How to Calculate Paint for Any Room
Start by measuring the perimeter of the room: add the length and width together and multiply by 2, then multiply by the ceiling height to get total wall square footage. For a 12x12 room with 8-foot ceilings, that is (12 + 12) x 2 x 8 = 384 square feet of wall area. Subtract about 20 square feet per standard door and 15 square feet per standard window. If you are painting the ceiling too, add length times width. Divide the total paintable area by 350 (coverage per gallon) and multiply by the number of coats.
For open floor plans or irregularly shaped rooms, measure each wall section individually and add them together. Closet walls, niches, and built-in shelving units all add paintable area that homeowners often forget. Stairwells are particularly tricky since the wall height varies. Measure the tallest point and use that for the full wall section, then subtract any large openings. When planning a full interior repaint, a wallpaper calculator helps compare the cost of wallpapering accent walls versus painting them.
Interior Paint Cost Guide
Interior paint costs $15-70 per gallon depending on quality tier. Economy paint at $15-25 per gallon works for rental properties or temporary spaces but typically requires three coats and shows wear within 2-3 years. Mid-range paint at $30-45 per gallon from brands like Behr or Valspar offers good coverage in two coats and lasts 5-8 years. Premium paint from Benjamin Moore or Sherwin-Williams at $50-70 per gallon provides the best coverage, color retention, and washability, lasting 10+ years. Most rooms need 1-2 gallons, so the difference between economy and premium is $30-90 per room.
Beyond paint itself, budget for supplies: rollers ($5-10 each), trays ($3-5), painter's tape ($5-8 per roll), drop cloths ($10-20), brushes ($8-15 for a good angled brush), and primer if needed ($20-40 per gallon). A complete painting setup for one room costs $40-70 in supplies on top of paint. Professional painters charge $200-500 per room including materials, or $1.50-3.50 per square foot of wall area. For floor replacement alongside painting, our flooring calculator estimates that scope separately.
Paint Types and When to Use Each
Flat (matte) paint has no sheen and hides wall imperfections well, making it ideal for ceilings and low-traffic areas like bedrooms and dining rooms. However, it marks easily and cannot be cleaned without damaging the finish. Eggshell has a subtle sheen that resists staining better than flat and is the most popular choice for living rooms and hallways. Satin paint offers moderate sheen and good washability, making it the standard for kitchens, bathrooms, and children's rooms where walls need regular cleaning.
Semi-gloss paint has noticeable shine and excellent durability, traditionally used on trim, doors, and cabinets. It highlights surface imperfections, so proper wall preparation is critical. High-gloss paint creates a dramatic, reflective finish used primarily on cabinetry and furniture. For exterior painting projects, 100% acrylic latex paint is the industry standard because it expands and contracts with temperature changes without cracking. When painting exterior trim alongside siding, use a matching sheen level for a cohesive look.
Painting Preparation and Technique
Proper preparation takes longer than the actual painting but determines the quality of the result. Start by removing all hardware (switch plates, outlet covers, curtain rods) and patching holes and cracks with spackle. Sand patches smooth once dry, then wipe all walls with a damp cloth or TSP solution to remove grease and dust. Apply painter's tape to baseboards, trim, and ceiling edges, pressing it down firmly to prevent bleed-through. Cover floors and furniture with canvas drop cloths, not plastic, which can be slippery and does not absorb spills.
Cut in the edges first using a 2-inch angled brush: paint along the ceiling line, baseboards, corners, and around window and door frames. Then use a roller to fill in the main wall area with W-shaped strokes, overlapping each stroke by about 50%. Work in 3-4 foot sections from top to bottom. Maintain a wet edge by always overlapping wet paint with the next stroke. Apply the second coat after the first is dry to the touch, typically 2-4 hours for latex paint. For ceilings, use an extension pole and work across the short dimension of the room to minimize fatigue.
Common Paint Calculation Mistakes
The biggest mistake is not accounting for texture and surface condition. Rough plaster, stucco, or unpainted drywall absorbs significantly more paint than previously painted smooth walls. Always buy 20-30% more than calculated for textured surfaces or raw drywall. The second mistake is forgetting to factor in primer, which you need on new drywall, dark-to-light color changes, and stained or smoke-damaged walls. Primer covers 300-400 sq ft per gallon and costs less than regular paint.
Not mixing all gallons together before starting is a subtle but costly error. Even with computerized color matching, there can be slight variations between cans. Pour all your paint into a 5-gallon bucket and mix thoroughly to ensure uniform color across the entire room. Similarly, buying just enough paint with no margin for error means you cannot fix mistakes or touch up damage later. Always keep at least a quart of leftover paint stored properly for future touch-ups.
Exterior vs Interior Painting Costs
Exterior painting costs $1,500-4,000 for a typical 1,500 sq ft home, or $1.50-4.00 per square foot of exterior wall surface. The price varies dramatically based on siding material (wood costs more than vinyl to prep), number of stories, and local labor rates. Exterior paint costs $30-80 per gallon and covers 250-400 sq ft depending on surface porosity. Two coats are standard, and a quality exterior paint job should last 7-10 years. Pressure washing ($200-400) and scraping or sanding ($500-1,500) are usually required before painting.
Interior painting for the same home costs $2,000-6,000 for all rooms, or $200-500 per room professionally done. The price per room stays relatively constant because setup and cleanup take the same time regardless of room size. Bathrooms and kitchens cost 20-30% more due to moisture-resistant paint requirements and more trim work. If you are also planning a tile backsplash in the kitchen, do the tile work first and paint second so the fresh paint covers any grout splatter on the walls.
For related calculations, try the concrete slab to estimate materials for your project.