Paint & interior · updated 2026
Paint Gallons Calculator
The most common painting mistake is buying too little — one gallon covers about 350 sq ft, and a “quick bedroom refresh” usually needs two coats plus a ceiling. This paint calculator takes your room dimensions, subtracts doors and windows, and tells you exactly how many gallons to buy for walls and ceiling, with 2026 costs (about $35/gallon for quality interior paint). Buy 10% more than the math says; returning unopened gallons beats a mid-project store run.
Each door ≈ 21 sq ft
Each window ≈ 15 sq ft
Show the math
Wall area = 2 × (12 + 12) × 8 = 384.0 ft²
Minus openings = 384.0 − (1 × 21) − (2 × 15) = 333.0 ft²
Wall gallons = 333.0 ÷ 350 × 2 coats = 1.90
Ceiling = 12 × 12 ÷ 350 = 0
Buy = ceil(total) gallons × $30–$50 = 60 – 100.0
What the numbers mean
Wall area starts with the room perimeter multiplied by ceiling height, then subtracts standard allowances for doors and windows.
Gallons for walls can be fractional because the surface area is continuous. Total gallons to buy is rounded up to whole cans, and an included ceiling is added to the total.
The paint cost range uses the rounded-up gallon count and the seeded 2026 interior paint range. Primer and sundries are separate.
2026 material prices
| Material | 2026 average price | Note |
|---|---|---|
| Interior paint, gallon | $30.00 – $50.00 / gallon | Behr Premium Plus interior reference, Sep 2026 |
Material prices: Home Depot, Sep 2026. National averages; local prices vary. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- One gallon covers about 350 sq ft per coat on smooth walls; textured walls drink about 20% more.
- Two coats is the real answer for any color change — one coat only works for same-color refreshes.
- Prime new drywall and patched spots, or the finish will flash (dull spots).
- Paint the ceiling first, then walls, then trim — drips land on unpainted surfaces.
- Buy all gallons at once and box (mix) them together for large rooms to avoid shade bands.
- A 12×12×8 ft room with 2 coats needs about 2 gallons for walls.
Common questions
Frequently asked questions
How much paint for a 12×12 room?
About 2 gallons for two coats on the walls (8-ft ceilings, 1 door, 2 windows). Add 1 gallon if you’re painting the ceiling.
How many square feet does a gallon cover?
Roughly 350 sq ft per coat on smooth interior walls.
Is one coat of paint enough?
Only for same-color refreshes on previously painted walls. New drywall, repairs, or color changes need two.
How much primer do I need?
Use the same area math as paint — primer covers about 300 sq ft per gallon. One coat on new drywall or bold color changes is typical.
How much does it cost to paint a room?
DIY: $90–$200 in paint and supplies for a 12×12 room. Pro: $300–$800 depending on region.
Can I return extra paint?
Unopened gallons, yes — most retailers take them back within 30–90 days. Tinted paint is often final sale once opened.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.