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Paint Calculator — Room Paint & Coverage Calculator

Calculate paint needed and costs for your room

Paint Calculator: Room Paint and Coverage Estimator

Table of Contents - Paint

How to Use This Calculator - Paint

Enter your Room Dimensions: length, width, and height in feet or meters.

Enter the Number of Doors (standard size: 21 sq ft each) and Windows (standard size: 16 sq ft each). Adjust for non-standard sizes if needed.

Enter the Paint Coverage Rate from your paint can label—typically 350-400 sq ft/gallon for smooth surfaces.

Select the Number of Coats—typically 2 for walls and ceilings.

Optionally enter the Paint Price per gallon for cost estimation.

Click "Calculate" to see results. The output displays:

  • Total wall area in square feet
  • Paintable area (after subtracting doors and windows)
  • Gallons of paint needed (rounded up)
  • Total estimated cost

The Core Principle: Surface Area and Coverage

Estimating paint requires two calculations: how much surface you're covering and how far a gallon of paint spreads.

Surface area for a rectangular room: Wall area = 2 × (Length + Width) × Height

This calculates the perimeter times height—all four walls.

Net paintable area: Subtract door area (typically 21 sq ft each) and window area (typically 16 sq ft each) from total wall area.

Paint needed: Gallons = (Net Area × Number of Coats) / Coverage Rate

Coverage varies by surface texture, paint quality, and application method. Manufacturer specifications provide guidance, but actual coverage often falls 10-20% below stated rates.

Always round up and add 10% for waste, touch-ups, and inevitable second passes on missed spots.

How to Calculate Paint Needs Manually

Example: 12 × 14 ft bedroom, 8 ft ceiling

Step 1: Calculate wall area Perimeter = 2 × (12 + 14) = 52 ft Wall area = 52 × 8 = 416 sq ft

Step 2: Subtract openings 1 door = 21 sq ft 2 windows = 32 sq ft Openings = 53 sq ft Net wall area = 416 - 53 = 363 sq ft

Step 3: Add ceiling (if painting) Ceiling = 12 × 14 = 168 sq ft Total area = 363 + 168 = 531 sq ft

Step 4: Calculate paint needed Coverage = 375 sq ft/gallon Coats = 2 Gallons = (531 × 2) / 375 = 2.83 gallons

Step 5: Apply waste buffer With 10% buffer: 2.83 × 1.1 = 3.1 gallons

Result: Buy 4 gallons (round up to nearest gallon or purchase quarts for the remainder)

Real-World Applications

Whole-house painting estimate. Calculate each room separately, sum the results, and add 15% overall buffer for color matching between batches.

Accent wall calculation. Single wall area × 2 coats ÷ coverage. One gallon typically covers a 10×8 ft accent wall twice.

Exterior painting. Similar calculation but use exterior coverage rates (often lower: 250-350 sq ft/gallon due to texture and absorption).

Commercial space budgeting. Large spaces require bulk ordering. Accurate estimates prevent costly overages or delays for additional orders.

Trim and detail work. Calculate linear feet of trim, multiply by average width, then calculate paint needs separately from wall paint.

Multi-room color coordination. When using the same color in multiple rooms, buy all paint at once to ensure batch consistency.

Scenarios People Actually Run Into

The textured wall surprise. Your textured walls absorb 40% more paint than smooth surfaces. That 350 sq ft/gallon becomes 250 sq ft/gallon. Recalculate accordingly.

The color change challenge. Painting light over dark (or vice versa) requires a tinted primer plus 2-3 coats of paint. Standard 2-coat calculations underestimate needs.

The mid-project shortage. You calculated correctly but spilled some, repainted a section, and needed more touch-ups than expected. Always buy the extra quart or gallon.

The ceiling oversight. You calculated walls perfectly but forgot the 400 sq ft ceiling adds significantly to total needs.

The trim underestimate. Trim paint covers differently than wall paint. Calculate baseboards, crown molding, and door frames separately.

Trade-Offs and Decisions People Underestimate

Quality versus coverage. Premium paints often cover better (one coat possible) but cost more per gallon. Calculate total project cost both ways.

Primer decisions. Bare drywall, stains, or dramatic color changes need primer. Primer is often cheaper than paint—using it may reduce total coats needed.

Roller versus brush versus spray. Spray uses more paint (overspray waste) but is faster. Brush uses less but takes longer. Roller is the typical balance.

Batch consistency. Paint from different batches may not match exactly. For open floor plans, buy all paint at once from the same batch.

Leftover paint value. Buying slightly extra provides touch-up capability for years. A half-gallon leftover isn't wasted if stored properly.

Common Mistakes and How to Recover

Forgetting doors and windows. Don't paint what isn't there. Subtract openings from total wall area.

Using ideal coverage rates. Manufacturer claims assume perfect conditions. Reduce stated coverage by 10-20% for real-world estimates.

Single-coat assumptions. One coat rarely provides adequate coverage, especially over different colors. Always calculate for two coats minimum.

Ignoring surface preparation. Paint over patched or repaired areas absorbs differently. These spots may need extra attention.

Not accounting for waste. Spills, tray lining, roller absorption, and touch-ups consume paint. Add 10% to calculated needs.

Related Topics

Paint coverage rates. Flat finishes often cover better than glossy. Check manufacturer specifications for your specific product.

Surface preparation. Proper prep (cleaning, sanding, priming) affects coverage and finish quality more than paint quantity.

Primer selection. Bonding primer for slick surfaces, stain-blocking primer for water stains or smoke damage, tinted primer for color changes.

Paint sheen. Flat (hides imperfections), eggshell (slight sheen, wipeable), satin/semi-gloss (durable, for trim and kitchens), gloss (highly durable, bright).

VOC content. Low-VOC and zero-VOC paints reduce fumes but may affect coverage and dry time.

How This Calculator Works

Wall area calculation:

wallArea = 2 × (length × height) + 2 × (width × height)
        = 2 × (length + width) × height

Opening deductions:

doorArea = numberOfDoors × 21 (or custom size)
windowArea = numberOfWindows × 16 (or custom size)
openings = doorArea + windowArea

Net paintable area:

netArea = wallArea - openings

Ceiling (if included):

ceilingArea = length × width
totalArea = netArea + ceilingArea

Paint needed:

paintArea = totalArea × numberOfCoats
gallonsNeeded = paintArea / coverageRate
withBuffer = gallonsNeeded × 1.10
finalGallons = ceiling(withBuffer)

Cost estimate:

totalCost = finalGallons × pricePerGallon

All calculations happen locally in your browser.

FAQs

How accurate is this calculator?

Very accurate if you input correct dimensions and use realistic coverage rates. The 10% buffer accounts for normal waste and variation.

What if my room isn't rectangular?

Calculate each wall separately: height × width for each wall. Sum all walls, then proceed with the standard calculation.

Do I need to include the ceiling?

Only if you're painting it. Ceilings typically need the same number of coats as walls.

Should I buy primer separately?

Yes—if painting new drywall, covering stains, or making dramatic color changes. Paint-and-primer combinations work for repaints over similar colors.

What's the standard coverage rate?

Most quality paints cover 350-400 sq ft/gallon on smooth, primed surfaces. Reduce to 250-300 sq ft/gallon for textured surfaces.

Why do I need two coats?

Two coats ensure uniform color, complete coverage over previous colors, and consistent sheen. One coat often looks patchy.

How long does leftover paint last?

Properly sealed and stored in moderate temperatures, latex paint lasts 2-10 years. Oil-based lasts up to 15 years.

How do I calculate for trim?

Measure linear feet of trim, multiply by average width (typically 3-6 inches), convert to square feet, then calculate separately from walls.

What's the best order for painting a room?

Ceiling first, then walls, then trim. This order allows you to overlap edges slightly at each stage, hiding any imperfections at the transitions.

How do I estimate time for a painting project?

A typical room (12×12 ft) takes 4-6 hours for two coats including prep. Add time for primer, complex trim work, or difficult access areas.

What's the difference between flat, eggshell, satin, and semi-gloss?

Flat hides imperfections but doesn't clean easily. Eggshell has slight sheen and wipes clean. Satin is durable for high-traffic areas. Semi-gloss is for trim, kitchens, and bathrooms—very durable and moisture-resistant.

How do I ensure color consistency across a large project?

Buy all paint at once from the same batch. If that's not possible, mix all cans together ("boxing") before starting to ensure uniform color throughout.

Should I use a brush, roller, or sprayer?

Rollers are fastest for walls. Brushes are for cutting in edges and trim. Sprayers are fastest for large areas but use more paint and require extensive masking.

How do I store leftover paint?

Seal the can tightly, store at moderate temperatures (not in garages that freeze or get very hot), and label with the room and date. Properly stored latex paint lasts years.

What's the difference between oil-based and latex paint?

Latex (water-based) cleans up with water, dries quickly, and has less odor. Oil-based is more durable, levels better, but requires solvent cleanup and has strong fumes. Most interior work uses latex.

How do I paint over wallpaper?

Generally, remove wallpaper first for best results. If you must paint over it, prime with shellac-based primer and expect potential issues with peeling or texture showing through.

What causes paint to peel or bubble?

Moisture trapped under paint, poor surface preparation, painting over contaminated surfaces, or incompatible paint layers. Fix the underlying issue before repainting.

How do I calculate paint for exterior projects?

Similar process but use lower coverage rates (250-350 sq ft/gallon for textured siding). Don't forget to calculate square footage of all surfaces including gables and soffits.

What prep work is required before painting?

Clean surfaces, repair holes and cracks, sand glossy surfaces for adhesion, prime bare wood or drywall, and tape edges. Good prep is 80% of a quality paint job.

How do I handle painting high ceilings?

Use extension poles for rollers (saves ladder climbing), proper scaffolding for cut-in work, and work in sections. Safety is more important than speed with heights.

What's the best way to avoid brush marks?

Use quality brushes, don't overload with paint, lay off strokes in one direction, maintain a wet edge, and work in manageable sections. Some paints self-level better than others.

Additional Notes

Accurate paint estimation prevents both costly overbuying and frustrating mid-project shortages. Take your time measuring, account for all surfaces, and always buy a bit extra for touch-ups. Remember that preparation is as important as the painting itself—a well-prepared surface ensures paint adhesion and durability. Quality results come from quality preparation and patience. Good planning saves time, money, and frustration.