dopecalc

Paint Calculator

Estimate the amount of paint needed for a room by calculating total wall area minus doors and windows. Accounts for multiple coats and standard paint coverage rates.

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About this Calculator

Estimate the amount of paint needed for a room by calculating total wall area minus doors and windows. Accounts for multiple coats and standard paint coverage rates.

Formula & Calculations

Formula

Wall Area = 2 × (Length + Width) × Height; Net Area = Wall Area − (Doors × 21 + Windows × 15); Gallons = Net Area × Coats / 350
Where:
  • L=Length of the room in feet
  • W=Width of the room in feet
  • H=Ceiling height in feet
  • Doors=Number of doors (assumes ~21 sq ft per door)
  • Windows=Number of windows (assumes ~15 sq ft per window)
  • Coats=Number of paint coats to apply

Assumptions

  • Standard door opening is approximately 21 square feet (3 ft × 7 ft).
  • Standard window opening is approximately 15 square feet (3 ft × 5 ft).
  • One gallon of paint covers approximately 350 square feet per coat.
  • Ceiling paint is not included in this calculation.

Calculation Examples

Example 1

Inputs:Room: 12 ft × 10 ft × 8 ft, 1 door, 2 windows, 2 coats
Result:Net area ~301 sq ft, ~1.72 gallons (buy 2 gallons)

Wall area = 2 × (12+10) × 8 = 352 sq ft. Subtract 1 door (21) + 2 windows (30) = 301 sq ft. 301 × 2 coats / 350 = 1.72 gallons.

Example 2

Inputs:Room: 15 ft × 12 ft × 9 ft, 2 doors, 3 windows, 1 coat
Result:Net area ~423 sq ft, ~1.21 gallons (buy 2 gallons)

Wall area = 2 × (15+12) × 9 = 486 sq ft. Subtract 42 + 45 = 87, net = 399 sq ft. 399 × 1 / 350 ≈ 1.14 gallons.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

On average, one gallon covers 350-400 square feet per coat on smooth surfaces. Rough or textured surfaces may reduce coverage. Always check the manufacturer's label.

Should I buy extra paint for touch-ups?

Yes. It's wise to keep at least one quart of each color for future touch-ups. Paint colors can be difficult to match exactly later.

Does this calculator include the ceiling?

No, this calculator estimates wall paint only. To include the ceiling, add the room's square footage (Length × Width) divided by 350 to your gallon count.