Paint Color Mixing Calculator

Paint quantity calculation ensures you buy the right number of cans for a complete job without wasteful excess or frustrating mid-project shortages. The calculation accounts for wall area, ceiling area, number of coats, and paint coverage rate.

Wall area formula: Wall Area = Perimeter × Wall Height − Door and Window Area

Where:

• Perimeter = 2 × (Room Length + Room Width)
• Door area ≈ 21 sq ft each (standard 3×7 door); subtract for doors, subtract or not for windows depending on whether you paint the wall behind blinds
• Window area ≈ 15 sq ft average; subtract if painting around, or leave if painting behind window treatments

Ceiling area: Ceiling Area = Room Length × Room Width

Total paint needed: Gallons = (Total Area × Number of Coats) ÷ Coverage per Gallon

Where:

• Coverage per Gallon — typically 350–400 sq ft per gallon on smooth surfaces; 250–350 sq ft on textured or porous surfaces; always use the lower end for dark-to-light color changes
• Number of Coats — 1 coat for same-color touch-up; 2 coats standard; 3 coats for dramatic color changes or primer-skipped surfaces

What each variable means:

• Walls vs. ceiling — typically different paint sheens: flat/matte for ceilings, eggshell/satin for walls, semi-gloss for trim
• Primer vs. paint — over bare drywall, primer first (1 coat) then paint (2 coats); over existing paint in similar color, 2 coats paint only is often sufficient
• Round up — always round up to the next full can; running out mid-wall creates visible lap lines when the paint dries

Reference: paint sheen guide:

• Flat/Matte: Ceilings and low-traffic areas; hides imperfections, not washable
• Eggshell: Living rooms, bedrooms; slightly washable
• Satin: Kitchens, hallways; more washable, light sheen
• Semi-gloss: Bathrooms, trim, doors; highly washable, noticeable sheen
• High-gloss: Cabinets, furniture; extremely durable and washable

Worked example: Room: 12ft × 14ft, 9ft ceiling. 1 door, 2 windows. 2 coats on walls, 1 coat on ceiling.

• Perimeter = 2 × (12 + 14) = 52 ft
• Wall area = 52 × 9 = 468 sq ft − (21 + 30) = 417 sq ft
• Ceiling area = 12 × 14 = 168 sq ft
• Wall paint = (417 × 2) ÷ 350 = 834 ÷ 350 = 2.38 gallons → buy 3 gallons
• Ceiling paint = (168 × 1) ÷ 400 = 0.42 gallons → buy 1 quart (32 oz)