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Calculate Exactly How Much Paint You Need

Input your room dimensions for instant gallon estimates, opening deductions, and material cost breakdowns.

Accurate Gallon Estimator · Zero Guesswork

Paint Coverage & Cost Estimator

Presets:

1. Room Dimensions

+144 sq ft

2. Doors & Windows Deducted

21 sq ft / door
15 sq ft / win

3. Number of Coats

2 Coats Standard
Estimated Paint Needed
2 Gallons
Exact volume needed: 1.90 gal
Recommended purchase: 2 × 1-Gallon Cans
Surface Area Breakdown 333 sq ft
Gross Wall Surface: 384 sq ft
Less Doors & Windows: -51 sq ft
Total Applied Area (2 coats): 666 sq ft
Estimated Paint Cost $90

Estimated from your entered price of $45/gal for 2 gallons. Actual retail price varies by brand, sheen, and local sales tax.

For a 12' × 12' room with 8' ceilings, 1 door, 2 windows, and 2 coats of paint, your net paintable area is 333 sq ft per coat (666 sq ft total). You will need 1.90 gallons of paint—we recommend purchasing 2 gallons.

Key Paint Calculator Features

Color Visualizer

🎨 Room Color Visualizer

Mix and match colors for your main wall, accent wall, and trim. Tap any surface to apply your chosen color.

Brush: Modern Sage #7A8E7D
Target surface:

📊 Room Paint Requirements

Based on standard 12' × 12' bedroom (8' ceilings, 333 sq ft net walls, 2 coats @ 350 sq ft/gal).

Main Wall
Unpainted Primer
~226 sq ft · ~1.3 gal
Accent Wall
Unpainted Primer
~107 sq ft · ~0.6 gal
Trim & Baseboard
White Enamel
~30 linear ft · 1 quart
📐 Calculate Paint for Your Room

How Paint Coverage is Calculated (The Exact Mathematical Formula)

Accurately estimating paint requirements prevents two frustrating project outcomes: running out of paint midway through a second coat (which causes visible lap marks when blending fresh batches) and overpurchasing costly specialty coatings that sit unused in a garage. Professional painting contractors and architectural coating manufacturers rely on a standardized mathematical formula based on gross wall dimensions, standard aperture deductions, and substrate spread rates.

Standard Paint Formula Architectural Standard
1. Net Paintable Surface
(Gross Wall Area − Deductions) + Ceiling Area
2. Total Paint Volume Needed
(Net Area × Number of Coats) ÷ Spread Rate

To break down each variable:

  • Perimeter & Gross Wall Area: Calculated as 2 × (Length + Width) × Height. For a standard 12-by-12-foot room with 8-foot ceilings, the perimeter is 48 linear feet, yielding a gross vertical surface of 384 square feet.
  • Aperture Deductions: Architectural standard interior doors measure 30 to 36 inches wide by 80 to 84 inches high (averaging 21 square feet including jamb trim). Standard residential double-hung windows average 36 by 60 inches (15 square feet). Subtracting these openings yields the net paintable surface area.
  • Ceiling Inclusion: If you are applying the same coating to the ceiling, add Length × Width (144 sq ft for a 12×12 room).
  • Material Cost Budgeting: By entering your estimated retail price per gallon or liter, our integrated cost calculator instantly projects total paint material expenses so you can accurately budget DIY renovations before heading to the store.

Understanding Coverage Rates: Why One Gallon Doesn't Always Cover 400 Sq Ft

Most premium latex, acrylic, and enamel paint manufacturers state an average theoretical spread rate of 350 to 400 square feet per gallon (approximately 8.6 to 9.8 square meters per liter). However, actual real-world spread rate is heavily dictated by wall texture, substrate porosity, and application method:

Smooth Repainted Drywall

380–400 sq ft/gal

Smooth satin or eggshell repaints experience minimal liquid absorption, maximizing roller transfer efficiency (~9.3–9.8 m²/L).

Standard Primed Drywall

350 sq ft/gal

The industry benchmark for residential rooms with light texture and a dry PVA primer coat (~8.6 m²/L).

Unprimed Bare Drywall

275–300 sq ft/gal

Raw paper and joint compound act as thirsty sponges (~6.7–7.4 m²/L). Always apply a primer before topcoat to avoid wasting paint.

Textured & Rough Masonry

200–250 sq ft/gal

Knockdown, heavy trowel, brick, and stucco expand physical surface area by up to 35%, requiring more paint (~4.9–6.1 m²/L).

Coats & Buying Rules: Avoiding Overages Without Running Short

A single coat is rarely sufficient unless you are applying an identical shade over a pristine existing coating. Professional paint specifications recommend two full finish coats for depth of color, uniform film build, and proper scrubbability. When executing dramatic color transitions (such as navy blue over beige, or light grey over dark red), two coats of topcoat over a tinted primer are required.

The “Boxing” Rule for Multi-Can Projects

When your calculation calls for 2 or more separate 1-gallon cans, manufacturers always advise “boxing” your paint—pouring all gallon cans together into a clean 5-gallon bucket and stirring thoroughly. Because slight tinting variations can occur between individual cans dispensed by automated paint store carousels, boxing ensures uniform color uniformity across every wall in your room.

Quick Reference Coverage by Room Size (8 ft Ceilings, 2 Coats)

Powder Room (6' × 8')
188 sq ft (376 sq ft 2 coats)
2 Gallons
1.07 gal exact
Small Bedroom (10' × 10')
269 sq ft (538 sq ft 2 coats)
2 Gallons
1.54 gal exact
Standard Bedroom (12' × 12')
333 sq ft (666 sq ft 2 coats)
2 Gallons
1.90 gal exact
Master Bedroom (14' × 16')
429 sq ft (858 sq ft 2 coats)
3 Gallons
2.45 gal exact
Open Living Room (15' × 20')
509 sq ft (1,018 sq ft 2 coats)
3 Gallons
2.91 gal exact

Whole-House Estimating: 1,000 to 2,000 Sq Ft Paint Requirements

When estimating for an entire home, square footage of floor space correlates closely with interior wall surface area (typically 2.5 to 3.0× floor area in gross vertical drywall):

1,000 Sq Ft House
• Interior Walls (2 coats): 14–16 gallons
• Exterior Siding (2 coats): 10–12 gallons
1,500 Sq Ft House
• Interior Walls (2 coats): 20–24 gallons
• Exterior Siding (2 coats): 15–18 gallons
2,000 Sq Ft House
• Interior Walls (2 coats): 26–30 gallons
• Exterior Siding (2 coats): 20–24 gallons

Is It Better to Roll or Spray Exterior Paint?

Spraying is up to 4× faster and provides seamless coverage on rough textures like stucco, lap siding, and brick. However, airless paint sprayers produce 20% to 30% overspray loss into the air. When spraying, add a 25% buffer to your gallon purchase. Rolling has virtually zero transfer loss and forces paint deeper into porous wood grains.

Frequently Asked Questions

Paint Calculation & Project Estimation FAQs

Clear, contractor-grade answers to the most common questions on room sizes, spread rates, coats, primers, and touch-up buffers.

For a standard 12' × 12' bedroom with 8-foot ceilings, 1 standard entry door, and 2 standard windows, the net paintable wall surface is approximately 333 square feet. With the recommended 2 coats (666 total sq ft at a standard 350 sq ft/gal spread rate), you will need 1.90 gallons of paint. We recommend purchasing 2 one-gallon cans.
One standard US gallon of quality interior paint covers between 350 and 400 square feet with a single coat on previously painted, smooth drywall. On unprimed bare drywall or porous masonry, coverage drops to 250–300 square feet per gallon due to surface absorption.
Professional painters recommend 2 full coats for virtually all interior walls to achieve uniform sheen, true color depth, and full scrub resistance. 1 coat is only sufficient when applying the exact same brand and shade over an existing clean, undamaged coat.
At a standard coverage rate of 350 sq ft per gallon, 1,000 square feet requires ~2.85 gallons for 1 coat, or ~5.7 gallons for 2 coats. In retail packaging, you should buy 1 five-gallon pail plus 1 one-gallon can (or 6 individual gallon cans).
'Paint + Primer' products are high-solids paints that work well over existing paint in good condition. However, you still need a dedicated stain-blocking or PVA primer when painting raw unprimed drywall, bare wood, heavily patched spackle, water stains, or making dramatic dark-to-light color transitions.
Standard residential interior doors average 21 square feet (3' × 7' opening including casing). Standard double-hung windows average 15 square feet (3' × 5'). Our calculator automatically deducts these openings from the gross wall area so you do not overbuy paint.
Ceiling area is calculated as Room Length × Room Width (e.g., 12' × 12' = 144 sq ft). Because ceilings are typically painted in ultra-flat white to hide imperfections, you can toggle ceiling calculation on or off depending on whether you are using the same wall color or separate ceiling paint.
The industry rule of thumb is a 10% buffer for roller absorption, minor spills, and leftover paint for future scuff touch-ups. When your exact math results in fractional gallons (like 1.85 gal), purchasing 2 full gallons automatically builds in this healthy 10% reserve.
For a 1,500 sq ft home interior (2 coats on all walls), plan on approximately 20 to 24 gallons of paint. For a 2,000 sq ft home interior, plan on 26 to 30 gallons (typically five to six 5-gallon buckets). For exterior siding, a 1,500 sq ft house requires 15–18 gallons, and a 2,000 sq ft house requires 20–24 gallons.
A typical 10' × 10' bedroom with 8-foot walls, 1 standard door, and 1 window has approximately 284 square feet of net wall area. For 2 coats of paint (568 sq ft total applied surface at 350 sq ft/gal), you need 1.62 gallons of paint. You should purchase two 1-gallon cans, which leaves a small buffer for cutting-in and future touch-ups.
A single standard room wall measuring 12 feet wide by 8 feet high is 96 square feet. Because 1 US quart of paint covers approximately 85–100 square feet with a single coat, one 1-quart can is sufficient for a single coat. If applying 2 coats or changing to a bold accent color, purchase 1 gallon or 2 quarts.
A standard residential door (3' × 7') is approximately 21 square feet per side. Painting both sides with 2 full coats requires about 84 square feet of coverage. One 1-quart can of exterior or semi-gloss trim paint is enough to paint 2 standard doors inside and out.
For a small 5' × 8' bathroom with an 8-foot ceiling (excluding bathtub/shower tile deductions), the net paintable area is about 120–150 sq ft, requiring 1 gallon of moisture-resistant satin or semi-gloss paint for 2 coats. For average kitchen cabinets (approx. 20–25 doors and drawers), budget 2 quarts of stain-blocking bonding primer and 1 to 1.5 gallons of specialized cabinet enamel for 2 coats.
Spraying is much faster and penetrates textured stucco and lap siding efficiently, but produces 20–30% overspray loss into the air. If spraying, budget 25% extra paint. Rolling has zero transfer loss and is best for smooth trim, doors, and smaller siding projects.