Paint Calculator

How paint is calculated

Paintable area = wall area + ceiling (if applicable) − doors − windows. Multiply by number of coats to get total square footage. Divide by coverage rate (350 ft²/gal standard) to get gallons. Round up to the nearest half-gallon — most retailers sell in pints, quarts, half-gallons, and gallons.

Wall area = perimeter × ceiling height. Perimeter for a 15×12 ft room = 2 × (15 + 12) = 54 ft. With 8 ft ceilings, wall area = 432 ft². Subtract ~21 ft² for one door and ~30 ft² for two windows: 381 ft² paintable. Two coats = 762 ft² total. ÷ 350 = 2.18 gallons. Round to 2.5 gallons.

Coverage by paint type

• Latex flat / matte: 350–400 ft²/gal. Best on ceilings and low-traffic walls.
• Latex eggshell / satin: 350 ft²/gal. Most common for living spaces.
• Latex semi-gloss / gloss: 350 ft²/gal. Best for trim, doors, kitchens, bathrooms.
• Oil-based: 350–400 ft²/gal. Slower drying. Used less today.
• Primer: 200–300 ft²/gal. Less than topcoat coverage.
• Textured / popcorn ceiling paint: 200–250 ft²/gal. Eats more paint.
• Masonry / concrete paint: 100–200 ft²/gal. Porous surfaces drink paint.
• Premium paint with high solids: 400–450 ft²/gal. The label usually states it.

Choosing the right finish

• Flat / matte: hides imperfections best, hardest to clean. Use on ceilings and low-traffic adult bedrooms.
• Eggshell: subtle sheen, moderately washable. Most popular for living rooms, bedrooms, hallways.
• Satin: slightly more sheen than eggshell, more washable. Kids' rooms, family rooms.
• Semi-gloss: shiny, very washable, durable. Trim, doors, kitchens, bathrooms.
• Gloss: maximum shine, ultra-durable. Front doors, cabinetry. Shows every imperfection — surface prep matters.

When you need more (or less) paint

Add 10–20% extra when:

• Walls are textured (knockdown, orange peel, rough plaster).
• You're going from a dark to a light color (some bleed-through requires extra coats).
• Walls have many corners, niches, or architectural details that slow application.
• You're a first-time painter (more drips, mistakes, touch-ups).

Buy less when:

• Refreshing the same color over already-painted walls (often 1 coat works).
• The current and new paint are both premium, similar colors.
• You're using paint-and-primer combos on smooth, lightly-soiled walls.

Surface prep — bigger impact than paint quality

The two paint jobs that fail are the one with great paint on bad prep, and the one with cheap paint on good prep. Don't skip:

1. Clean walls — TSP or warm soapy water. Especially in kitchens (grease) and bathrooms (mildew).
2. Repair holes and dings — spackle, sand smooth.
3. Sand glossy surfaces — paint won't bond to slick finishes without scuffing.
4. Prime where needed — new drywall, repairs, stains, major color changes.
5. Tape edges — painter's tape, applied carefully, removed before paint fully dries (or once it cures, by scoring with a utility knife).
6. Drop cloths — canvas (not plastic) for floors. Cover furniture or move out.

Practical tips

• Buy all paint for one project from one batch — paint stores can mix in 5+ gallon buckets to ensure color match. Different batches of the same color number can be subtly different.
• Test the color first — paint a 2×2 ft swatch and view at different times of day. Wall colors look different than the chip.
• Don't skimp on rollers — quality 9-inch rollers shed less, hold more paint, and finish smoother. $5 vs $15 covers a real quality difference.
• Cut in first, roll second — paint edges with a brush before rolling the main field. Maintain a wet edge to avoid lap marks.
• Save leftover paint — label the can with date, room, and brand/color. Touch-ups will need it.

For other home calculations: Square Footage Calculator for any area, Concrete Calculator for slabs/footings, and Unit Converter for any measurement conversions you need.