Interior Paint Calculator
How much interior paint do you need? This free interior paint calculator gives DIYers and remodeling pros instant gallon totals for walls, ceilings, and trim — separately — with doors and windows automatically subtracted from your room dimensions.
Buying the wrong paint quantity is one of the most common renovation overpays. A typical bedroom needs 1.5 gallons per coat, but most homeowners buy 2 full gallons because that is the next size up — leaving $25 of unusable paint on a shelf for years. Underbuying is worse: lap marks show wherever you stop and reload.
Calculates primer + 2 coats per PDCA coverage standards, with separate ceiling estimates by texture.
Interior Paint Calculator
Professional estimation based on industry standards.
Room Measurements
Room 1
Paint Specifications
Wall Paint
💡 Standard Practice: New/unpainted surfaces get 1 coat primer + 2 coats paint. Previously painted surfaces typically need 2 coats of paint only.
Ceiling Paint
Interior paint covers 350-400 sq ft per gallon on smooth drywall. Textured walls reduce coverage to 300-350 sq ft/gal. Primers cover 200-300 sq ft/gal. Deduct 20 sq ft per door and 15 sq ft per window.
🎨 Coverage Rates by Surface Type
| Surface Condition | Paint (sq ft/gal) | Primer (sq ft/gal) | Notes |
|---|---|---|---|
| Smooth Drywall (Painted) | 350-400 | 200-300 | Best coverage, similar colors |
| Smooth Drywall (Bare) | 250-300 | 200-300 | First coat absorbs more |
| Textured Walls (Knockdown) | 300-350 | 200-300 | 15-20% more paint needed |
| Textured Walls (Orange Peel) | 300-350 | 200-300 | Moderate texture |
| Popcorn Ceiling | 250-300 | 200-300 | Heavy texture, high absorption |
| Dark to Light Color | 300-350 | 200-300 | May need 3 coats or tinted primer |
Primer Coverage
All primers cover 200-300 square feet per gallon regardless of surface type per PCA standards. This reduced coverage ensures proper sealing, stain blocking, and adhesion for topcoats.
📐 Standard Deductions
Doors
Windows
Large Openings
🧮 Room Calculation Example
12' × 14' Bedroom (8' Ceilings)
Step 1: Calculate wall area
Perimeter: (12 + 14) × 2 = 52 feet
Wall area: 52 × 8 = 416 sq ft
Step 2: Deduct openings
Door: 20 sq ft
Window: 15 sq ft
Paintable walls: 416 - 20 - 15 = 381 sq ft
Step 3: Add ceiling
Ceiling: 12 × 14 = 168 sq ft
Total area: 381 + 168 = 549 sq ft
Step 4: Calculate paint needed
Coverage: 375 sq ft/gal (smooth drywall)
Per coat: 549 ÷ 375 = 1.5 gallons
Two coats: 1.5 × 2 = 3 gallons
With 10% waste: 3 × 1.10 = 3.3 → 1 gallon walls + 3 quarts ceiling
Alternative (walls only):
Walls: 381 ÷ 375 = 1.02 gallons per coat
Two coats with waste: 1.02 × 2 × 1.10 = 2.2 → 1 gallon + 1 quart
🔢 How Many Coats Do You Need?
One Coat Acceptable:
- • Same color refresh (good condition)
- • Light color over light color
- • High-quality paint with primer built-in
- • Previously primed surface
Two Coats Required:
- • New drywall (1 primer + 2 paint)
- • Color change (light to dark or reverse)
- • Stain blocking needed
- • Professional/warranty requirements
Dark to Light Colors
When painting light colors over dark walls, use tinted primer (gray for covering dark colors) plus 2 coats, or plan for 3 coats of paint without tinted primer.
📚 Industry Standards
PCA Standards (formerly PDCA)
The Painting Contractors Association sets coverage standards: 350-400 sq ft/gal for smooth surfaces, 200-300 sq ft/gal for primers. Recommends 10% material overage for waste.
MPI (Master Painters Institute)
Provides architectural paint specifications and performance standards. Defines proper surface preparation, primer requirements, and coating systems for various substrates.
ASTM D3276 - Standard Guide for Painting Inspectors
Establishes qualifications and responsibilities for paint inspectors. Covers film thickness, coverage rates, and quality control procedures.
Manufacturer Specifications
Major brands (Sherwin-Williams, Benjamin Moore, Behr) provide technical data sheets with coverage rates, dry times, and application instructions. Always follow manufacturer guidelines for warranty compliance.
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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Frequently Asked Questions
How much paint does one gallon cover?
One gallon covers 350-400 square feet per coat on smooth drywall per PCA standards. Textured walls cover 300-350 square feet per gallon. Bare drywall or dramatic color changes require 250-300 square feet per gallon for first coat due to higher absorption.
How much does primer cover per gallon?
Primers universally cover less surface area than topcoats, standardizing at 200-300 square feet per gallon across all manufacturers. This reduced coverage ensures proper sealing and adhesion on bare or previously unpainted surfaces.
Should I deduct doors and windows from calculations?
Deduct openings larger than standard door size (20 square feet). Small windows (15 square feet) are often not deducted as they offset areas missed in corners and edges. For large window walls, deduct 50% of opening area as a practical compromise between accuracy and paint waste.
How many coats of interior paint do I need?
Most interior projects need 2 coats for uniform coverage and durability. Light colors over white primer may need 1-2 coats. Dark or bold colors require 2-3 coats. New drywall needs 1 coat primer plus 2 finish coats per manufacturer recommendations.
Should I paint my walls or use wallpaper?
Both are valid wall finishes; the choice usually comes down to durability, repair cost, and pattern. Paint is faster to apply, easier to spot-repair, and tolerates moisture better — preferred for bathrooms with poor ventilation, basements, and kitchens. Wallpaper hides minor wall imperfections (drywall texture, hairline cracks) and offers pattern and texture that paint can't match, but seams and dye-lot risk make repair work harder. Bedrooms, dining rooms, and feature walls often favor wallpaper; high-traffic and wet areas favor paint. Cost per square foot at the material level is similar for paint and mid-tier wallpaper. If you're considering wallpaper, the Wallpaper Calculator handles bolt (double-roll) counts, pattern-repeat math, and adhesive coverage.