How to Calculate Paint
To determine how much paint you need, you first measure the total length of the walls you want to paint and multiply by the ceiling height to get the gross wall area. Then, you subtract the area of any doors and windows that will not be painted.
Formula:
Gross Wall Area = Total Wall Length (ft) × Wall Height (ft)
Deductions = (Doors × Door Area) + (Windows × Window Area)
Net Paintable Area = Gross Wall Area - Deductions
Gallons Needed = (Net Paintable Area × Number of Coats) ÷ Coverage per Gallon
Example Calculation
A room has a total wall perimeter of 40 ft and an 8 ft ceiling. It has 1 door (21 sq ft) and 2 windows (12 sq ft each).
- Gross Wall Area: 40 × 8 = 320 sq ft
- Deductions: 21 + 12 + 12 = 45 sq ft
- Net Paintable Area: 320 - 45 = 275 sq ft
- Total Area for 2 Coats: 275 × 2 = 550 sq ft
- Gallons Needed: 550 / 350 sq ft coverage = 1.57 gallons
You would need approximately 1 Gallon and 3 Quarts to complete the job.
How Much Should I Buy?
Always buy slightly more than your estimate. If you run out of paint halfway through a wall, the new can might have a slightly different color tint, and blending it will be difficult.
Frequently Asked Questions
How much paint do I need?
A standard gallon of paint covers about 350 to 400 square feet with one coat. Measure your wall length and height, subtract doors and windows, and divide the total by the paint's coverage rate.
How many coats of paint do I need?
Most interior painting projects require two coats of paint for a smooth, durable finish. If you are painting a very light color over a dark wall, you may need three coats or a coat of primer.
Does primer cover the same area as paint?
Primer often covers less area than premium paint, especially on fresh, porous drywall. A gallon of primer might only cover 200 to 300 square feet. Check the label on your specific product.
Should I subtract doors and windows?
Yes. A standard interior door takes up about 21 square feet, and a standard window is about 12 square feet. If a room has multiple doors and windows, failing to subtract them could result in buying an entire extra gallon of paint.