How many gallons of paint do I need for a room?

One gallon of paint covers approximately 350-400 square feet with one coat. For a standard 12x12 room with 8-foot ceilings (384 sq ft of wall area), you need about 1 gallon per coat, or 2 gallons for two coats. Textured walls or porous surfaces may require 15-20% more paint.

How much does it cost to paint a room professionally?

Professional interior painting costs $2-$6 per square foot in 2025, or $400-$800 for a standard bedroom. This includes prep work, primer, two coats of paint, and cleanup. Costs increase for high ceilings, extensive prep work, or premium paints.

Do I need primer before painting?

Primer is recommended when painting over dark colors, new drywall, stains, or when switching between paint types (oil to latex). Self-priming paints work well for color changes on already-painted surfaces in good condition. Budget 1 gallon of primer per 300-400 sq ft.

What is the difference between interior and exterior paint costs?

Exterior paint costs $25-$70 per gallon compared to $20-$50 for interior paint. Exterior painting labor is also higher at $3-$8 per square foot due to ladder work, surface prep, and weather considerations. Exterior paint must withstand UV, moisture, and temperature extremes.

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Typical Paint Costs by Finish Type

Paint Finish	Material $/gallon	Installed $/sq ft	Best Use
Interior Flat	$20 – $50	$2.00 – $4.50	Ceilings, low-traffic rooms
Interior Eggshell / Satin	$25 – $58	$2.50 – $5.00	Living rooms, bedrooms, hallways
Interior Semi-Gloss	$28 – $65	$3.00 – $6.00	Kitchens, bathrooms, trim
Exterior (Acrylic Latex)	$35 – $75	$3.50 – $7.00	Siding, fascia, soffits
Primer (Interior/Exterior)	$22 – $45	$1.00 – $2.50	New drywall, color changes, stain blocking

Installed costs include 2 coats of paint plus primer on prepared surfaces. Premium brands (Benjamin Moore, Sherwin-Williams) at the higher end.

Assumptions & Sources

Assumptions

Coverage rate: 350 sq ft/gallon for standard paint on smooth surfaces. Textured, porous, or previously unpainted walls reduce effective coverage to 250-300 sq ft/gallon.
Coat count: 2 coats standard. Color changes (dark to light) or new drywall may require 3 coats or a tinted primer coat.
Labor rates: $1.50/sq ft national average for professional interior painting. Exterior, high walls, and detailed trim add 20-50% to labor.
Waste factor: 10% for standard rooms. Cuts, roller loading, tray waste, and touch-ups account for most loss.
Regional multiplier: ZIP-code-based adjustment using RSMeans 2025 regional cost indices applied to both materials and labor.

Last updated: February 2026

Pro Tips from a Construction PM

Primer isn't optional: Going dark-to-light or painting new drywall without primer means 3 coats minimum. That $30 gallon of primer saves you $100+ in extra paint and another half-day of labor.
Real coverage vs. label claims: The can says 400 sq ft/gallon. On textured walls or porous surfaces? Closer to 300. Always buy one extra gallon — use it for touch-ups or return it.
Prep is 80% of the job: Rolling takes a couple hours. Taping, caulking, patching, sanding, and priming? That's your whole first day. Budget accordingly.
Exterior weather windows: 50-85°F, no rain for 24 hours, low humidity. Paint in direct sun and it dries too fast for proper leveling. Early morning or late afternoon on the shaded side is the sweet spot.
Cut in then back-roll fast: Cut edges with a 2.5" angled brush, then roll within 10 minutes. If cut lines dry before rolling, you get visible lap marks that won't blend out.

Related Calculators

Drywall Calculator — Estimate drywall sheets, mud, and tape if you're repairing walls before painting.
Flooring Calculator — Calculate flooring costs if you're renovating the whole room — paint walls before installing floors.
Insulation Calculator — Figure insulation needs if you're opening up walls for repair before repainting.

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