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
Walls and ceiling are figured separately — see how the take-off splits them
Wall Paint
💡 Standard Practice: New/unpainted surfaces get 1 coat primer + 2 coats paint. Previously painted surfaces typically need 2 coats of paint only.
How do paint quality and surface texture change coverage? See the diagram
Why does spraying need more paint than rolling? See the comparison
Ceiling Paint
What drives your interior-paint estimate
Three inputs move the gallon count more than people expect. These engineering-style diagrams show why spraying buys far more paint than rolling, how wall texture and paint grade change how far a gallon goes, and why walls and the ceiling are figured as two separate surfaces.
The roll-versus-spray comparison is why the calculator adjusts paint volume for application method. A sprayer loses roughly a third of the paint to overspray, so spraying the same wall buys noticeably more material than rolling. Method, not just area and coats, is why two identical rooms can need different gallons.
The texture-coverage chart explains why wall texture changes the gallon count. Texture is hidden surface area, so knockdown or popcorn spreads a gallon far less than smooth drywall, and paint grade shifts coverage too. Telling the calculator the texture is what keeps the estimate from assuming a flat wall.
The walls-versus-ceiling breakdown is why the calculator treats them as two surfaces. Walls are the perimeter times height minus openings, the ceiling is length times width, and each gets its own paint, sheen, and coats. You paint the trim, not the door slab or the glass, which is why openings deduct a fixed area rather than the whole opening.
Calculation Formulas
Calculate the total square footage of all four walls in a rectangular room.
Example:
Room 12ft × 15ft with 8ft ceilings: 2 × (12 + 15) × 8 = 432 sq ft
Standard door is 7ft × 3ft = 21 sq ft. Standard window is 3ft × 5ft = 15 sq ft.
Example:
2 doors + 3 windows = (2 × 21) + (3 × 15) = 87 sq ft deducted
Standard coverage is 350-400 sq ft per gallon. Varies by surface texture and paint quality.
Example:
345 sq ft ÷ 350 sq ft/gal = 0.99 gallons (round up to 1 gallon)
Most projects require 2 coats. Primer adds 1 additional coat. Dark-to-light color changes may need 3 coats.
Example:
1 gallon × 2 coats = 2 gallons total
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Standard Coverage (Flat/Matte) | 400 sq ft/gal | Best coverage - absorbs least, smoothest application |
| Eggshell/Satin Coverage | 375 sq ft/gal | Moderate coverage - slight sheen reduces coverage |
| Semi-Gloss Coverage | 350 sq ft/gal | Lower coverage - higher resin content |
| High-Gloss Coverage | 325 sq ft/gal | Lowest coverage - thicker formula for durability |
| Standard Door Size | 21 sq ft | 7ft height × 3ft width (industry standard) |
| Standard Window Size | 15 sq ft | 5ft height × 3ft width (average residential) |
| Recommended Coats | 2 | PDCA standard for new paint over existing similar color |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
PDCA - Painting & Decorating Contractors of America Standards(P1-P20 Standards)
View StandardIndustry standards for surface preparation, application methods, and quality benchmarks. Defines coverage rates, drying times, and coating specifications for professional painting.
Key Requirements:
- •Surface prep: Walls must be clean, dry, and smooth before painting
- •Primer required for: New drywall, repairs, dark-to-light color changes, stains
- •Minimum 2 coats recommended for uniform coverage and durability
- •24-hour drying time between coats at 70°F and 50% humidity
Master Painters Institute (MPI) Standards(MPI Gloss Level Standards)
View StandardDefines paint sheen levels and appropriate applications. Level 1 (Flat) to Level 7 (High Gloss). Higher gloss = more durable but shows imperfections more easily.
Key Requirements:
- •Level 1-2 (Flat/Matte): Ceilings, low-traffic walls - 400 sq ft/gal
- •Level 3-4 (Eggshell/Satin): Living areas, bedrooms - 375 sq ft/gal
- •Level 5-6 (Semi-Gloss): Kitchens, bathrooms, trim - 350 sq ft/gal
- •Level 7 (High-Gloss): Cabinets, doors, high-wear areas - 325 sq ft/gal
ASTM D3276 - Guide for Painting Inspectors(ASTM D3276-15)
View StandardStandard for paint inspection and quality control. Covers film thickness measurement, adhesion testing, and defect identification.
Key Requirements:
- •Wet film thickness: 4-6 mils per coat typical
- •Dry film thickness: 1.5-2 mils per coat minimum
- •Coverage uniformity: Visual inspection for holidays (missed spots)
- •Adhesion test: Cross-hatch or pull-off test for critical applications
Paint Quality Institute (PQI) Guidelines(Consumer Education Standards)
View StandardProvides paint quality ratings and application best practices. Premium paints offer better coverage (fewer coats) and durability than economy paints.
Key Requirements:
- •Premium paints: 350-400 sq ft/gal, better hide, one-coat coverage possible
- •Mid-grade paints: 300-350 sq ft/gal, adequate hide, two coats standard
- •Economy paints: 250-300 sq ft/gal, poor hide, may require 3+ coats
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Humidity and Temperature Effects
Affects drying time and coverage rates
High humidity and extreme temperatures impact paint application, drying times, and coverage. Coverage rates assume 70°F and 50% relative humidity.
Regional Examples:
Surface Texture Variations
Texture significantly impacts paint requirements
Rough or textured surfaces require substantially more paint than smooth drywall. Coverage rates assume standard smooth or light texture drywall.
Regional Examples:
VOC Regulations
State-specific volatile organic compound limits
Some states restrict paint formulations to reduce air pollution. Low-VOC paints may have slightly different coverage rates and application properties.
Regional Examples:
Historic District Requirements
Preservation guidelines may dictate paint types and colors
Properties in historic districts often have paint color and finish restrictions. Some areas require breathable paints for historic plaster.
Regional Examples:
Rental Property Regulations
Some jurisdictions have specific requirements for rental units
Certain cities require specific paint standards for rental properties, particularly regarding lead paint in pre-1978 construction.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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
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🧮 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.
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Want to Learn More?
Interior painting guide — prep, priming, cutting in, rolling technique, and cleanup. Covers order of operations, mistakes, and finish selection.
Read the Complete Interior Painting Guide
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.