Calculate exactly how many gallons of paint you need. Handles walls, ceilings, doors, windows and multiple coats.
Buying too little paint mid-project means an emergency store run and potentially a slightly-off color match from a new batch. Buying too much wastes 30 to 70 dollars per unused gallon. This paint calculator eliminates the guesswork by computing exact gallons needed based on your room's dimensions, number of doors and windows, coats required, and the coverage rate of your specific paint. The math is simple but easy to get wrong when done in your head, and the cost of getting it wrong scales with the size of your project.
Paint coverage is measured in square feet per gallon. Standard interior latex paint covers 350 to 400 square feet per gallon with one coat on a smooth, primed surface — most manufacturers print this on the label. That coverage rate assumes ideal conditions: previously painted smooth drywall, similar base color, moderate temperature and humidity. Real-world coverage often runs 10 to 20 percent lower because most surfaces have some texture (orange peel, knockdown, or heavy texture), fresh drywall absorbs paint into the porous surface, and applying paint over a different color requires more paint to fully cover. Premium paints like Behr Marquee, Benjamin Moore Aura, and Sherwin-Williams Emerald advertise "one-coat coverage" but even these usually need two coats when changing color significantly. This calculator uses 375 sqft per gallon as the default, which is a realistic middle ground for typical interior walls. Our concrete calculator uses similar area-based math for driveways and slabs, and our tile calculator handles flooring calculations for renovation projects.
| Room Size | Wall Area (8 ft ceiling) | Gallons for 2 Coats | Est. Paint Cost |
|---|---|---|---|
| 8 x 10 (small bedroom) | 288 sq ft | 1.5 gallons | $45-100 |
| 10 x 12 (bedroom) | 352 sq ft | 2 gallons | $60-140 |
| 12 x 12 (standard bedroom) | 384 sq ft | 2 gallons | $60-140 |
| 12 x 15 (large bedroom) | 432 sq ft | 2.5 gallons | $75-175 |
| 16 x 20 (living room) | 576 sq ft | 3 gallons | $90-210 |
| 20 x 20 (great room) | 640 sq ft | 3.5 gallons | $105-245 |
| Full house (2,000 sqft home) | ~4,000 sq ft walls | 15-20 gallons | $450-1,400 |
The number of coats you need matters more than most homeowners realize. One coat only produces a uniform, durable finish when you are repainting exactly the same color with a premium paint on a surface in good condition. In every other scenario, two coats is the correct answer. Two coats is required when: changing colors even slightly, painting over primer, covering stains or water spots, applying flat or matte finishes (which show single-coat unevenness more than semi-gloss), or when painting over a different sheen than the previous finish. Three coats is needed when transitioning from dark to light colors (navy to white, dark red to cream), covering strongly pigmented colors (red, deep blue, yellow, orange), or when the substrate has significant texture variations. Painters and manufacturers both build "two coats" into their assumptions — the "coverage per gallon" number on the label refers to single-coat coverage. For accurate paint math: total square footage x number of coats = total coverage needed, divided by coverage per gallon = gallons needed. Our area calculator handles the geometry side of measuring irregular spaces.
For accurate paint estimates, subtract the area of doors and windows from your total wall area — these areas won't receive wall paint. Standard door dimensions and windows have predictable sizes: an interior door averages 21 sq ft (3 ft wide by 7 ft tall), a standard window is 15 sq ft (3 ft x 5 ft), a large picture window is 25 to 40 sq ft, a small bathroom window is 6 to 9 sq ft, and a sliding glass door assembly is 40 to 50 sq ft. On a typical bedroom with one door and one window, you subtract about 36 sq ft from your wall total. Not subtracting these leads to overbuying paint by 5 to 15 percent, which is fine as buffer but wastes money on larger projects. If you plan to paint doors and trim, budget separately: interior doors need about half a quart each (both sides), and trim (baseboards, casings, crown molding) for a standard room uses about 1 quart total. Trim paint is often a different sheen (semi-gloss or gloss) than wall paint so you buy separately anyway.
| Feature | Standard Size | Square Feet |
|---|---|---|
| Interior door | 3 ft x 7 ft | 21 sq ft |
| Standard window | 3 ft x 5 ft | 15 sq ft |
| Large picture window | 5 ft x 6 ft | 30 sq ft |
| Bathroom window | 2 ft x 3 ft | 6 sq ft |
| Sliding glass door | 6 ft x 7 ft | 42 sq ft |
| French doors | 6 ft x 7 ft | 42 sq ft |
| Standard closet door | 2.5 ft x 7 ft | 17.5 sq ft |
Ceilings and walls typically get different paint. Ceiling paint is usually flat (dead matte) to hide imperfections and reduce glare — most ceilings are painted with dedicated ceiling paint that is thicker (less drip) and specifically formulated to cover in one coat. Wall paint comes in eggshell, satin, semi-gloss, or matte finishes depending on the room. If you are painting the ceiling, add the room's floor area (length x width) to your paint calculation — a 12 x 12 room adds 144 sq ft of ceiling. Ceiling paint often covers slightly better than wall paint (400 to 450 sq ft per gallon) because it's flat and thicker. For a standard room, one gallon of ceiling paint covers a 12 x 15 ceiling with one coat easily. Bathroom and kitchen ceilings should use satin or semi-gloss for moisture resistance. Our electricity bill calculator can help you budget for room renovations that also involve lighting changes.
Interior paint prices vary widely by quality tier. Budget paints (Behr Premium Plus, Valspar) cost 20 to 35 dollars per gallon and work for rental properties or areas that will be repainted often. Mid-range paints (Behr Ultra, Valspar Reserve, Sherwin-Williams SuperPaint) cost 35 to 60 dollars per gallon and provide the best value for most homeowners — good coverage, easy application, reasonable durability. Premium paints (Behr Marquee, Benjamin Moore Aura, Sherwin-Williams Emerald) cost 60 to 90 dollars per gallon and offer superior coverage (often one coat with same-color repaints), better washability, and longer color retention. Beyond paint itself, budget 30 to 80 dollars per room for supplies (brushes 15 dollars, rollers and covers 15 dollars, tape 8 dollars, drop cloths 12 dollars, primer if needed 30 dollars, tray 5 dollars). Buying enough paint upfront matters more than most buyers realize — paint mixed in the same batch has slightly different tint than paint mixed later, so the last gallon can show subtle color differences on the wall. Always round up to whole gallons rather than trying to precisely match calculated amounts. Our concrete calculator follows the same "always round up" principle for similar reasons — you can't easily buy exactly what you need mid-project.
Deciding between DIY and hiring a painter is largely economic. DIY interior painting typically costs 90 to 190 dollars per room in materials for a standard bedroom, plus 6 to 12 hours of your time per room including prep, painting, and cleanup. Professional interior painting costs 2 to 6 dollars per square foot of wall area, so a 12 x 12 room costs 700 to 2,000 dollars professionally painted, depending on prep work needed and paint quality. Whole-house interior painting professionally costs 3,000 to 9,000 dollars for a 2,000 sqft home. Professional painters bring speed, skill (cleaner lines, less spatter, better technique for tall walls), and remove the labor burden entirely. DIY makes financial sense for confident amateurs with time, when doing single rooms rather than whole houses, when you own the tools already, and for rental properties. Hiring professionals makes sense when the project is large (whole houses), involves tall ceilings requiring scaffolding, includes complex trim work, or when your time is worth more than the labor savings. Whatever route you choose, the paint calculation stays the same — knowing exactly how much paint the job needs prevents scope surprises whether you buy the paint or the painter does.
Paint quality determines the finish, but prep work determines whether that finish lasts. Clean walls with mild soap and water and let dry fully — dust, oils and residue prevent adhesion, causing peeling and flaking within months. Fill nail holes and cracks with lightweight spackle, sand smooth once dry, then spot-prime patched areas so they do not "flash" (show through as duller spots) after painting. Sand any glossy previous paint lightly to dull the surface — new paint does not adhere well to gloss. Tape trim, ceilings, and adjacent surfaces with quality painter's tape (Frog Tape or 3M Blue) and remove within 24 hours to avoid pulling paint off with the tape. Use drop cloths, not plastic sheets, on floors — plastic gets slippery when spattered and traps drips that transfer to shoes. Skipping prep to save an hour costs you a repaint within 2 to 3 years instead of 8 to 10.