How Much Paint Do I Need? Paint Coverage Calculator Guide
Nothing kills painting momentum like running out of paint halfway through a wall. Buy too much paint and you're stuck storing gallon cans you'll never use. Here's a simple paint calculator formula to estimate how much interior paint you need.
Disclaimer: Paint Coverage May Vary
The calculations below are estimates only. Actual paint coverage depends on surface texture, porosity, application method, paint brand, color, and individual technique. Always check manufacturer specifications on your specific paint can for coverage guidelines. When in doubt, buy a little extra.
Paint Coverage Calculator: The Basic Formula
Use this step-by-step method to estimate paint for any room.
Step 1: Measure Total Wall Square Footage
Multiply the length of each wall by ceiling height, then add walls together.
- Example: A 12' x 10' room with 8' ceilings
- Wall calculation: (12 + 12 + 10 + 10) x 8 = 352 square feet of wall space
Step 2: Subtract Doors and Windows
Reduce your total square footage for openings you won't paint.
- Standard door: ~20 sq ft
- Standard window: ~15 sq ft
- Example: 352 - 20 (door) - 30 (2 windows) = 302 sq ft of paintable wall area
Step 3: Divide by Paint Coverage Per Gallon
Most interior paints cover approximately 350–400 square feet per gallon on smooth, primed surfaces. Coverage rates vary by brand and product.
- Example: 302 sq ft ÷ 350 = 0.86 gallons for one coat
Step 4: Multiply by Number of Coats
Two coats of paint is standard for most interior painting projects.
- Example: 0.86 x 2 coats = 1.72 gallons needed
- Round up: Buy 2 gallons of paint
Paint Calculator Quick Reference Chart
Estimates based on standard 8' ceilings and average coverage rates. Your results may vary.
| Room Size | Estimated Gallons (2 Coats) |
|---|---|
| Small bathroom | 1 gallon |
| Bedroom (10x12) | 2 gallons |
| Living room (15x20) | 3 gallons |
| Open concept main floor | 5–7 gallons |
Factors That Increase Paint Coverage Needs
You may need more gallons of paint when working with:
- Porous surfaces: New drywall, bare wood, and textured walls absorb more paint.
- Dramatic color changes: Going from dark to light colors may require a third coat or tinted primer.
- Spray application: Airless sprayers use 20–40% more paint material due to overspray.
- Heavy texture: Knockdown, orange peel, or popcorn surfaces require significantly more paint.
- Application technique: Heavier coats use more material than thin, even passes.
Factors That Decrease Paint Coverage Needs
You may need less paint when working with:
- Primed or previously painted walls in good condition
- Similar color changes (light to light, neutral to neutral)
- Smooth wall surfaces with low paint absorption
- High-quality paints with better coverage and hiding power
Interior Paint Buying Tips
- Always buy extra paint for future touch-ups. A quart set aside now beats a mismatched batch six months later.
- Save the paint can label or photograph the color formula. Paint stores can remix your exact color.
- When your paint calculator lands between sizes, round up. Unopened paint cans can usually be returned.
- Check the coverage specs on your specific paint—premium paints often cover more per gallon.
The Bottom Line
This paint coverage calculator gives you a solid starting estimate: measure walls, subtract openings, divide by 350, multiply by coats, and round up. Every project is different, so treat these numbers as a guide, not a guarantee. When in doubt, grab an extra quart. It's cheaper than a second trip to the store.