Free Paint Calculator — Gallons & Primer Estimator

How to Use the Paint Calculator

How to Calculate Paint for a Room

Paint coverage is calculated with the formula Total Wall Area ÷ Coverage Rate per Gallon × Number of Coats. Measure each wall's width and height, multiply to get the square footage, then subtract the area of doors and windows to find your net paintable surface. Divide that number by the paint's coverage rate — typically 350 to 400 square feet per gallon — and multiply by the number of coats to get the total gallons required.

For example, a standard 12' × 12' room with 8' ceilings has four walls totaling 384 square feet. Subtract one standard door (20 sq ft) and two standard windows (24 sq ft total) and you get approximately 340 square feet of paintable area. At 350 square feet per gallon with two coats, you need about 1.94 gallons — so purchase two gallons. This calculator handles these measurements and deductions automatically, letting you enter dimensions in feet-inches-fractions, decimal feet, or metric formats.

Understanding Paint Coverage Rates

Not all paint finishes cover the same area per gallon. The sheen level directly affects how far a gallon will spread because glossier paints contain more resins and fewer solids per volume. Flat and matte finishes offer the best coverage at roughly 400 square feet per gallon, making them the top choice for ceilings and low-traffic rooms. Eggshell and satin finishes cover approximately 350 square feet per gallon and are the most popular choice for living rooms, bedrooms, and hallways because they balance washability with coverage. Semi-gloss and gloss finishes cover around 300 square feet per gallon but provide superior moisture resistance, making them ideal for kitchens, bathrooms, and trim work.

Premium paints from brands like Benjamin Moore Regal Select and Sherwin-Williams Duration often achieve coverage at the higher end of these ranges due to better pigment density and binder formulations. Budget paints may fall 10–15% below the rated coverage, especially on porous or textured surfaces. When in doubt, use the lower coverage rate in your estimate to avoid a mid-project trip to the paint store.

Room Mode vs Area Mode

This calculator offers two input modes to handle different painting scenarios. Room mode is designed for standard rectangular rooms — you enter the room length, width, and wall height, and the calculator computes the total wall perimeter and paintable area automatically. It also lets you specify the number of doors and windows to subtract from the total. Room mode is fastest for bedrooms, offices, and other spaces with a simple floor plan.

Area mode is for non-standard situations: accent walls, exterior siding, vaulted ceilings, L-shaped rooms, or any surface where you already know the square footage. Enter the total paintable area directly and the calculator skips the room geometry step entirely. Use area mode when you have already measured your walls with a laser measure or when the room shape does not fit a simple rectangle.

Accounting for Doors and Windows

Every door and window you subtract from the wall area reduces the amount of paint you need. The standard deductions used by professional painters are 20 square feet per door (a standard 3'-0" × 6'-8" interior door) and 12 square feet per window (a typical 3' × 4' double-hung window). These are conservative estimates that account for the fact that you usually need to cut in carefully around the trim anyway, which uses some paint.

For rooms with oversized openings — sliding glass doors (40–50 sq ft), picture windows (25–30 sq ft), or French doors (30–40 sq ft) — switch to area mode and measure the actual paintable wall surface directly. The standard deductions work well for most bedrooms and living rooms, but they will underestimate openings in rooms with large windows or multiple exterior doors.

Primer: When You Need It

Primer is a separate coating applied before paint to improve adhesion, block stains, and create a uniform base color. You need primer in three main situations: new drywall or bare wood (the porous surface will absorb paint unevenly without a sealer coat), dark-to-light color changes (primer blocks the old color from bleeding through and reduces the number of topcoats needed from three or four down to two), and stain blocking (water stains, smoke damage, knots in wood, and tannin bleed all require a shellac-based or high-hide primer before painting).

If you are repainting a wall that is already in good condition with a similar or darker color, you can skip the primer entirely. Many paint-and-primer-in-one products work well for these same-color repaint scenarios. However, for new construction or dramatic color changes, a dedicated primer coat will save you money by reducing the total number of paint coats required. Primer typically covers 300–400 square feet per gallon depending on the formula.

Pro Tips for Accurate Paint Estimation

• The second coat always uses less paint than the first. The first coat seals the surface and absorbs into the substrate, while the second coat glides over a sealed surface. Expect about 10–15% less paint consumption on the second coat compared to the first.
• Add 10% to your final estimate for touch-ups, cutting in around trim, and future repairs. A quart of leftover paint stored properly will last years and is invaluable for covering scuffs, nail holes, and minor wall damage.
• Vaulted and cathedral ceilings increase wall area significantly. A room with a 12' vaulted peak has 50% more wall area than the same room with standard 8' ceilings. Use area mode and measure the actual wall height at multiple points along the slope.
• Textured walls — knockdown, orange peel, or heavy stipple — consume 15–25% more paint than smooth drywall. The texture creates peaks and valleys that increase the effective surface area. Lower your coverage rate estimate accordingly.
• Ceiling paint is typically flat white and costs less per gallon than wall paint. Calculate ceiling area separately (length × width of the room) and use the flat finish coverage rate of 400 square feet per gallon. One gallon covers a 20' × 20' ceiling in a single coat.