Paint Calculator

Calculate Paint Coverage for Interior and Exterior Projects

Running out of paint mid-project is frustrating—unfinished walls, wasted time, and the hassle of color-matching a new batch. Buying too much wastes $30-60 per gallon on paint that sits in the garage until it goes bad. This calculator determines exactly how many gallons you need based on wall area, number of coats, and paint type.

Understanding Paint Coverage Rates

Manufacturers claim 350-400 sq ft per gallon, but real-world coverage varies:

• Smooth drywall: 400 sq ft/gallon. Ideal conditions with primer already applied.
• Textured walls: 300-350 sq ft/gallon. Texture increases surface area requiring more paint.
• Bare drywall (unprimed): 250-300 sq ft/gallon. Porous surface absorbs more paint.
• Dark-to-light color changes: Expect to use 25-50% more paint or add an extra coat. Primer is cheaper than extra topcoat—always prime when going from dark to light.
• Exterior surfaces: Wood siding: 250-350 sq ft/gal. Stucco: 200-300 sq ft/gal (very porous). Brick: 150-250 sq ft/gal.

Measuring Walls for Paint

1. Calculate wall area: Measure room perimeter and multiply by ceiling height. Standard 8-foot ceilings in a 12×15 room = (12+12+15+15) × 8 = 432 sq ft.

2. Subtract openings: Deduct 20 sq ft per door, 15 sq ft per standard window. For rooms with many windows/doors, this reduces paint needs significantly—a room with 3 windows and 2 doors saves 90 sq ft (nearly ¼ gallon).

3. Don't forget ceiling: Ceiling area = room length × width. Use the same paint or separate ceiling paint (flat white is common). Ceilings need 1-2 coats depending on existing color and paint quality.

4. Account for coats needed: Most projects need 2 coats for even color and durability. Primed surfaces or similar color changes may only need 1 coat with quality paint.

Choosing the Right Paint

Primer first: Always prime new drywall, bare wood, or when making drastic color changes (dark to light or vice versa). Primer costs $20-30/gallon vs $30-60 for paint, and it improves topcoat coverage dramatically.

Sheen matters:

• Flat/matte: Hides imperfections, not washable, best for ceilings and low-traffic areas.
• Eggshell: Slight sheen, washable, good for living rooms and bedrooms.
• Satin: Durable, washable, ideal for kitchens, bathrooms, and hallways.
• Semi-gloss/gloss: Very durable, highlights imperfections, use for trim, doors, and cabinets.

Quality tiers: Budget paint ($20-30/gal) requires 3+ coats and doesn't last. Mid-grade ($30-45/gal) offers good coverage and durability. Premium ($50-75/gal) covers in 1-2 coats, lasts longer, and is worth it for large projects or high-traffic areas.

Painting Best Practices

Prep is 80% of the job: Clean walls (TSP or degreaser), fill holes with spackle, sand smooth, caulk gaps where trim meets walls. Bad prep shows through the best paint.

Cut in edges first: Use a brush to paint a 2-3 inch border around ceiling, trim, and corners. Then roll the center sections. Cutting in while the edge is wet prevents lap marks.

Use quality rollers: ⅜" nap for smooth walls, ½" for light texture, ¾" for heavy texture or exterior. Cheap rollers shed fibers and create poor finishes. Spend $8-12 on a good roller cover.

Don't overload the roller: Dip ½ of roller in paint, roll excess on tray grid. Too much paint causes drips and uneven coverage. Thin, even coats dry faster and look better than thick, drippy coats.

Common Painting Mistakes

Skipping primer: Topcoat alone on bare drywall or dark colors requires 3-4 coats to look right. One coat of primer + two coats of paint costs less time and money than four coats of paint.

Painting in wrong conditions: Don't paint below 50°F or above 85°F. High humidity (>70%) prevents proper drying and causes adhesion problems. Ideal conditions: 60-75°F, 40-60% humidity.

Not stirring paint thoroughly: Pigments settle to the bottom. Stir for 2-3 minutes before using and occasionally during painting. Unstirred paint causes color inconsistency.

Rushing between coats: Follow manufacturer's dry time (usually 2-4 hours for latex). Painting over tacky paint causes peeling and uneven finish. When in doubt, wait longer.