Enter your room's length, width, and wall height, then specify the number of doors and windows and how many coats you plan to apply. The calculator computes total wall area, subtracts doors and windows, and tells you exactly how many gallons (or quarts) to buy. It uses a coverage rate of 350 sq ft per gallon — typical for quality interior paint on smooth walls.
Measure the length and width of the room at floor level, and measure wall height from floor to ceiling. The calculator handles the perimeter math automatically — it multiplies (length + width) × 2 × height to get total wall area. If your room isn't rectangular, break it into sections and add the results.
A standard door is about 20 sq ft and a standard window about 15 sq ft. These seem small, but they add up — a room with 2 doors and 4 windows loses over 100 sq ft of paintable area. Our calculator subtracts them so you don't overbuy.
We use 350 sq ft per gallon as our baseline — typical for quality interior latex on smooth drywall. If you're painting rough plaster, textured walls, or using a paint sprayer (which uses more paint), coverage may drop to 250–300 sq ft. Buy a little extra in those cases.
A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area. After one door and one window, roughly 349 sq ft. One gallon covers ~350 sq ft — so 1 gallon per coat, or 2 gallons for two coats.
Most projects need 2 coats for even coverage. Use 1 coat if you're painting over a similar color with high-quality paint. Use 3 coats for drastic color changes or painting over dark colors.
One gallon typically covers 350–400 sq ft on smooth surfaces with one coat. Rough or textured surfaces may reduce coverage to 250–300 sq ft per gallon.
Yes, if painting over bare drywall, making a drastic color change, or covering stains. Many modern paint-and-primer combos work for minor color changes without separate primer.