Paint Square Footage Calculator — Wall Area, Coverage, and Gallons Needed
Paint calculation is wall area, not floor area. The two numbers are completely different for the same room. A 12'×14' room with 8 ft ceilings has a floor area of 168 sq ft but a total wall area of 416 sq ft before subtracting doors and windows. Buying paint based on floor area produces a severe shortage. The correct method: calculate the perimeter, multiply by ceiling height, subtract openings, then divide by the paint's coverage rate.
This calculator handles the measurement side. Enter your room's dimensions to get both the floor area (for reference) and the room dimensions needed to calculate wall area. Use the formula below to arrive at the number of gallons for your specific paint and surface type.
Paint Coverage Rates
Paint labels state theoretical coverage — what one gallon covers under perfect conditions on a primed, smooth surface. Real-world coverage is always lower due to absorption, surface texture, and multiple coats.
- Smooth, previously painted drywall (2nd coat): 350–400 sq ft per gallon
- New drywall, primed (1st finish coat): 300–350 sq ft per gallon
- Primer on raw drywall: 250–300 sq ft per gallon
- Textured (orange peel or knockdown) walls: 250–300 sq ft per gallon
- Heavy texture (popcorn ceiling): 150–200 sq ft per gallon
- Exterior siding (rough cedar or OSB): 200–250 sq ft per gallon
- Exterior masonry/stucco: 100–150 sq ft per gallon (first coat)
Calculating Gallons Needed
Formula: Gallons = (Wall Area − Openings) × Number of Coats ÷ Coverage Rate
Example: A living room 16'×18' with 9 ft ceilings. Wall area = 2×(16+18)×9 = 612 sq ft. Subtract 2 doors (42 sq ft) and 3 windows (45 sq ft) = 525 sq ft net. Two coats on previously painted smooth walls at 350 sq ft/gallon: 525 × 2 ÷ 350 = 3.0 gallons. Buy 4 gallons — always round up and keep extra for touch-ups.
Ceiling Paint Calculation
Calculate ceiling paint separately from wall paint — ceilings typically use flat white paint at a different coverage rate than wall paint. Ceiling area = room length × room width. Add 10% for cut-in overlap and touch-ups. A 16'×18' ceiling = 288 sq ft. At 350 sq ft/gallon: 1 gallon covers one coat. For two coats: 2 gallons. Buy an extra quart for touch-ups.
Trim and Door Paint
Trim paint (baseboards, door casings, window sills, crown moulding) is ordered separately and usually in a higher sheen (semi-gloss or gloss) than wall paint. Measure the linear feet of all trim. One gallon of trim paint covers approximately 200 linear feet of 3.5" baseboard at two coats. Door paint: one quart covers one door (both sides) at two coats.
Related tools: wall calculator · room calculator · house calculator · cost per sq ft
Paint Sheen Selection and Surface Area Impact
Paint sheen level affects how much paint you need and how you calculate wall area. Flat and matte finishes hide surface imperfections and absorb light, meaning a gallon goes further on smooth walls but requires more coats on rough surfaces. Semi-gloss and gloss finishes are self-levelling and reflect light, requiring fewer coats but showing every imperfection — which means properly primed, skim-coated, and sanded walls before calculating the final coat area. For a room switching from flat to semi-gloss: prime the walls first (1 coat), then apply semi-gloss (typically 2 coats). Three total coats means buying 3× the single-coat gallon estimate. For a 400 sq ft wall area at 350 sq ft/gallon per coat: 400/350 × 3 = 3.43 gallons — buy 4 gallons. Keeping a consistent sheen level eliminates mid-project trips to the store when one coat bleeds through.