Paint Calculator

This calculator determines the total liquid volume of paint (in gallons or liters) and the number of 1-gallon (or 1-liter) cans needed to coat interior or exterior walls. It calculates gross wall area, deducts standard door and window openings, multiplies by your chosen number of coats, and applies a waste/touch-up allowance.

1. Enter the combined Total Wall Length (perimeter) of the room (in feet or meters).
2. Enter the Wall Height from finished floor to ceiling (in feet or meters).
3. Enter the Number of Doors (deducts 21 sq ft or 2 m² per door).
4. Enter the Number of Windows (deducts 16 sq ft or 1.5 m² per window).
5. Select the Number of Coats (default is 2 coats).
6. Enter the paint's Coverage rate (default: 350 sq ft/gal or 10 m²/L).
7. Adjust the Waste % (default 10%) for roller absorption and touch-ups.

The tool calculates the net paintable area, multiplies by the coats, divides by coverage rate, and applies your waste percentage.

A room with 40 feet of total wall perimeter, 8-foot ceilings, 1 door, 2 windows, 2 coats of paint at 350 sq ft/gal coverage, and 10% waste:

• Gross Wall Area: 40 × 8 = 320 sq ft
• Opening Deductions: (1 door × 21) + (2 windows × 16) = 21 + 32 = 53 sq ft
• Net Area: 320 − 53 = 267 sq ft
• Coated Area (2 Coats): 267 × 2 = 534 sq ft
• Base Paint Volume: 534 ÷ 350 = 1.526 gallons
• Primary Output (Gallons with 10% waste): 1.526 × 1.10 = 1.7 gallons
• Secondary Output (Cans to Buy): Ceil(1.7) = 2 (1-gal cans)
• Waste Margin: 0.2 gallons

In metric mode: A room with 12 m perimeter, 2.5 m ceiling height (30 m² gross), 1 door (2 m²), 1 window (1.5 m²), 2 coats, 10 m²/L coverage, and 10% waste yields 26.5 m² net × 2 = 53 m² coated ÷ 10 = 5.3 L × 1.10 = 5.8 Liters (purchasing 6 one-liter cans).

Substrate porosity and texture significantly alter spread rates. Unprimed fresh drywall, textured plaster, or masonry will drink up paint, reducing actual coverage to 250–300 sq ft per gallon. Dramatic color transformations (e.g. going from navy to white) will require a dedicated primer coat in addition to two topcoats.

A 10% overage covers roller nap saturation, tray residue, wall edge cutting, and leaves a small leftover container for scuff repairs over the life of the paint job.

• Planning for only one coat. Even high-end self-priming paints require two coats for rich, uniform sheen and color depth.
• Forgetting the ceiling. This calculator estimates vertical walls. If the ceiling is being painted the same color, calculate ceiling square footage separately and add it to your total.
• Not "boxing" multiple cans. When using more than one can of tinted paint, mix them together in a 5-gallon bucket to ensure seamless shade consistency across walls.

Cut in wall edges, corners, and ceiling lines with a 2.5-inch angled sash brush before rolling the open field. Keep a wet edge with your roller to avoid visible lap marks.