Paint Calculator Calculators

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Estimating how much paint to buy for a project is a practical calculation that prevents costly over- or under-purchasing. Standard interior latex paint covers approximately 350–400 square feet per gallon for a single coat on a smooth, primed surface. The total area to paint — walls minus windows and doors — divided by coverage rate gives gallons needed, multiplied by the number of coats planned. Porous surfaces (bare drywall, masonry), textured walls, and darker colors requiring multiple coats all increase paint consumption beyond standard estimates.

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Basic Paint Coverage Formula

Gallons needed = (Total area in sq ft × number of coats) / coverage rate

Standard coverage is 350–400 sq ft/gallon for interior latex paint. For a 12 × 14 ft room with 9 ft ceilings:

  • Wall area: 2(12+14) × 9 = 468 sq ft
  • Subtract doors/windows: −80 sq ft = 388 sq ft
  • Two coats: 388 × 2 = 776 sq ft
  • At 380 sq ft/gal: 776 / 380 ≈ 2.04 gallons → buy 3 gallons

Factors That Affect Coverage

  • Surface texture: Smooth walls: 400 sq ft/gal; orange peel: 350; knockdown: 300; rough masonry: 150–200
  • Primer coat: Unprimed drywall absorbs significantly more paint — always prime new drywall
  • Color change: Covering dark with light or changing from saturated to white may require 3+ coats
  • Paint quality: High-hide, self-priming paints provide better coverage in fewer coats

Ceiling and Trim Estimation

Ceiling area = length × width. One gallon covers approximately 400 sq ft of flat ceiling. Trim (baseboards, door/window casings) is often estimated at 1 gallon per 400 linear feet of baseboard for standard 3.5-inch baseboard. Doors are approximately 20 sq ft per door face.

Always Round Up

After calculating the required gallons, always round up to the nearest whole gallon or quart. Slight overestimation is preferable — touch-ups, missed spots, and future damage repairs need matching paint from the same batch.

Glossary

Coverage Rate
The area that one gallon of paint covers at the recommended dry-film thickness; typically 350–400 sq ft/gallon for interior latex paint on smooth surfaces.
Hide (Hiding Power)
A paint's ability to obscure the underlying color or surface; measured by contrast ratio; high-hide paints require fewer coats when changing colors.
Primer
A preparatory coating applied before paint to improve adhesion, block stains, and seal porous surfaces; reduces total paint consumption and improves final appearance.

Frequently Asked Questions

For a 12×12 room with standard 8 ft ceilings: wall area = 2(12+12) × 8 = 384 sq ft. Subtract a typical door and window (about 50 sq ft): 334 sq ft. For one coat at 380 sq ft/gallon: about 0.88 gallons — buy 1 gallon. For two coats: 1.76 gallons — buy 2 gallons. Always check the specific paint's coverage rate on the label, as it varies by brand and formula.

Yes. Multiply the total wall area by the number of coats before dividing by the coverage rate. Most interior repaints need 2 coats for a solid, uniform finish. Primed or previously painted surfaces in similar colors may be fine with 1 coat of premium paint. Covering dark colors with light, or changing from one saturated color to another, typically requires 2–3 coats regardless of paint quality.

Standard interior latex paint covers 350–400 square feet per gallon on smooth, previously painted surfaces. Exterior paint typically covers 250–350 sq ft/gallon depending on surface porosity. High-build or self-priming paints may cover less per gallon due to higher solids content. Rough or porous surfaces like bare wood, unprimed drywall, or masonry can reduce coverage to 150–250 sq ft/gallon.

Yes. Subtract the area of windows and doors from total wall area. A standard interior door is about 20 sq ft per face. A typical window is 15–20 sq ft. For rough estimates: subtract 20 sq ft per door and 15 sq ft per window from your total wall area. Precision matters most for large open areas; for small rooms, buying an extra quart as buffer is simpler than precisely measuring every opening.