Paint Coverage Calculator
Calculate how many gallons of paint you need for walls, ceilings, and trim, including waste factor and number of coats.
I've made the paint-buying mistake in both directions, running short mid-wall and buying so much I had a half-gallon sitting in the garage for five years. Taking two minutes to calculate before buying is always worth it, especially with paint at $50–80 per gallon now.
Paint coverage varies significantly by surface texture, paint quality, and applicator type. Smooth drywall with a roller: 350–400 sq ft/gallon. Textured or porous surfaces: 250–300 sq ft/gallon. Always round up to the next full gallon.
How paint coverage is calculated
The formula starts with total wall area, perimeter of the room multiplied by ceiling height. Subtract deductions for doors (approximately 20 sq ft each) and windows (approximately 15 sq ft each). Multiply by the number of coats. Divide by the paint's coverage rate (listed on the can, typically 350–400 sq ft per gallon for quality interior paints on smooth surfaces). Ceilings are calculated separately as length times width.
Buying strategy, rounding up
Always round up to the next full gallon. Running out of paint mid-wall forces a second store trip and risks a color match that's slightly off if you buy from a different batch. Leftover paint stores well for touch-ups, label the can with the room and color code. One extra quart is better than needing to make an emergency run. For two-coat jobs, many painters buy enough for both coats upfront.
When you need more coats
One coat works when you're repainting the same or very similar color in good condition. Two coats is standard for most interior painting projects and for any color change. Three coats may be needed when covering dark colors with light paint, painting over red or orange (notoriously difficult to cover), painting new drywall (which absorbs the first coat heavily), or achieving a very flat, professional finish.
Primer considerations
Primer is not accounted for in this calculator but is often necessary. New drywall needs primer before paint. Drastic color changes benefit from a tinted primer close to the final color, which reduces the coats of finish paint needed. Stains, knots in wood, and water damage usually require shellac or oil-based stain-blocking primer before any paint application.
Paint types and sheen
Flat/matte paint hides imperfections but is hard to clean, good for ceilings and low-traffic bedrooms. Eggshell and satin are the most popular choices for walls, washable with a slight sheen. Semi-gloss is used for trim, doors, and bathrooms where moisture resistance matters. Gloss is used for cabinetry and high-wear surfaces.
Frequently asked questions
How much does a gallon of paint cover?
Quality interior paint covers 350–400 square feet per gallon on smooth surfaces with one coat. Rough, textured, or porous surfaces absorb more paint and cover less, budget for 250–300 sq ft/gallon. Exterior paint typically covers 250–350 sq ft/gallon due to surface irregularities.
Can I use leftover wall paint for the ceiling?
Technically yes, but ceiling paint is formulated differently, it's flatter to avoid light reflection and thicker to reduce dripping. If you have significant leftover wall paint in a neutral color, it can work for the ceiling, but purpose-made ceiling paint produces better results.
How do I calculate paint for an accent wall?
Calculate just that wall: height × width × number of coats ÷ coverage rate. For a 9-ft ceiling, 14-ft wide accent wall with 2 coats: 9 × 14 × 2 = 252 sq ft ÷ 350 = 0.72 gallons. One quart (covers ~100 sq ft) plus one gallon would be sufficient.