Paint Area Calculator
Paintable Area
Gallons Needed
Room Paint Diagram
InteractiveHow to Calculate Paint Area in Square Feet
Buying the right amount of paint means calculating the total paintable surface area, subtracting openings, and accounting for multiple coats. Too little paint forces an mid-project store run (and risks color batch mismatches). Too much paint wastes money and storage space.
Step 1: Calculate Gross Wall Area
Add the lengths of all four walls (perimeter) and multiply by the ceiling height. For a 15×12 ft room with 9 ft ceilings: perimeter = 2(15+12) = 54 ft. Gross wall area = 54 × 9 = 486 sq ft.
Paint Area Formula
FormulaExample: (54 ft × 9 ft) − 45 sq ft openings = 441 sq ft paintable area
Step 2: Subtract Openings
A standard interior door is about 21 sq ft (3×7 ft). A standard window is about 12 sq ft (3×4 ft). Subtract each opening from the gross wall area. Don't skip this — in a room with 2 doors and 3 windows, you'd over-buy by nearly 80 sq ft worth of paint.
Step 3: Multiply by Coats
Most interior painting requires 2 coats for uniform coverage. Painting over dark colors with light paint may need 3 coats. Multiply your net paintable area by the number of coats, then divide by the paint coverage (typically 350–400 sq ft per gallon) to get gallons needed.
Paint Coverage Reference by Surface Type
| Surface Type | Coverage per Gallon | Notes |
|---|---|---|
| Smooth Drywall | 350–400 sq ft | Best coverage, standard assumption |
| Textured Drywall | 300–350 sq ft | Texture absorbs more paint |
| New/Unprimed Drywall | 250–300 sq ft | Primer coat needed first |
| Wood / Trim | 300–350 sq ft | Grain absorbs paint unevenly |
| Brick / Masonry | 200–300 sq ft | Porous surface, heavy absorption |
| Stucco (Exterior) | 150–250 sq ft | Very rough texture |
| Metal | 350–500 sq ft | Non-porous, good coverage |
| Concrete Floor | 250–350 sq ft | Depends on sealer and porosity |
Room-by-Room Paint Estimation Guide
Standard Bedroom (12×12 ft, 9 ft ceiling)
Perimeter = 48 ft. Gross wall area = 432 sq ft. Subtract 1 door (21 sq ft) + 2 windows (24 sq ft) = 387 sq ft. With 2 coats and 350 sq ft/gallon coverage: 387 × 2 ÷ 350 = 2.2 gallons → buy 3 gallons.
Living Room (15×18 ft, 9 ft ceiling)
Perimeter = 66 ft. Gross wall area = 594 sq ft. Subtract 2 doors + 3 windows = 78 sq ft → 516 sq ft. With 2 coats: 516 × 2 ÷ 350 = 2.9 gallons → buy 3 gallons. Add a 4th gallon if painting the ceiling (270 sq ft).
Bathroom (5×8 ft, 8 ft ceiling)
Perimeter = 26 ft. Gross wall area = 208 sq ft. Subtract 1 door + 1 window = 33 sq ft → 175 sq ft. With 2 coats: 175 × 2 ÷ 350 = 1 gallon. Bathrooms need moisture-resistant paint — slightly more expensive per gallon.
Ceilings
Ceiling area = room length × width. A 15×12 room has 180 sq ft of ceiling. Ceiling paint is thicker to prevent dripping and typically covers 350–400 sq ft per gallon. One gallon with 2 coats handles rooms up to 175 sq ft ceiling area.
Frequently Asked Questions
Measure the room's perimeter (sum of all wall lengths) and multiply by ceiling height to get gross wall area. Subtract doors and windows. Multiply by number of coats. Divide by 350 sq ft/gallon and round up to the nearest whole gallon.
One gallon covers approximately 350–400 square feet on smooth drywall with one coat. Textured walls, raw wood, or masonry absorb more paint and reduce coverage to 200–300 sq ft per gallon. Always check the manufacturer's label for specific coverage.
A 12×12 room with 9 ft ceilings has ~387 sq ft of paintable wall area (after door/window deductions). With 2 coats: 774 sq ft ÷ 350 = 2.2 gallons. Buy 3 gallons. Add 1 more gallon if you're painting the ceiling too.
Yes. A standard door (21 sq ft) and window (12 sq ft) each save measurable paint. In a small bathroom, skipping the deduction would overestimate by 15–20%. Use 21 sq ft per door and 12 sq ft per standard window as defaults.
Two coats is standard for most interior walls. Use 3 coats when covering dark colors with light paint, painting over patched areas, or using low-coverage colors like reds and yellows. One coat may suffice for touch-ups with matching paint.
Primer is recommended for new drywall, stain-blocking, or drastic color changes. Primer coverage is similar to paint (300–400 sq ft/gallon). Calculate primer separately — it's an additional gallon count on top of your paint coats.
Measure each exterior wall's width and height, sum all wall areas, and subtract windows, doors, and garage openings. Exterior surfaces (siding, stucco, brick) have lower coverage rates — use 250–350 sq ft per gallon depending on texture.
Measure the linear feet of trim and multiply by the trim width (typically 3.5–5.5 inches = 0.3–0.46 ft). A room with 40 linear feet of 4-inch baseboard has 13 sq ft of trim. One quart covers 75–100 sq ft — usually enough for 2 coats of all trim in an average room.