How Much Paint Do I Need? A Room-by-Room Guide

Calculating Paint Requirements the Professional Way

Whether you're a professional painter bidding on jobs or a homeowner tackling a DIY project, accurately calculating paint requirements saves money, prevents delays, and ensures you have enough material to complete the work. Running out of paint mid-project means extra trips to the store, potential color-matching issues, and wasted time. Buying too much ties up capital and creates disposal problems. Getting it right requires understanding coverage rates, surface preparation, paint types, and the specific characteristics of your project.

This comprehensive guide walks through every aspect of paint calculation, from basic wall measurements to complex multi-room estimates, with real-world examples and professional techniques that ensure accurate material takeoffs every time.

Understanding Paint Coverage Rates

Coverage rate is the area a gallon of paint will cover in a single coat, typically measured in square feet per gallon. This number varies significantly based on paint type, sheen, surface texture, application method, and surface porosity.

Coverage by Paint Sheen

Different sheens have different coverage characteristics due to their formulation and the way they flow and level on the surface:

| Paint Sheen | Coverage Rate (sq ft/gallon) | Typical Applications | |-------------|------------------------------|---------------------| | Flat/Matte | 350-400 sq ft | Ceilings, low-traffic walls, hiding imperfections | | Eggshell | 350 sq ft | Living rooms, bedrooms, dining rooms | | Satin | 300-350 sq ft | Kitchens, bathrooms, hallways, trim | | Semi-Gloss | 300-350 sq ft | Trim, doors, cabinets, high-moisture areas | | Gloss/High-Gloss | 300-350 sq ft | Doors, trim, furniture, high-traffic areas |

Flat paint typically offers the best coverage because it doesn't flow and level as much as glossier paints, creating a thicker, more opaque film. Glossier paints spread more thinly to achieve their smooth, reflective finish, reducing coverage per gallon.

First Coat vs. Second Coat Coverage

Your first coat of paint will always use more material than subsequent coats because the surface is more porous and absorbent. This is particularly true on new drywall, bare wood, or previously unpainted masonry.

First coat on porous surfaces: Coverage may drop to 250-300 sq ft per gallon as the paint soaks into the substrate. This is why primer is so important - it seals the surface and prevents the finish coat from being absorbed unevenly.

Second and subsequent coats: Coverage typically increases to 350-450 sq ft per gallon on a properly primed, sealed surface because the paint sits on top rather than soaking in.

For estimating purposes, use the conservative coverage rates listed in the sheen table above for all coats. This builds in a safety margin and accounts for texture, application inefficiency, and touch-up needs.

New Drywall vs. Previously Painted Surfaces

New drywall is extremely porous and absorbent. The paper facing and joint compound create a varied surface with different absorption rates. Without primer, you might need three or even four coats of paint to achieve uniform coverage and color.

New drywall requirements:

• One coat PVA (polyvinyl acetate) drywall primer or primer-sealer
• Two coats finish paint minimum
• Total paint needed: Primer at 300-350 sq ft/gal + finish paint at 350-400 sq ft/gal for coat one, 400+ sq ft/gal for coat two

Previously painted surfaces in good condition:

• No primer typically needed if the existing paint is sound and clean
• Two coats finish paint for color change
• One coat may suffice for minor refresh in the same color
• Coverage: 350-400 sq ft per gallon for most applications

Previously painted surfaces requiring repair:

• Spot prime any bare areas, repairs, or stains
• Two coats finish paint over the entire surface
• Prime and paint all repairs before applying finish coats to the full surface

Textured Walls and Reduced Coverage

Textured surfaces like knockdown, orange peel, and skip trowel finishes have significantly more surface area than smooth walls. The peaks and valleys create additional surface to coat, reducing effective coverage by 15-25% depending on the texture depth.

Texture coverage adjustments:

• Light orange peel texture: Reduce coverage by 10-15%
• Medium knockdown texture: Reduce coverage by 15-20%
• Heavy skip trowel or popcorn: Reduce coverage by 20-30%

Example: A smooth wall requiring 2 gallons might need 2.3-2.4 gallons with medium knockdown texture. For a room with heavy texture, budget an extra 20-25% paint beyond smooth wall calculations.

Heavily textured ceilings like popcorn texture are particularly paint-hungry. Spray application is typically required, which also increases material usage due to overspray loss.

Calculating Wall Paint Requirements

The basic formula for wall area is straightforward, but the details matter for accuracy.

Step 1: Measure Room Dimensions

Measure the room perimeter by adding the length of all walls:

• Measure each wall length at floor level
• Add all walls together for total perimeter
• Measure floor-to-ceiling height at multiple points (settling can create variations)

Example room:

• Two walls: 14 feet each
• Two walls: 12 feet each
• Perimeter: 14 + 14 + 12 + 12 = 52 feet
• Height: 8 feet

Step 2: Calculate Gross Wall Area

Multiply perimeter by height:

Gross wall area = Perimeter × Height 52 feet × 8 feet = 416 square feet

Step 3: Deduct Openings

Subtract the area of doors, windows, and other openings that won't be painted.

Standard opening sizes:

• Standard interior door (single, 3'0" × 6'8"): 21 square feet
• Standard exterior door (single, 3'0" × 6'8"): 21 square feet
• Double door/French door: 42 square feet
• Standard window (3' × 5'): 15 square feet
• Large window (4' × 6'): 24 square feet
• Sliding glass door (6' × 6'8"): 40 square feet

For irregular openings, measure width × height and calculate the actual area.

Example room deductions:

• One standard door: 21 sq ft
• Two standard windows: 30 sq ft (15 sq ft each)
• Total deductions: 51 sq ft

Net paintable area = 416 - 51 = 365 square feet

Step 4: Calculate Paint Required

Divide net area by coverage rate and multiply by number of coats:

Paint needed = (Net Area ÷ Coverage Rate) × Number of Coats

For our example room with satin paint (coverage: 325 sq ft/gallon), two coats: Paint needed = (365 ÷ 325) × 2 = 2.25 gallons

Round up to 2.5 gallons (two gallons plus one quart) or simply purchase 3 gallons to ensure adequate material for touch-ups.

Should You Deduct Openings?

For professional accuracy, yes. For quick estimating on small residential projects, some painters skip this step and use the extra paint as a built-in safety margin for waste and touch-ups.

Deduct openings when:

• Bidding commercial or large residential projects where material cost is significant
• Working with expensive specialty paints
• Estimating for client approval and budget accuracy

Skip deducting openings when:

• Painting small rooms (under 200 sq ft) where the difference is minimal
• Using economy paint where the cost difference is negligible
• You want maximum safety margin for touch-ups and future repairs

Ceiling Paint Calculations

Ceiling calculations are simpler than walls because there are typically no openings to deduct (except skylights).

Basic Ceiling Formula

Ceiling area = Length × Width

For our example room (14' × 12'): Ceiling area = 168 square feet

Ceiling Paint Coverage

Ceilings are almost always painted with flat paint, which provides the best coverage and hides imperfections in the ceiling surface. Flat ceiling paint typically covers 350-400 sq ft per gallon.

Paint needed for two coats: (168 ÷ 375) × 2 = 0.90 gallons

One gallon provides enough paint for two coats with material left for touch-ups.

Vaulted and Cathedral Ceilings

For vaulted ceilings, you need to calculate the actual sloped area, not just the floor area. This requires measuring the slope.

Method 1 - Direct measurement: Measure the actual length of the sloped ceiling and multiply by width.

Method 2 - Pythagorean calculation: If you know the horizontal run and the rise (height difference), calculate the slope length: Slope length = √(run² + rise²)

Example: 12-foot horizontal run with 4-foot rise: Slope length = √(144 + 16) = √160 = 12.65 feet

This 12-foot wide ceiling section with a 4-foot rise has 12.65 × 12 = 151.8 sq ft, compared to 144 sq ft floor area - about 5% more paint required.

Textured Ceilings and Popcorn

As noted earlier, texture significantly increases paint consumption. Popcorn ceilings are particularly challenging:

• Often require spray application due to the delicate texture
• Coverage typically 200-250 sq ft per gallon (nearly double the paint)
• Spray application adds 20-40% waste from overspray
• May need special popcorn ceiling paint or texture-bridging primer

For a 168 sq ft popcorn ceiling: Paint needed = (168 ÷ 225) × 2 × 1.3 (overspray factor) = 1.94 gallons

Budget 2-2.5 gallons for this small ceiling, compared to 1 gallon for smooth ceiling.

Trim and Baseboard Paint Calculations

Trim requires a different calculation approach based on linear feet rather than square footage.

Measuring Trim

Measure the linear footage of all trim to be painted:

• Baseboards
• Door casings (both sides of opening)
• Window casings
• Crown molding
• Chair rail
• Any other decorative trim

Standard door casing: Each door has two vertical casings (6-7 feet each) plus header casing (3-4 feet) = approximately 16-18 linear feet per door.

Standard window casing: Similar calculation, typically 12-16 linear feet per window depending on size.

Example room trim:

• Baseboard: 52 feet (follows wall perimeter, minus door opening width)
• One door casing: 18 feet
• Two window casings: 28 feet (14 feet each)
• Total trim: 98 linear feet

Trim Paint Coverage Rule of Thumb

A general rule for trim is:

One quart covers approximately 200 linear feet for two coats (assuming standard 3-4 inch baseboard or casing width)

For our 98 linear feet of trim: Paint needed = 98 ÷ 200 = 0.49 quarts, round up to 1 quart

Alternative Trim Calculation Method

For more precision, calculate actual trim area:

Trim area = Linear feet × Average width (in feet)

For 98 linear feet of trim averaging 3.5 inches (0.29 feet) wide: Trim area = 98 × 0.29 = 28.4 square feet

With semi-gloss trim paint (coverage: 325 sq ft/gallon), two coats: Paint needed = (28.4 ÷ 325) × 2 = 0.17 gallons = 0.68 quarts

One quart provides adequate coverage with material for touch-ups.

Wide Trim and Crown Molding

Crown molding and wide baseboards require more paint due to their greater width:

Crown molding (3-5 inches): One quart per 150-175 linear feet, two coats Wide baseboards (6-8 inches): One quart per 100-125 linear feet, two coats

Complex trim profiles with many details may reduce coverage further due to the increased surface area.

When Primer is Mandatory

Primer isn't always necessary, but certain situations require it for acceptable results.

New Drywall - Always Prime

New drywall must be primed. The paper facing on drywall and the joint compound used on seams absorb paint at different rates, creating a blotchy appearance known as "flashing" if you skip primer. PVA (polyvinyl acetate) drywall primer is specifically formulated to seal new drywall uniformly.

New drywall primer requirements:

• One coat PVA primer: 300-350 sq ft per gallon
• Covers uniformly despite varied absorption of paper and mud
• Essential for professional finish
• Cost-effective compared to using extra coats of expensive finish paint

Bare Wood - Always Prime

Bare wood, whether new construction lumber or stripped furniture, must be primed to seal the wood grain and prevent tannin bleed-through.

Wood primer types:

• Oil-based primer: Best stain blocking, excellent sealing
• Shellac-based primer (BIN, Cover Stain): Superior stain blocking for knots and tannins
• Latex primer: Acceptable for most softwoods and previously painted wood

Knots, sap, and tannins in wood can bleed through even multiple coats of paint if not sealed with appropriate primer. This is particularly problematic with cedar, redwood, pine, and oak.

Stain Blocking - Always Prime

Water stains, smoke damage, marker, crayon, and other stains will bleed through paint if not sealed with stain-blocking primer.

Stain-blocking primer types:

• Shellac-based (BIN): Blocks everything including smoke, water stains, odors
• Oil-based stain blocker: Excellent for water and tannin stains
• Latex stain blocker: Effective for light stains, easier cleanup

For severe staining, shellac-based primers like Zinsser BIN provide the most reliable sealing. They dry in 45 minutes and block virtually all stains from bleeding through.

Dark to Light Color Changes - Prime Recommended

Changing from dark to light colors is notoriously difficult without primer. Deep reds, purples, blues, and blacks can show through even three or four coats of lighter paint.

Options for dark-to-light transitions:

• Use tinted primer in a medium gray tone
• Apply one coat primer plus two coats finish paint
• Alternative: Apply three to four coats of finish paint (expensive and time-consuming)

The primer approach saves money and time. A gallon of gray-tinted primer plus two gallons of light finish paint costs less and performs better than four gallons of finish paint.

Glossy Surfaces - Prime or Sand

Paint adheres poorly to glossy surfaces. Existing semi-gloss or gloss paint should be either sanded dull or primed with adhesion-promoting primer before applying new paint.

Options for glossy surfaces:

• Sand with 120-150 grit sandpaper to dull the gloss, then paint
• Apply bonding primer designed for slick surfaces
• Combination: Light sanding plus primer for maximum adhesion

Deglosser (liquid sandpaper) is a chemical alternative to mechanical sanding, but primer still provides better adhesion for critical applications.

When Primer Can Be Skipped

You can typically skip primer in these situations:

• Repainting sound, clean, previously painted surfaces in similar colors
• Touching up walls in good condition
• Refreshing trim painted within the past few years
• Color-to-color changes in the same value range (medium to medium)

Even when primer isn't mandatory, it often improves final results and can reduce the number of finish coats needed.

Paint Coverage by Application Method

The method you use to apply paint affects coverage rates and waste.

Roller Application

Rollers are the standard for walls and ceilings, offering good coverage with moderate waste.

Roller coverage factors:

• Standard 3/8" nap roller: Best coverage, 350-400 sq ft/gallon on smooth surfaces
• 1/2" nap roller: Medium texture surfaces, 325-375 sq ft/gallon
• 3/4" nap roller: Heavy texture, 275-325 sq ft/gallon
• Waste: 5-10% from loading roller and tray

Use the shortest nap that fully contacts your surface. Longer nap holds more paint but spreads it less efficiently, reducing coverage.

Brush Application

Brushing is necessary for trim, cutting in, and detail work but is the least efficient application method.

Brush coverage factors:

• Coverage: 275-325 sq ft/gallon due to thicker application
• Waste: 10-15% from loading brush and cleaning
• Best for: Trim, doors, cabinets, detail work
• Plan extra material when brushing large areas

Spray Application

Spraying provides the smoothest finish but uses significantly more paint due to overspray.

Spray coverage factors:

• Coverage: 200-300 sq ft/gallon depending on technique
• Overspray waste: 20-40% additional paint
• Total efficiency: 60-80% compared to rolling
• Best for: Cabinets, furniture, textured ceilings, new construction

Example: A wall requiring 1 gallon with roller might need 1.3-1.5 gallons when sprayed. However, spray application is faster for large projects and produces superior finish quality on smooth surfaces.

HVLP (High Volume Low Pressure) sprayers are more efficient than airless sprayers, reducing overspray waste to 15-25% while maintaining excellent finish quality.

Room-by-Room Paint Estimates

Pre-calculated estimates for common rooms help you quickly estimate projects.

Standard Bedroom (12' × 12', 8' ceilings)

Walls:

• Perimeter: 48 feet
• Wall area: 384 sq ft
• Less one door (21 sq ft) and one window (15 sq ft): 348 sq ft
• Paint needed (satin, two coats): 2.2 gallons

Ceiling:

• Area: 144 sq ft
• Paint needed (flat, two coats): 0.8 gallons

Trim:

• Baseboard (48 ft) + door casing (18 ft) + window casing (14 ft) = 80 linear feet
• Paint needed (semi-gloss, two coats): 1 quart

Total: 3 gallons wall paint, 1 gallon ceiling paint, 1 quart trim paint

Master Bedroom (16' × 14', 8' ceilings)

Walls:

• Perimeter: 60 feet
• Wall area: 480 sq ft
• Less one door (21 sq ft) and two windows (30 sq ft): 429 sq ft
• Paint needed (satin, two coats): 2.7 gallons

Ceiling:

• Area: 224 sq ft
• Paint needed (flat, two coats): 1.2 gallons

Trim:

• Linear feet: approximately 100 feet
• Paint needed (semi-gloss, two coats): 1 quart

Total: 3 gallons wall paint, 1.5 gallons ceiling paint, 1 quart trim paint

Living Room (20' × 16', 8' ceilings)

Walls:

• Perimeter: 72 feet
• Wall area: 576 sq ft
• Less two doorways (42 sq ft) and three windows (45 sq ft): 489 sq ft
• Paint needed (eggshell, two coats): 2.9 gallons

Ceiling:

• Area: 320 sq ft
• Paint needed (flat, two coats): 1.8 gallons

Trim:

• Linear feet: approximately 140 feet (includes door casings, window casings, baseboards)
• Paint needed (semi-gloss, two coats): 1 quart

Total: 3 gallons wall paint, 2 gallons ceiling paint, 1 quart trim paint

Kitchen (12' × 10', 8' ceilings)

Walls:

• Perimeter: 44 feet
• Wall area: 352 sq ft
• Less one door (21 sq ft), one window (15 sq ft), and cabinet areas (approximately 80 sq ft): 236 sq ft
• Paint needed (satin, two coats): 1.5 gallons

Ceiling:

• Area: 120 sq ft
• Paint needed (flat, two coats): 0.7 gallons

Cabinets (if painting):

• 20 linear feet of cabinets, both sides of doors: approximately 160 sq ft
• Paint needed (semi-gloss, two coats): 1 gallon

Total: 2 gallons wall paint, 1 gallon ceiling paint, 1 gallon cabinet paint

Bathroom (8' × 6', 8' ceilings)

Walls:

• Perimeter: 28 feet
• Wall area: 224 sq ft
• Less one door (21 sq ft), one window (10 sq ft), tub surround/shower (30 sq ft): 163 sq ft
• Paint needed (satin, two coats): 1 gallon

Ceiling:

• Area: 48 sq ft
• Paint needed (flat, two coats): 0.3 gallons

Total: 1 gallon wall paint, 1 quart ceiling paint (or use wall paint on ceiling)

Two-Car Garage (20' × 20', 8' ceilings)

Walls:

• Perimeter: 80 feet
• Wall area: 640 sq ft
• Less garage door (140 sq ft), entry door (21 sq ft), window (15 sq ft): 464 sq ft
• Paint needed (satin, two coats): 3 gallons

Ceiling (if painting):

• Area: 400 sq ft
• Paint needed (flat, two coats): 2.2 gallons

Total: 3-4 gallons wall paint, 2.5 gallons ceiling paint if finishing ceiling

Exterior Paint Considerations

Exterior painting requires additional considerations beyond interior work.

Exterior Coverage Differences

Exterior surfaces are typically more porous and textured than interior surfaces, reducing coverage rates:

Exterior coverage by substrate:

• Smooth wood siding: 300-350 sq ft/gallon
• Rough-sawn wood siding: 200-250 sq ft/gallon
• Stucco (smooth): 250-300 sq ft/gallon
• Stucco (textured): 150-200 sq ft/gallon
• Brick (painted): 200-250 sq ft/gallon
• Vinyl or aluminum siding: 300-400 sq ft/gallon
• Concrete block: 200-250 sq ft/gallon

Exterior Paint System

Exterior painting typically involves primer plus two coats:

• Prime all bare wood, repaired areas, and problem surfaces
• First finish coat: Base coverage rate
• Second finish coat: Increased coverage rate (better than first coat)

Trim and Detail Work

Exterior trim, shutters, doors, and window frames require separate calculations:

• Measure linear feet of all trim
• Calculate door and shutter areas separately
• Use semi-gloss or gloss exterior paint for trim
• Budget for two coats on all exterior wood trim

Paint Waste Factors and Extra Material

No painting project achieves 100% efficiency. Account for waste and extra material needs.

Recommended Waste Factors

Add these percentages to your calculated paint requirements:

10% waste factor for:

• Professional painters using roller application
• Simple rooms with few cuts and transitions
• High-quality paint with good coverage

15% waste factor for:

• DIY painters with less experience
• Rooms with multiple colors or complex cuts
• Spray application (add to the base overspray loss)
• Textured surfaces

20% waste factor for:

• Very complex jobs with many colors and detail work
• Highly textured surfaces
• First-time DIYers
• Insurance against running short

Touch-Up and Future Repair Material

Always keep extra paint for touch-ups and future repairs:

• Label cans with room name and date
• Store in climate-controlled area
• Keep sealed and protected from freezing
• Latex paint stores 2-3 years if properly sealed
• Oil-based paint stores 5-10 years if properly sealed

Having touch-up paint available saves significant time and money compared to repainting entire walls for minor damage.

How to Store Leftover Paint

Proper storage extends paint life and maintains quality.

Storage Best Practices

Container selection:

• Store in original can if at least 1/3 full
• Transfer to smaller container if less than 1/3 full (less air = less drying)
• Use glass jars, plastic paint containers, or clean, dry cans

Sealing:

• Clean paint from rim before sealing
• Place plastic wrap over opening before replacing lid
• Tap lid down firmly with rubber mallet (not hammer)
• Turn can upside down briefly to create air-tight seal, then store right-side up

Labeling:

• Write room name on lid
• Note date painted
• Record paint brand, color name, and formula number
• Include sheen type (flat, satin, semi-gloss)

Storage location:

• Climate-controlled area (avoid freezing or extreme heat)
• Off ground to prevent rust on metal cans
• Away from children and pets
• Avoid direct sunlight

When to Dispose of Old Paint

Discard paint when it:

• Has been frozen (latex paint)
• Shows mold growth
• Has hardened or developed thick skin
• Smells rancid or off
• Won't remix to smooth consistency
• Is more than 5 years old (latex) or 10 years old (oil-based)

Many communities have hazardous waste collection days for proper paint disposal.

Complete Example: Calculating Paint for an Entire House

Let's calculate paint requirements for a complete 1,500 sq ft ranch house with three bedrooms, two bathrooms, living room, kitchen, and hallways.

Room Breakdown

Master bedroom (16' × 14'): 3 gallons wall, 1.5 gallons ceiling Bedroom 2 (12' × 12'): 3 gallons wall, 1 gallon ceiling Bedroom 3 (12' × 10'): 2.5 gallons wall, 1 gallon ceiling Master bathroom (10' × 8'): 1.5 gallons wall Hall bathroom (8' × 6'): 1 gallon wall Living room (20' × 16'): 3 gallons wall, 2 gallons ceiling Kitchen (12' × 10'): 2 gallons wall, 1 gallon ceiling Hallways (40 linear feet, 8' ceilings): 2 gallons wall, 1 gallon ceiling

Paint Summary

Wall paint (all rooms using same color):

• Total calculated: 18 gallons
• Add 15% waste: 20.7 gallons
• Round up to: 21 gallons

Ceiling paint (flat white throughout):

• Total calculated: 7.5 gallons
• Add 15% waste: 8.6 gallons
• Round up to: 9 gallons

Trim paint (semi-gloss white throughout):

• Approximately 400 linear feet total trim
• Calculated: 2 quarts
• Add 15% waste and round up: 3 quarts

Doors (6 interior doors, both sides):

• Each door: approximately 40 sq ft both sides
• Total: 240 sq ft
• Paint needed (semi-gloss, two coats): 1.5 gallons
• Round up to: 2 gallons

Final Order

• 21 gallons wall paint (or 20 gallons plus 4 quarts)
• 9 gallons ceiling paint
• 2 gallons trim/door paint plus 3 quarts additional trim paint
• Consider ordering 22-23 gallons wall paint if using same color throughout for maximum safety margin and future touch-ups

Cost Estimate

At $35 per gallon for quality paint:

• Wall paint: 21 gallons × $35 = $735
• Ceiling paint: 9 gallons × $35 = $315
• Trim paint: 2.75 gallons × $40 (semi-gloss typically costs more) = $110
• Total paint cost: approximately $1,160

This estimate assumes mid-grade quality paint. Premium paints cost $50-75 per gallon, while economy paints may be $20-25 per gallon.

Professional Tips for Accurate Estimates

These field-tested techniques improve accuracy:

Measurement Tips

• Always measure twice for critical projects
• Use a laser measuring tool for speed and accuracy
• Sketch room layouts with dimensions noted
• Take photos of complex areas for reference
• Account for soffits, bulkheads, and architectural details

Estimation Shortcuts

• For quick estimates: Total square footage of house × 2.5 = total wall and ceiling area (rough approximation)
• Standard 8' ceiling room: Perimeter × 2 gives approximate wall area after deductions
• Round all measurements up to next foot for safety margin
• When in doubt, buy an extra gallon - most stores accept returns of unopened paint

Bidding Considerations for Professionals

When bidding painting jobs:

• Calculate material precisely, then add 10-15% waste
• Account for primer separately in material and labor
• Include cost of supplies (tape, plastic, rollers, brushes, trays)
• Verify whether ceiling painting is included
• Clarify who provides paint (contractor or homeowner)
• Document number of coats in proposal
• Note any areas excluded from bid (closets, unfinished spaces)

Conclusion and Key Takeaways

Accurate paint calculation prevents costly mistakes, project delays, and wasted material. The keys to success are:

Essential calculation steps:

• Measure room perimeter and height to calculate gross wall area
• Deduct standard openings: doors (21 sq ft), windows (15 sq ft average)
• Use appropriate coverage rates: flat (350-400 sq ft/gal), satin (300-350 sq ft/gal), semi-gloss (300-350 sq ft/gal)
• Account for two coats on walls and ceilings for professional results
• Calculate trim separately using linear feet (1 quart per 200 linear feet for two coats)

When primer is mandatory:

• New drywall requires PVA primer
• Bare wood requires stain-blocking primer
• Stain blocking situations require shellac or oil-based primer
• Dark to light color changes benefit from tinted primer
• Glossy surfaces need bonding primer or sanding

Coverage adjustments:

• New drywall: Reduce coverage by 15-20% on first coat
• Textured surfaces: Reduce coverage by 15-25% depending on texture depth
• Spray application: Add 20-40% for overspray waste
• First coat on porous surfaces: Plan for lower coverage

Material management:

• Add 10-15% waste factor to all calculations
• Buy extra for touch-ups and future repairs
• Store properly labeled in climate-controlled area
• Keep leftover paint 2-3 years for touch-ups

By following these guidelines and using the formulas and tables provided, you'll accurately estimate paint requirements for any project, ensuring you have adequate material without significant waste or expensive overages.