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Interior Paint & Primer Coverage Calculator
Calculate paint and primer gallons needed for interior walls and ceilings with precise feet and inches room dimensions, opening deductions, and coat counts.
Formulas, units and estimation rules
Perimeter = 2 × (Length + Width)
Gross Wall Area = Perimeter × Ceiling Height
Net Wall Area = Gross Wall Area - (Doors × 21 ft² + Windows × 15 ft² + Extra Deductions)
Ceiling Area = Length × Width (if included)
Net Paintable Area = Net Wall Area + Ceiling Area
Purchasing Area = Net Paintable Area × (1 + Waste % ÷ 100)
Paint Total Area = Purchasing Area × Paint Coats
Exact Paint Gallons = Paint Total Area ÷ Paint Spreading Rate (350 ft²/gal)
Paint Gallons to Buy = ceil(Exact Paint Gallons)
Primer Total Area = Purchasing Area × Primer Coats
Exact Primer Gallons = Primer Total Area ÷ Primer Spreading Rate (300 ft²/gal)
Primer Gallons to Buy = ceil(Exact Primer Gallons)
- Standard US Spreading Rates: According to the Painting and Decorating Contractors of America (PDCA) and major paint manufacturers, one US gallon (128 fluid ounces) covers approximately 350 to 400 square feet on smooth, sealed interior drywall. Textured walls (such as knockdown, orange peel, or rough plaster) increase surface area by 15% to 25%, reducing effective coverage to 280–320 square feet per gallon.
- Primer vs Paint Coverage: Raw drywall paper and spackled compound are highly porous and absorb more liquid, yielding approximately 250 to 300 square feet per gallon for primer. Once sealed, finish coats spread farther at 350 to 400 square feet per gallon.
- Standard Door & Window Opening Deductions: A typical residential interior door measures 36 inches wide by 84 inches high (3×7 ft = 21 sq ft). A standard double-hung bedroom window measures 36 inches wide by 60 inches high (3×5 ft = 15 sq ft). Subtracting these openings prevents purchasing excess paint.
- Two-Coat Standard: Applying two coats of finish paint ensures complete color hide, even gloss/sheen uniformity, and the physical film thickness required for scrub resistance and stain washability.
How to measure room surfaces and prep walls for painting
Follow these standard painter preparation and application steps using our Interior Paint & Primer Coverage Calculator to achieve a flawless, streak-free interior finish:
- Measure room perimeter and ceiling height. Measure the length and width of the room in feet and inches, then multiply the total perimeter by ceiling height to get gross wall square footage.
- Deduct doors, windows, and unpainted openings. Subtract 21 sq ft for each standard 3'x7' interior door and 15 sq ft for each 3'x5' window to determine the net paintable wall surface area.
- Inspect, wash, and patch wall imperfections. Wash dusty or greasy walls with mild TSP substitute detergent. Fill drywall gouges, nail holes, and cracks with spackling compound, letting it cure before sanding flush with 150-grit sandpaper.
- Mask trim, baseboards, and protect flooring. Lay canvas drop cloths across floors and press professional painter's blue tape along baseboards, window casings, door frames, and crown moulding.
- Apply one full coat of primer sealer. On bare drywall, patched areas, or when making major color transitions, roll one uniform coat of high-hide bonding primer (at ~300 sq ft per gallon) and allow 2 to 4 hours drying time.
- Cut in edges and roll two coats of finish paint. Cut in corners and perimeter trim with a 2-1/2" angled sash brush, then roll two uniform coats of interior latex paint with a 3/8" nap roller sleeve, allowing 4 hours dry time between coats.
Recommended premium paints & primers (Home Depot & Amazon)
Affiliate Disclosure: When you purchase paint, primer, or tools through the links below, XayDung US may earn an affiliate commission at no extra cost to you. We recommend materials from major home improvement retailers including The Home Depot and Amazon based on recognized painting contractor standards. Learn more in our affiliate disclosures.
Order quality paints and primers in advance to ensure consistent dye lots, optimal hide, and lasting scrub resistance:
BEHR PREMIUM PLUS & ULTRA Interior Paint & Primer
100% acrylic latex paint and primer in one, delivering outstanding hide, durable scrubbability, and low-VOC air quality. Available in flat, eggshell, satin, and semi-gloss sheens with superior fade resistance.
Prestige Paints & Glidden One-Coat Interior Acrylic Latex
Ready-to-use premium interior latex paints offering smooth roller application, excellent single-pass coverage, and simple soap-and-water cleanup. Shipped securely in 1-gallon and 5-gallon contractor containers.
KILZ 2 All-Purpose & Zinsser Bulls Eye 1-2-3 Primer Sealer
Water-based interior primer sealers engineered to seal raw drywall paper, block light water and smoke stains, and promote adhesion between topcoat paint and sanded spackle repairs.
KILZ Premium High-Hide Stain-Blocking Interior Primer
Zero-VOC, low-odor heavy-duty latex primer sealer. Equalizes porosity across patched walls, covers dark previous wall paints, and guarantees a uniform sheen without flashing.
Essential room painting & surface prep tools
Having contractor-grade roller covers, angled sash brushes, and clean-release painter's tape saves hours of labor and eliminates roller stippling:
Wooster & Purdy Pro 9-Piece Roller, Sash Brush & Tray Kit
Contractor-grade 9-inch roller frames, 3/8-inch microfiber nap roller covers, 2.5-inch angled sash brushes, and heavy-duty metal paint trays with disposable ribbed liners for smooth, lint-free wall finishes.
ScotchBlue Painter's Tape, Canvas Drop Cloths & Sanding Sponges
Multi-surface 1.88-inch painter's masking tape with 14-day clean removal, 9×12 ft heavyweight canvas drop cloths that prevent paint bleed-through, and medium/fine 150-grit sanding blocks for flush feathering.
Frequently asked questions
How many square feet does 1 gallon of paint cover?
One standard US gallon of interior wall paint covers between 350 and 400 square feet on smooth, pre-primed drywall. On textured walls (knockdown or orange peel) or unprimed porous surfaces, coverage decreases to approximately 280 to 320 square feet per gallon.
Do I really need 2 coats of interior wall paint?
Yes. Two coats are the industry standard for interior living spaces. The second coat fills microscopic roller voids, establishes true color depth, ensures uniform sheen across lap marks, and provides the film build necessary for washability and long-term durability.
When is primer necessary before painting walls?
Primer is essential whenever you paint bare drywall, joint compound patches, stained areas (water, smoke, grease), glossy surfaces, or when transitioning from a dark color to a light color. Primer seals the porosity of the substrate so the topcoat does not flash or absorb unevenly.
How much paint do standard doors and windows deduct from a room?
A standard interior door (3 ft by 7 ft) deducts approximately 21 square feet. A standard residential window (3 ft by 5 ft) deducts 15 square feet. In a standard bedroom with one door and two windows, deducting 51 square feet prevents ordering unnecessary excess paint.
Should I buy paint in single gallons or 5-gallon buckets?
If your calculation shows you need 4 or more gallons of the same paint color and sheen, buying a 5-gallon bucket saves 15% to 20% over individual gallon cans. It also eliminates slight color variations between individual factory can tintings ("boxing" paint).
How do I calculate paint quantity for the ceiling?
Ceiling area equals room length multiplied by room width (e.g. 14.5 ft × 12 ft = 174 sq ft). Ceilings typically require 1 or 2 coats of flat ceiling white paint rolled with a thicker 1/2-inch nap roller to minimize surface glare and hide framing imperfections.
What paint sheen should I choose for each room?
Use Flat/Matte for ceilings and master bedrooms to hide wall imperfections; Eggshell or Satin for living rooms, hallways, and kids' rooms for cleanability with subtle sheen; and Semi-Gloss for moisture-prone bathrooms, kitchens, doors, and trim.
Limits and project considerations
- Substrate Porosity: Unsealed drywall mud and raw drywall paper absorb significantly more paint than previously painted walls. Always budget for 1 coat of dedicated primer sealer before applying finish paint.
- Wall Texture: The calculator assumes smooth drywall surfaces. Heavy knockdown, popcorn, or masonry textures increase surface area by up to 25%; adjust the spreading rate downward to 280–300 sq ft/gal in the form fields.
- Ventilation and Safety: Always maintain adequate cross ventilation when applying interior latex coatings and primers. Wear safety goggles and an N95 dust mask when sanding drywall repairs prior to painting.