Interior vs Exterior Paint Coverage: Key Differences

Interior and exterior paints are formulated differently, and their coverage rates reflect these differences. Understanding these distinctions helps you choose the right paint and estimate quantities accurately.

Why Interior and Exterior Paints Differ

Interior paint is designed for scrubbing, stain resistance, and low VOC emissions inside living spaces. Exterior paint is formulated to resist UV radiation, moisture, temperature swings, and mildew. These different formulations affect how the paint spreads and absorbs:

  • Interior paint has a smoother, finer pigment blend that covers evenly on smooth drywall.
  • Exterior paint has more resin and UV inhibitors, making it slightly thicker and sometimes reducing coverage on rough surfaces.

Coverage Rate Comparison

Paint TypeSmooth SurfaceRough Surface
Interior flat/matte350-400 sq ft/gal250-300 sq ft/gal
Interior eggshell/satin300-350 sq ft/gal200-250 sq ft/gal
Exterior acrylic latex300-400 sq ft/gal200-280 sq ft/gal
Exterior on stucco/brick250-300 sq ft/gal150-200 sq ft/gal

When Coverage Drops Significantly

Exterior surfaces often have rougher textures than interior walls. Here's how much coverage can drop:

  • Stucco: Reduces coverage by 25-35% (absorbs into texture)
  • Brick: Reduces coverage by 30-40% (mortar joints absorb heavily)
  • Rough-sawn siding: Reduces coverage by 20-30%
  • Pressure-treated wood (new): Reduces coverage by 15-25% (moisture content)

How to Adjust Your Estimate

When using the Exterior Paint Calculator, enter a lower coverage rate for rough surfaces. For stucco or brick, use 200-250 sq ft/gal instead of the default 350. For interior walls, the default 350 is usually accurate on smooth, primed drywall.

Can You Use Interior Paint Outside?

No. Interior paint lacks UV inhibitors and mildewcides. It will fade, peel, and grow mildew within months on an exterior surface. Always use exterior-rated paint for outdoor surfaces.

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