Paint Calculator

Professional painters spend 60 to 70% of total project time on preparation. Fill nail holes and small cracks with lightweight spackle, let it dry, then sand smooth with 120-grit paper. Wash walls with a damp cloth or TSP substitute to remove grease and dust — paint will not adhere well to a dirty or glossy surface. Tape edges along trim, windows, and doors carefully with painter's tape, pressing the tape edge down firmly to prevent bleed-through.

For new drywall, never skip primer. Raw drywall is extremely porous and will absorb the first coat of paint unevenly, creating a blotchy finish even with two topcoats. A single coat of PVA drywall primer seals the surface, reduces total paint consumption by 30 to 40%, and produces a dramatically more even finish. On walls with smoke stains, water stains, or marker, use a shellac-based stain-blocking primer to prevent the stains from bleeding through all your topcoats.

Two coats is the standard for most interior color changes. A single coat rarely provides full, even coverage — the old color telegraphs through, especially at lower sheen levels like flat and eggshell. When painting over a dramatically different color — dark to light, or a very saturated color to a neutral — apply a tinted primer first. Tint the primer halfway between the old color and the new color; this gray-shifts the base so the topcoats cover in two coats rather than three or four.

Ceiling paint is typically applied in two coats as well, and ceiling white is a different formulation than wall paint — thicker, with higher hiding power per coat to minimize drips when applied overhead. Using wall paint on ceilings is possible but often requires an extra coat. For trim, doors, and windows, semi-gloss or gloss paint is standard and typically requires two coats over a properly primed or sanded surface. Skimping to one coat on trim is noticeable at every angle and in every type of light.

The formula starts with total wall area: Wall Area = 2 × (Length + Width) × Height. This calculates the combined area of all four walls in a rectangular room. Door and window areas are subtracted to get net paintable area. Gallons needed is then: (Paintable Area × Number of Coats) / Coverage per Gallon. Coverage per gallon defaults to 350 sq ft, which is the conservative end of the 350 to 400 sq ft range stated on most latex paint labels.

Use the conservative end of the coverage range for rough or textured surfaces where the paint sinks into the texture and requires more product per square foot. Smooth drywall and previously painted walls in good condition can stretch toward the 400 sq ft end. The calculator rounds up to the nearest whole gallon — it is better to have a partial gallon left over for touch-ups than to run short mid-wall and have to buy an additional gallon at a different store or batch, which risks slight color variation from lot-to-lot differences in manufacturing.