🪣Paint Calculator — How many gallons of paint do I need for a room?

The Paint Calculator estimates how many gallons or liters of paint you need for any painting project. Enter your room dimensions or total wall area, the coverage rate for your paint, number of coats, and the calculator returns gallons needed with a recommended purchase buffer — so you buy enough paint without massive leftover.

Accurately estimating paint quantity saves money and avoids mid-project supply runs. Most standard interior paints cover 350–400 square feet per gallon per coat. Flat and matte finishes often require 2 coats for full opacity, while high-quality eggshell and satin paints can achieve good coverage in one coat on primed walls. Exterior paints typically cover 250–350 sq ft per gallon because porous wood and textured surfaces absorb more product.

For rooms, the calculator handles standard rectangular layouts (4 walls + ceiling option) with automatic door and window deductions. A standard interior door is approximately 21 sq ft (7 ft × 3 ft) and a standard window is approximately 15 sq ft. Deducting these prevents over-buying for surfaces that will not be painted.

For custom projects — murals, trim, furniture, cabinets, ceilings, or exterior siding — use the custom area input to enter total square footage directly. The calculator scales with any coverage rate, so you can enter your specific paint brand's specification in sq ft per gallon or sq m per liter.

Practical buying advice: always add a 10-15% buffer above the calculated amount for touch-ups, drips, and coverage variation on porous surfaces. If you are painting new drywall that has never been primed, expect actual coverage to be 15-20% lower than the rated specification. Save the remaining paint in a tightly sealed container for future touch-ups — keeping the batch consistent avoids noticeable color patches.

For multi-color projects, the calculator supports separate areas per color. Accent walls, trim colors, and ceiling tones all get separate quantity calculations so you can create a shopping list with specific amounts per color rather than a single aggregate.

Professional painters and contractors can use this tool to generate client-facing material estimates before job costing. Accurate paint quotes prevent under-ordering, which delays projects, or over-ordering, which wastes material budget.

Paint quantity follows a direct coverage formula with adjustments for room geometry.

Step 1: Calculate total paintable area For rooms: Wall Area = 2 × (Length + Width) × Height Subtract: Door openings (typically 21 sq ft each) + Window openings (typically 15 sq ft each) Add ceiling if needed: Length × Width

Step 2: Calculate paint needed Gallons needed = (Paintable Area × Number of Coats) / Coverage rate (sq ft per gallon)

Step 3: Apply purchase buffer Recommended purchase = Gallons needed × 1.10 (10% buffer for touch-ups and variation)

Worked example Room: 12 ft × 14 ft, 9 ft ceiling, 2 doors, 1 window Wall area = 2 × (12 + 14) × 9 = 468 sq ft Deductions = (2 × 21) + (1 × 15) = 57 sq ft Paintable area = 468 − 57 = 411 sq ft With 2 coats: 411 × 2 = 822 sq ft to cover At 350 sq ft/gallon: 822 / 350 = 2.35 gallons With 10% buffer: 2.35 × 1.10 = 2.59 → buy 3 gallons