🖌️ Paint Calculator

How Much Paint Do You Need?

Estimating paint quantity accurately saves you money (avoiding over-purchase) and prevents the frustration of running short mid-project (forcing multiple store trips and risking batch color variations). This calculator computes the paintable wall area of a rectangular room, subtracts standard door and window areas, and divides by the industry-standard coverage rate of 350 square feet per gallon — then multiplies by the number of coats required.

The wall area formula for a rectangular room is: Wall Area = 2 × (Length + Width) × Height. Each standard interior door occupies approximately 21 sq ft (3 ft × 7 ft); each standard window approximately 15 sq ft (3 ft × 5 ft). These are subtracted from the total before calculating paint quantity.

Coverage Rates: What to Expect

Paint coverage varies based on several factors. The 350 sq ft/gallon standard is a reasonable baseline, but actual coverage may differ:

• High-quality paints: Some premium products cover 400–450 sq ft per gallon due to higher pigment density and better formulation. Worth paying for when you need fewer coats or exceptional durability.
• Budget/economy paints: May cover only 250–300 sq ft per gallon, often requiring more coats to achieve full coverage — potentially costing more in the end.
• Textured surfaces: Popcorn ceilings, rough stucco, or heavily textured walls absorb significantly more paint — expect 200–300 sq ft per gallon for rough textures.
• Porous surfaces (new drywall, bare plaster): Absorb large amounts of paint into the substrate. Always prime new drywall before painting — it dramatically reduces how much finish paint is needed and prevents uneven sheen.
• Dark to light color changes: Going from dark to light colors may require 3 or more coats for full coverage. Consider tinting your primer close to the finish color to reduce the number of finish coat applications needed.

Do You Need Primer? Understanding When to Prime

• Always prime when: Painting over new drywall or bare plaster (primer seals the porous surface); making a dramatic color change (dark to light or vice versa); painting over stains, smoke damage, or water marks (use a stain-blocking primer like Zinsser BIN or Kilz); painting over glossy surfaces (scuff-sand and prime for adhesion).
• You can skip primer when: Painting the same color over previously painted walls in good condition; using a "paint + primer in one" product for same-color touch-ups or recoats; the manufacturer specifically states no primer needed for your use case.
• Self-priming paints: "Paint and Primer in One" products have improved significantly. They work well for maintenance repaints in the same or similar color. For true priming challenges (stains, new drywall, dramatic color changes), a dedicated primer is still superior.

Estimating Paint for Ceilings and Trim

This calculator focuses on walls, but most rooms also require paint for ceilings and trim:

• Ceiling area: Length × Width. A 15×12 ft room has a 180 sq ft ceiling — approximately 0.5–0.6 gallons for one coat. Ceiling paint is typically flat finish and often requires only one coat if it's a recoat (same color).
• Trim (baseboards, door casings, window casings): Difficult to calculate precisely. For a 15×12 ft room, allow approximately 0.5 gallons for trim paint — semi-gloss or gloss finish for durability and cleanability.
• Separate paint types: Walls, ceilings, and trim typically use different finishes (matte/eggshell for walls, flat for ceilings, semi-gloss for trim). Buy separate products for each.

Choosing the Right Paint Finish

• Flat/Matte: No sheen, hides surface imperfections best. Good for low-traffic areas and ceilings. Not washable — marks are difficult to clean without damaging the paint.
• Eggshell: Slight sheen, more washable than flat. The most popular finish for living rooms, bedrooms, and dining rooms. Balances appearance and durability.
• Satin: Medium sheen, more durable than eggshell. Good for hallways, kids' rooms, and any high-traffic area. Resists moisture better.
• Semi-Gloss: High sheen, very durable and washable. Standard for trim, doors, kitchens, and bathrooms where cleaning is frequent.
• Gloss: Highest sheen, maximum durability. Used for exterior trim, furniture, and surfaces that need maximum protection. Highlights surface imperfections.

Frequently Asked Questions

How much extra should I buy for touch-ups? Purchase 10–15% more than calculated. Store leftover paint in a tightly sealed container, labeled with the room name and paint color code. It keeps 2–5 years properly sealed and is invaluable for touch-ups from scuffs, nail holes, and dings — and exact color matching is guaranteed.

Can I use leftover paint to touch up years later? Yes, if stored properly. Stir well before using — paint separates over time. If a skin has formed on top, strain the paint through a mesh paint strainer before use. Test on an inconspicuous spot first since the sheen may have changed slightly from the original application.

How long does paint take to dry and cure? Dry time (touch dry) is typically 1–4 hours for latex paint. Recoat time (when you can apply the next coat without disturbing the first) is usually 4 hours for latex. Full cure time (when the paint reaches maximum hardness and washability) is 2–4 weeks. Avoid washing painted surfaces or placing objects against freshly painted walls for at least 2 weeks after completion.