Paint Calculator: How to Never Buy Too Much Paint Again

Americans waste $100-300 million on unused paint annually. Most DIYers either buy too much (wasting money) or too little (making frustrating mid-project store runs). A paint calculator solves both problems while saving you money and time.

The Basic Paint Coverage Formula

One gallon of paint covers approximately 350-400 square feet with one coat on smooth, primed surfaces. This is the industry standard, but reality varies based on several factors.

Standard Formula: (Wall Width × Wall Height) = Square Footage Square Footage ÷ 350 = Gallons Needed

Example Room: 12' × 10' room with 8' ceilings

• Wall 1: 12' × 8' = 96 sq ft
• Wall 2: 10' × 8' = 80 sq ft
• Wall 3: 12' × 8' = 96 sq ft
• Wall 4: 10' × 8' = 80 sq ft
• Total: 352 sq ft ÷ 350 = 1.01 gallons

You need 1 gallon for one coat, or 2 gallons for two coats (recommended for color changes).

Windows and Doors: Should You Subtract Them?

Professional painters have a rule: Don't subtract windows and doors under 20 square feet. This accounts for:

• Paint absorbed by wall edges around openings
• Touch-ups and mistakes
• Slight coverage variations

Subtract These Large Areas:

• Sliding glass doors (20+ sq ft each)
• Picture windows (15+ sq ft)
• French doors (18+ sq ft)

Don't Subtract These:

• Standard doors (20 sq ft, but requires edge painting)
• Standard windows (12-15 sq ft)
• Trim and baseboards

Surface Texture Impact on Coverage

Smooth Surfaces (drywall, primed walls): 400 sq ft per gallon coverage

Semi-Smooth (textured drywall, painted wood): 350 sq ft per gallon coverage

Rough Surfaces (stucco, brick, concrete block): 250-300 sq ft per gallon coverage

Example: Same 352 sq ft room with textured walls: 352 ÷ 300 = 1.17 gallons needed vs. 1.01 for smooth walls

Rough surfaces absorb significantly more paint. Always round up on these surfaces.

Paint Color and Primer Considerations

Dramatic Color Changes:

• Light to dark: 2-3 coats required
• Dark to light: 2-3 coats required + primer
• Similar shades: 2 coats usually sufficient

Primer Coverage: One gallon covers 200-300 sq ft (less than paint because it's more absorbent). Always use primer when:

• Painting over bare drywall
• Covering dark colors with light
• Painting over glossy surfaces
• Dealing with stains or water damage

Paint Sheen Affects Coverage

Different sheens have different coverage rates:

Flat/Matte: 400+ sq ft/gallon (most coverage, hides imperfections) Eggshell: 400 sq ft/gallon (slight sheen, good for living rooms) Satin: 350-400 sq ft/gallon (semi-gloss, good for high-traffic areas) Semi-Gloss: 350 sq ft/gallon (durable, kitchens and bathrooms) Gloss: 300-350 sq ft/gallon (least coverage, most durable)

Higher-sheen paints are thicker and provide less coverage but more durability.

Ceiling Paint Calculations

Ceilings are straightforward—no windows or doors to account for:

Formula: Room Length × Room Width = Ceiling Square Footage Ceiling Square Footage ÷ 350 = Gallons Needed

Example: 14' × 12' room = 168 sq ft ceiling 168 ÷ 350 = 0.48 gallons

Round up to 1 gallon (minimum purchase size). One gallon will cover this ceiling with 2 coats and leave extra for touch-ups.

Pro Tip: Ceiling paint is typically flat white and covers better than wall paint (400+ sq ft/gallon).

Trim, Baseboards, and Molding

Trim requires a different calculation approach:

Method 1: Linear Foot Method Measure total linear feet of trim × height of trim = square footage

Example:

• 12' × 10' room = 44 linear feet perimeter
• 6" baseboards = 0.5 feet height
• 44 × 0.5 = 22 sq ft
• 22 ÷ 350 = 0.06 gallons

One quart (0.25 gallons) covers 87.5 sq ft, enough for multiple rooms' baseboards.

Method 2: Rule of Thumb One quart of trim paint covers trim in 2-3 average rooms.

Real Project Examples with Calculations

Example 1: Living Room Makeover

• Room: 15' × 18' with 9' ceilings
• 2 windows (15 sq ft each)
• 1 door (20 sq ft)
• Wall texture: Smooth
• Color change: Beige to gray (2 coats needed)

Calculation:

• Total wall area: (15+18+15+18) × 9 = 594 sq ft
• Subtract large window: 594 - 30 = 564 sq ft
• Don't subtract door (accounts for edges and touch-ups)
• Coverage: 564 ÷ 350 = 1.61 gallons per coat
• Two coats: 1.61 × 2 = 3.22 gallons total

Buy: 2 gallons (covers 1.24 coats) + 1 gallon (covers remaining 0.76 coats + extra) = 3 gallons

Example 2: Bedroom Refresh

• Room: 12' × 12' with 8' ceilings
• 1 window, 1 door, textured walls
• Same color refresh (2 coats)

Calculation:

• Wall area: (12+12+12+12) × 8 = 384 sq ft
• Textured coverage: 384 ÷ 300 = 1.28 gallons per coat
• Two coats: 2.56 gallons
• With 20% buffer for touch-ups: 2.56 × 1.2 = 3.07 gallons

Buy: 3 gallons minimum, or 4 gallons if you want extra for future touch-ups

Example 3: Kitchen Cabinet Painting

• 20 cabinet doors (30" × 18" average)
• Inside and outside painting
• Primer + 2 coats

Calculation:

• Each door: 2.5' × 1.5' × 2 sides = 7.5 sq ft
• 20 doors: 150 sq ft
• Primer: 150 ÷ 250 = 0.6 gallons (1 quart)
• Paint (2 coats): 150 ÷ 350 per coat = 0.43 gallons per coat
• Total paint: 0.86 gallons (1 gallon sufficient)

Buy: 1 quart primer + 1 gallon paint

Professional Painter's Tips to Minimize Waste

Tip #1: Buy Quality Paint Premium paints cover better (closer to 400 sq ft/gallon) and typically require fewer coats. The extra $10-15 per gallon is offset by using less.

Tip #2: Proper Surface Prep Patched holes, sanded surfaces, and primed walls require less paint than unprepared surfaces.

Tip #3: Technique Matters Proper roller technique (N-pattern or W-pattern) distributes paint evenly, providing better coverage per gallon.

Tip #4: Buy Slightly More Than Calculated Add 10-15% buffer to your calculation for:

• Touch-ups
• Painting mistakes
• Future touch-ups
• Slight miscalculations

Tip #5: Keep Records Write on the paint can lid:

• Room painted
• Date
• Number of coats
• Square footage covered

This helps with future touch-ups and repaints.

Paint Can Size Economics

Quart: $8-15, covers 100 sq ft Gallon: $25-50, covers 350-400 sq ft 5-Gallon Bucket: $100-200, covers 1,750-2,000 sq ft

Cost Per Square Foot:

• Quart: $0.08-$0.15 per sq ft
• Gallon: $0.07-$0.14 per sq ft
• 5-Gallon: $0.05-$0.10 per sq ft

For whole-house projects (2,000+ sq ft), 5-gallon buckets save $40-100 compared to buying individual gallons.

Storing Leftover Paint Properly

If you do have extra paint:

Storage Steps:

1. Remove paint skin from top (don't mix it in)
2. Pour paint into smallest container that fits (less air = longer shelf life)
3. Tap lid securely with rubber mallet (not hammer)
4. Store upside down to create air-tight seal
5. Label with room, color name, and date
6. Store in climate-controlled space (50-80°F)

Properly stored paint lasts 2-10 years:

• Latex/water-based: 2-5 years
• Oil-based: 5-10 years

Common Paint Calculator Mistakes

Mistake #1: Not Accounting for Multiple Coats Always paint 2 coats minimum for even color and durability. Multiply your single-coat calculation by 2.

Mistake #2: Forgetting Ceiling Paint Ceilings are often overlooked in DIY projects. Calculate them separately—they use different paint.

Mistake #3: Ignoring Primer Primer is required for new drywall, stain blocking, and dramatic color changes. Calculate it separately at 250-300 sq ft/gallon.

Mistake #4: Not Measuring Accurately "Eyeballing" room dimensions can be off by 20-30%. Use a tape measure for accurate square footage.

Mistake #5: Assuming Perfect Coverage Manufacturers' coverage rates are ideal conditions. Real-world application typically achieves 85-90% of stated coverage.

Frequently Asked Questions

How much paint do I need for a 12×12 room? For walls only with 8' ceilings: approximately 2 gallons for 2 coats. Add 1 gallon for ceiling. Total: 3 gallons for complete room.

Does one gallon of paint cover 400 square feet? On smooth, primed surfaces, yes. On textured or rough surfaces, expect 300-350 sq ft coverage. Always check your specific paint's label.

Should I subtract doors and windows from calculations? Only subtract large openings (sliding doors, picture windows). Don't subtract standard doors and windows—this accounts for edges, touch-ups, and natural variation.

How many coats of paint are needed? Two coats minimum for all projects. Three coats for dramatic color changes (dark to light or light to dark). Always use primer for new drywall or covering dark colors.

Can I save leftover paint for touch-ups? Yes. Store in airtight containers, label clearly, and keep in climate-controlled space. Latex paint lasts 2-5 years; oil-based lasts 5-10 years when properly stored.

Calculate your exact paint needs with our Paint Calculator before your next project. Also check our Tile Calculator for bathroom remodels and our Carpet Calculator for flooring projects.