How to Use the Paint Calculator
- Select Your Measurement System: Choose between U.S. Customary (feet, inches, U.S. gallons) or Metric (meters, centimeters, liters). Dimensions and coverage convert smoothly between systems.
- Pick Your Surface Type: Select Interior Wall, Ceiling, Trim / Baseboards, Exterior Fence, or Exterior Deck. The calculator displays only the dimensions relevant to that surface.
- Enter Dimensions: For a full room, enter length, width, and ceiling height. For a single accent wall, fence, or deck, enter the length and width/height. For trim and baseboards, enter room perimeter or linear run length and face width in inches (or cm).
- Subtract Openings (Walls Only): Planning defaults subtract 20 sq ft (1.86 m²) per standard door and 15 sq ft (1.39 m²) per standard window. For sliding glass doors, French doors, archways, or large picture windows, enter the exact measured area in Custom Cutouts.
- Enter Coats and Coverage: Enter the coverage rating stated on your paint can's technical data label. Preset values are starting examples; actual coverage varies by brand, sheen, porosity, and application method.
Formulas and Calculation Methods
Surface geometry and required coating volume apply the following pure formulas:
- Full Room Walls (4 Walls):
Gross Area = 2 × (Length + Width) × Wall Height - Single Wall or Flat Surface:
Gross Area = Length × Height - Ceiling or Deck Surface:
Gross Area = Length × Width - Exterior Fence:
Gross Area = Length × Height × Number of Sides (1 or 2) - Trim and Baseboards:
U.S. Mode: Area = Linear Run (ft) × (Trim Face Width (in) ÷ 12)Metric Mode: Area = Linear Run (m) × (Trim Face Width (cm) ÷ 100) - Net Paintable Area (Single Pass):
Net Area = Gross Area − Door Deductions − Window Deductions − Custom Cutouts - Total Coated Area:
Total Coated Area = Net Area × Number of Coats - Paint Volume:
U.S. gallons = Total Coated Area (sq ft) ÷ Coverage per GallonLiters = Total Coated Area (m²) ÷ Coverage per Liter
U.S. planning mode rounds up to whole gallons. Metric mode leaves liters unrounded; package sizes vary by market and product.
Worked Examples
Example 1: Bedroom (All Four Walls)
Painting all four walls of a 12 ft × 10 ft bedroom with 8 ft ceilings, 1 default door deduction (20 sq ft), 2 default window deductions (30 sq ft total), applying 2 coats at an example coverage rate of 350 sq ft per U.S. gallon:
- Room Perimeter: 2 × (12 ft + 10 ft) = 44 ft
- Gross Wall Area: 44 ft × 8 ft = 352 sq ft
- Cutout Deductions: (1 door × 20) + (2 windows × 15) = 50 sq ft
- Net Single-Coat Area: 352 sq ft − 50 sq ft = 302 sq ft
- Total Coated Area (2 coats): 302 sq ft × 2 = 604 sq ft
- Volume Required: 604 ÷ 350 = 1.73 U.S. gallons → 2 U.S. Gallons (with 96 sq ft nominal coverage remaining).
Example 2: Exterior Wood Privacy Fence
Staining both sides of a 50 ft long by 6 ft tall cedar fence with 1 coat at an example coverage rate of 250 sq ft per gallon (rough-sawn wood absorbs more coating):
- Gross Area: 50 ft × 6 ft × 2 sides = 600 sq ft
- Total Coated Area (1 coat): 600 sq ft
- Volume Required: 600 ÷ 250 = 2.40 U.S. gallons → 3 U.S. Gallons (with 150 sq ft nominal coverage remaining).
To plan the fence structure and boards separately, use the Fence Calculator with measured runs and gate openings.
Example 3: Baseboard Trim Run
Painting 50 linear feet of 4-inch baseboard molding around a room with 2 coats at 400 sq ft per gallon:
- Trim Area (Single Pass): 50 ft × (4 ÷ 12 ft) = 16.67 sq ft
- Total Coated Area (2 coats): 16.67 sq ft × 2 = 33.33 sq ft
- Volume Required: 33.33 ÷ 400 = 0.08 U.S. gallons. Check the product's locally available package sizes and label coverage before buying.
Coverage examples and product data
The calculator's starting rates are examples for estimating arithmetic, not manufacturer specifications. These published ranges show why the rate should be checked for the exact coating, surface, and application:
| Published coverage example | U.S. Rate (sq ft / gal) | Metric Rate (m² / L) | Notes |
|---|---|---|---|
| General interior paint estimate | 350 – 400 | 8.6 – 9.8 | Sherwin-Williams describes this as typical; product requirements vary. |
| Exterior wood stain, first coat | 150 – 300 | 3.7 – 7.4 | Rough or weathered wood can fall toward the lower end. |
| Deck stain planning range | 250 – 350 | 6.1 – 8.6 | Depends on wood type and selected product. |
Sources: Sherwin-Williams paint calculator guidance(about 350–400 sq ft per gallon for a typical gallon, with surface and product variation),exterior wood stain coverage guidance(150–300 sq ft per gallon on the first coat), anddeck-stain planning guidance(250–350 sq ft per gallon). The example ranges do not replace the selected product's technical data. Reviewed 15 October 2026.
Assumptions & Technical Limitations
- Opening Deductions: Standard door (20 sq ft / 1.86 m²) and window (15 sq ft / 1.39 m²) deductions are editable planning estimates. Measure custom sliding doors, French doors, or large picture windows when precision is important.
- Separate Primer Coat: When changing between contrasting colors, covering dark pigment, or painting raw drywall or bare wood, calculate primer as a separate independent pass.
- Application Method: Application loss depends on equipment, technique, surface, environment, masking, overspray, and product instructions. Check the selected coating's technical data for coverage and application guidance; this calculator does not apply a universal transfer-loss percentage.
- Color and Sheen: Some colors or sheen changes may need additional coats for the intended appearance. Check the product instructions and test a small area.
Related Home Improvement Calculators
- For a renovation that also needs new drywall, estimate panels first with theDrywall Calculator.
- Flooring Calculator — Estimate plank, tile, or hardwood box counts and room waste allowance.
- Carpet Calculator — Calculate square footage and roll cuts for residential carpeting.
- Guide: How to Calculate Paint for a Room — Comprehensive step-by-step room measuring guide.