The Paint Coverage Formula Saves Money on Every Project

One gallon of paint covers approximately 350 to 400 square feet. That number is printed on every paint can and is wrong for most real situations. Porous surfaces textured walls and dark-to-light color changes all reduce coverage significantly.

The formula

Paint needed (gallons) = Total wall area (sq ft) / Coverage per gallon * Number of coats

Total wall area for a rectangular room:

function wallArea(length, width, height, doors, windows) {
  const perimeter = 2 * (length + width);
  const grossArea = perimeter * height;
  const doorArea = doors * 21; // Standard door: 3x7 = 21 sq ft
  const windowArea = windows * 12; // Standard window: ~12 sq ft
  return grossArea - doorArea - windowArea;
}

function gallonsNeeded(area, coveragePerGallon = 350, coats = 2) {
  return Math.ceil((area * coats) / coveragePerGallon);
}

// 12x15 room, 8ft ceiling, 2 doors, 3 windows
const area = wallArea(12, 15, 8, 2, 3);   // 396 sq ft
const gallons = gallonsNeeded(area);         // 3 gallons

Adjusting coverage

The 350 sq ft/gallon figure assumes:

• Smooth, primed drywall
• Similar existing color
• Quality paint applied with roller

Reduce coverage for:

• Textured surfaces: 250-300 sq ft/gallon
• Unprimed drywall: 250-300 sq ft/gallon
• Dark-to-light color change: add an extra coat
• Exterior surfaces: 250-300 sq ft/gallon

The cost of getting it wrong

Buying too little paint means a mid-project trip to the store, and the new batch might not match exactly (same color code, slightly different tint). Buying too much wastes money, though leftover paint is useful for touch-ups.

The safe approach: calculate the needed amount and round up to the next full gallon. For large projects, buy one extra gallon as reserve.

For calculating paint quantities with room dimensions and coverage adjustments, I built a calculator at zovo.one/free-tools/paint-calculator. Enter your room dimensions and surface conditions, and it tells you exactly how many gallons to buy.

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I'm Michael Lip. I build free developer tools at zovo.one. 500+ tools, all private, all free.