How to Calculate How Much Paint You Need: The Complete Guide

Basic Paint Coverage Formula

Most interior latex paint covers 350–400 sq ft per gallon. One coat is the standard assumption, but most rooms need two coats (especially when changing from dark to light colors). Formula: (wall square footage) ÷ (coverage per gallon) × (number of coats) = gallons needed.

Calculating Wall Square Footage

For a rectangular room: Perimeter × Wall height = Total wall area. Subtract 20 sq ft per standard door and 15 sq ft per standard window. Example: 12×15 ft room, 8ft ceiling, 1 door, 2 windows: Perimeter = (12+15+12+15) = 54 ft. Area = 54 × 8 = 432 sq ft. Minus door (20) and 2 windows (30) = 382 sq ft. Two coats at 350 sq ft/gal = 382 × 2 ÷ 350 = 2.2 gallons → round up to 3 gallons.

Coverage by Paint Finish

Flat/matte: 350–400 sq ft/gal — hides surface imperfections best
Eggshell/satin: 350–400 sq ft/gal — slightly washable, versatile
Semi-gloss/gloss: 300–350 sq ft/gal (slightly less coverage, more reflective)
Primer: 250–350 sq ft/gal (more absorption than finish paint)

Ceiling and Trim

Ceiling area = room length × room width. The same coverage formula applies. Trim is typically sold by linear feet: doors/windows have 15–20 lf trim each; baseboards are the room perimeter. Trim typically needs 1 quart per room for most average-sized rooms.

Buying Strategy

Round up to the nearest gallon and buy 10–15% extra. Same-batch paint avoids dye lot variation. Store extra paint for touch-ups (paint lasts 5–10 years properly stored — keep tightly sealed in a climate-controlled space above freezing).

Frequently Asked Questions

Do I always need two coats of paint?

Not always. One coat is sufficient when: repainting with the same color and similar sheen, using high-quality paint with built-in primer, or painting a light color over white. Two coats are needed when: changing colors significantly, going from dark to light, painting over raw drywall, or using lower-coverage paint. Most professional painters default to two coats for quality results.

What's the difference between paint and primer?

Primer is formulated for adhesion and sealing, not coverage. It fills in porous surfaces, blocks stains, and creates a uniform base for paint. Paint provides color and surface protection. For most repaints over same-color paint in good condition, primer is unnecessary. For new drywall, new wood, major color changes, or stain coverage, primer is essential.