3: Add ceiling area.
12 × 14 = 168 sq ft
Step 4: Total paintable area.
418 + 168 = 586 sq ft
Step 5: Divide by coverage rate and multiply by coats.
Using 350 sq ft/gallon (the room has flat drywall, moderately textured):
586 ÷ 350 = 1.67 gallons per coat × 2 coats = 3.34 gallons
Round up: buy 4 gallons. If the walls and ceiling are different colors (common), split the calculation: ~2.4 gallons for walls, ~1 gallon for ceiling — so two separate quarts for the ceiling and one full gallon, or two gallons of wall color and one of ceiling white.
1. Using floor area instead of wall area.
This is the most widespread error. A 12 × 14 ft room has 168 sq ft of floor — but over 400 sq ft of wall. People who calculate paint based on the room's square footage end up buying less than half what they need. Always measure walls, not floors.
2. Planning for one coat when two are needed.
Switching from a dark color to a light one almost always requires two coats — sometimes three. Going from white to white might need only one. Factor in coats before you buy, not after your first coat dries patchy and translucent.
3. Ignoring surface texture and porosity.
Bare drywall, brick, and heavily textured surfaces absorb significantly more paint than previously painted smooth walls. Using the 400 sq ft/gallon rate on new drywall without a primer coat can leave you 30–40% short. Prime first, then paint — or use a paint-and-primer product and budget for the lower coverage rate.
1. Painting a newly built or freshly drywalled room.
New drywall is highly absorbent. Calculate at 300–350 sq ft per gallon, plan for a dedicated primer coat, and increase your total paint estimate by 20–30% compared to a repaint job on smooth walls.
2. Repainting multiple rooms in the same color.
Combine all room calculations before buying, since larger quantities often allow you to step up to a 5-gallon bucket at a meaningfully lower cost per gallon — typically 15–20% less than buying five individual gallons.
3. Estimating for a rental property refresh.
Landlords painting between tenants often deal with mixed wall conditions — some scuffed, some patched, some in decent shape. Using 350 sq ft/gallon as a conservative across-the-board rate and calculating each room separately gives a reliable total without surprises.
Does paint brand affect coverage?
Yes, modestly. Premium paints with higher pigment and resin content often cover closer to 400 sq ft/gallon, while budget paints may land at 300–350 even on smooth surfaces. The spec sheet on the can (or manufacturer's website) lists the rated coverage — check it before assuming a standard rate.
Should I buy extra paint for touch-ups?
For freshly painted walls, save a quart from your project rather than buying extra. Label it with the room name, color code, and date. Touch-up paint from the same batch matches far better than a new can mixed months later, even from the same formula.
How much does a gallon of paint weigh, and does that affect anything?
A gallon of latex paint weighs approximately 10–12 lbs. It doesn't affect coverage, but it matters when estimating how much to carry up stairs or whether a single trip to the car is realistic.
What if my room isn't rectangular?
Break it into rectangles and calculate each wall segment separately. An L-shaped room, for instance, has six wall surfaces rather than four — measure each one, add them up, then subtract doors and windows as normal.
Calculate wall area (not floor area), choose the right coverage rate for your surface, and multiply by coats before buying. Those three steps give you an accurate gallon count every time.