Paint Calculator

How the Paint Calculator Works

This calculator estimates paint coverage based on the paintable wall area in your room, minus deductions for doors and windows. Most interior paints cover 350–400 square feet per gallon at recommended film thickness. This calculator uses 350 sq ft per gallon as a conservative estimate — a good buffer for porous surfaces and color changes.

The calculation:

1. Wall area: Perimeter (2 × length + 2 × width) × wall height
2. Subtract doors: Each standard interior door ≈ 20 sq ft (2.5' × 6.8')
3. Subtract windows: Each standard window ≈ 15 sq ft (3' × 5')
4. Divide by 350 to get gallons per coat, then multiply by number of coats
5. Ceiling (optional): Length × width ÷ 350
6. Primer (optional): Same coverage rate as paint, 1 coat

1 Coat or 2 Coats of Paint?

The number of coats depends on the project:

• 1 coat works when: You're using the same color (touch-up), applying paint + primer combo on white walls, or using a very high-hide formula
• 2 coats are standard for: Color changes, covering dark colors, new drywall, or achieving a uniform, professional finish on most projects
• 3 coats may be needed for: Painting over deep reds or dark blues, applying white over black, or porous unprimed surfaces

Professional painters almost always apply 2 coats. The second coat costs roughly 40% more paint but dramatically improves coverage uniformity, especially near edges and corners.

When Do You Need Primer?

Primer is a separate, purpose-built coating applied before paint. It improves adhesion, seals porous surfaces, and blocks stains. You need primer when:

• New drywall: Bare drywall paper is extremely porous — primer is essential, not optional
• Major color change: Going from dark to light (e.g., navy to white) often requires a tinted primer to reduce the number of topcoats needed
• Stains or water damage: A stain-blocking primer (like Zinsser BIN or KILZ) prevents bleed-through that topcoat alone can't stop
• Glossy surfaces: Painting over semi-gloss or gloss without primer causes adhesion failures
• Previously unpainted wood or metal: Always prime

"Paint + Primer in one" products are fine for minor updates on already-painted surfaces in similar colors. They're not a substitute for true primer on new drywall or stain situations.

Paint Types and Sheens Explained

Interior paint comes in several sheens, each with different durability and visual characteristics:

• Flat/Matte: No reflectivity. Hides surface imperfections best. Hard to clean — best for ceilings and low-traffic areas
• Eggshell: Slight sheen (10–25% gloss). The most common wall finish for living rooms and bedrooms. Wipes clean reasonably well
• Satin: Noticeably shiny (25–35% gloss). More durable and moisture-resistant. Ideal for hallways, children's rooms, and family rooms
• Semi-gloss: 35–70% gloss. Excellent for moisture and humidity. Standard for kitchens, bathrooms, and trim
• High-gloss: 70–90% gloss. Extremely durable and washable but reveals every wall imperfection. Used on doors, cabinets, and built-ins

A common approach: flat/matte on ceilings, eggshell or satin on walls, semi-gloss on trim and doors.

Interior vs. Exterior Paint

Never use interior paint outdoors. Interior and exterior paints use different binder formulations:

• Interior paint: Engineered for low VOCs, scrubability, and stable color indoors. Binders are harder and less flexible. Will crack, chalk, and fail outdoors within 1–2 seasons
• Exterior paint: Contains UV-resistant pigments, mildew inhibitors, and flexible binders that expand/contract with temperature changes. Higher VOC content. Generally more expensive ($45–80/gallon vs. $30–65/gallon for interior)

Exterior paint coverage is similar — roughly 300–400 sq ft per gallon depending on surface texture. Rough wood siding absorbs much more paint than smooth surfaces; budget 20–30% extra for rough or weathered surfaces.

How to Buy Paint in the Right Quantity

Once you know how many gallons you need, here's how to buy efficiently:

• Round up to the nearest gallon — running out mid-project and needing a second batch risks a color mismatch even with the same paint code
• Buy a test quart first for color — paint looks dramatically different on a real wall vs. a chip. Apply a 12"×12" patch and evaluate in both natural and artificial light
• Gallons vs. quarts: A quart costs ~60–70% of a gallon price but covers only 25% of the area. For projects over 100 sq ft, always buy gallons
• 5-gallon buckets: If you need more than 3 gallons of the same color, buy a 5-gallon bucket — it's 15–20% cheaper per gallon and reduces dye lot inconsistency
• Save the lid: Write the room, purchase date, and mix formula on the lid so you can match it perfectly for touch-ups 5 years later

Frequently Asked Questions

How many gallons do I need for a 12×12 room? For a standard 12×12 room with 9-foot ceilings, walls only, 1 door, and 1 window, you'll need about 1 gallon for 1 coat or 2 gallons for 2 coats.

Does a dark color require more paint? Dark colors don't necessarily require more gallons of finish coat — but they often require primer or more coats to achieve full coverage when going lighter. When going darker, coverage is generally fine at 2 coats.

How long does a gallon of paint last in storage? Properly sealed latex paint lasts 10 years; oil-based lasts up to 15. Store at room temperature — freezing destroys latex paint permanently. Stir well before use; if it's lumpy or smells sour, discard it.

What's the most popular interior paint brand? Sherwin-Williams (Emerald, Duration), Benjamin Moore (Aura, Regal), and Behr Premium Plus (available at Home Depot) are consistently top-rated. Sherwin-Williams and Benjamin Moore are professional-grade; expect to pay $65–85/gallon. Behr is a solid value at $35–55/gallon.