Construction

Paint Calculator

Estimate how many gallons of paint you need for any surface. Enter the total area and number of coats for an accurate quantity with a 10% waste allowance.

For a room: add up all wall areas (L × H for each wall)

Enter the surface area and number of coats to estimate paint needed.

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Paint Quantity Formula

Paint quantity is estimated from total surface area and number of coats, with a standard coverage of 350 sq ft per gallon and a 10% waste allowance.

Gallons = (Area × Coats) ÷ 350

With waste = Gallons × 1.10

Example: 400 ft² × 2 coats ÷ 350 = 2.29 gal × 1.10 = 2.5 gallons

Getting Paint Coverage Right

Paint coverage varies by product quality, surface porosity, and application method. Premium paints with higher pigment concentration typically cover 400 sq ft per gallon, while economy paints may only reach 300 sq ft. Always check the coverage spec on the specific paint can you plan to purchase.

Roller nap thickness also affects coverage — a thick 3/4-inch nap for textured surfaces holds more paint and wastes more, while a 3/8-inch nap for smooth walls is more efficient. Spray application typically increases waste by 20–30% compared to rolling.

Frequently asked questions

How much does one gallon of paint cover?
One gallon of standard interior or exterior paint covers approximately 350–400 square feet per coat on smooth surfaces. Rough or porous surfaces like brick or unprimed drywall may only yield 250–300 square feet per gallon due to greater absorption. This calculator uses 350 sq ft per gallon as a conservative standard estimate.
How many gallons of paint do I need for a room?
Add up the wall areas of the room (length × height for each wall) and subtract door and window areas. Divide the total by 350 for one coat. For a typical 12 ft × 12 ft room with 8 ft ceilings, the four walls total about 384 sq ft, which requires roughly 1.1 gallons per coat or 2.2 gallons for two coats.
Do I need two coats of paint?
Two coats are recommended in most situations for even color coverage and durability. One coat may suffice when painting a very similar color over an existing finish in good condition. Three coats are needed when covering dark colors with a lighter shade, using primer on bare surfaces, or applying specialty paints like chalk paint that have lower coverage.
Should I buy extra paint beyond what the calculator shows?
Yes — buy at least 10% more than calculated. This calculator already includes a 10% overage factor. The extra paint covers touch-ups, roller waste, and slight measurement errors. Keep leftover paint in an airtight container labeled with the room name and color for future touch-ups, as color matching is never perfect.
How do I calculate paint for ceilings and trim?
Calculate ceiling area separately (room length × width) and add it to your wall area if painting both. Trim, doors, and baseboards are typically painted with a semi-gloss or gloss finish paint sold separately from wall paint. A single gallon of trim paint is usually sufficient for a standard room's worth of baseboards, window trim, and one door.

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