Paint Calculator

Paint coverage depends on the paint type, surface texture, and color change. Most interior latex paints cover approximately 350 square feet per gallon on smooth surfaces. Textured walls, new drywall, and dramatic color changes (light to dark or vice versa) may require additional coats, reducing effective coverage.

The formula: calculate total wall area (perimeter × height), subtract openings (standard door ≈ 21 sq ft, standard window ≈ 15 sq ft), multiply by number of coats, then divide by paint coverage per gallon. Always round up — it's better to have leftover paint for touch-ups.

Measure wall area with our square footage calculator. For new walls, see our drywall calculator.

Paint Coverage by Type

Step-by-Step: How to Estimate Paint

Step 1: Measure the Walls

Measure the length and height of each wall in feet. Multiply length × height to get the area of each wall. For rectangular rooms, a shortcut: perimeter × ceiling height = total wall area. Perimeter = 2 × (length + width).

Step 2: Subtract Openings

Measure doors and windows. Standard deductions: a standard door is about 21 sq ft (3 × 7 ft) and a standard window is about 15 sq ft (3 × 5 ft). Subtract these from total wall area. For large picture windows or sliding doors, measure the actual dimensions.

Step 3: Add Ceiling and Trim (Optional)

For ceiling paint, add the ceiling area: length × width. For trim and baseboards, measure the linear feet around the room and multiply by the trim height (usually 4–6 inches). Crown molding typically adds 3–4 inches of paintable surface per linear foot.

Step 4: Calculate Gallons

Gallons = (paintable area × number of coats) ÷ coverage per gallon. Always round up to the next full gallon or quart. Having leftover paint is ideal for future touch-ups — colors vary slightly between batches.

Paint Coverage Chart by Room Size

This table shows estimated gallons needed for common room sizes with 8 ft ceilings, 1 standard door, and 2 standard windows — walls only:

Note: These assume 350 sq ft/gal coverage. Textured walls or dark-to-light color changes may reduce effective coverage by 20–30%.

How to Estimate Paint for Trim

Trim includes baseboards, door casings, window casings, chair rail, and crown molding. Estimating separately from walls ensures accuracy:

• Baseboards: Measure the room perimeter in linear feet. Height is typically 3.5–5.5 inches (0.29–0.46 ft). Multiply perimeter × height for total area.
• Door casings: Each standard door has about 17 linear feet of casing (both sides + top, inside and outside). At 3.5 inches wide, that's ~5 sq ft per door.
• Window casings: Each standard window has about 14 linear feet of casing. At 3.5 inches wide, that's ~4 sq ft per window.
• Crown molding: Room perimeter × crown height (typically 3–5 inches).

Tip: Semi-gloss or high-gloss paint is standard for trim — it's more durable and easier to clean than flat paint. One quart covers about 75–100 sq ft of trim.

Interior vs. Exterior Paint

Surface Preparation Guide

• New drywall: Apply 1 coat of PVA primer before painting. Primer seals the porous paper surface and reduces paint absorption — you'll use less paint and get better adhesion.
• Previously painted walls: Clean with TSP (trisodium phosphate) solution. Lightly sand glossy surfaces for adhesion. Fill nail holes and cracks with spackle.
• Textured surfaces: Expect 20–30% more paint than smooth walls. Use a thick-nap roller (¾ inch for medium texture, 1 inch for heavy texture).
• Stained or damaged walls: Use a stain-blocking primer (shellac-based for severe stains, latex-based for light stains) before painting.
• Exterior wood: Scrape loose paint, sand smooth, prime bare wood. Caulk gaps around trim, windows, and doors before painting.

2025 US Paint Cost Guide

Pro tip: Premium paint often covers in fewer coats, has better durability, and is easier to touch up. A $60/gal premium paint that covers in 1 coat can be cheaper overall than a $25/gal economy paint that needs 3 coats.