Interior Paint Calculator
Get an accurate interior paint estimate by entering your room's length, width, ceiling height, number of doors and windows, number of coats, and finish type. The calculator computes net wall area (after subtracting a standard 21 sq ft per door and 15 sq ft per window), ceiling area, and converts to gallons using coverage rates based on finish: flat/matte covers 350 sq ft per gallon, semi-gloss covers 400 sq ft per gallon. Primer is calculated separately at 300 sq ft per gallon because it's thicker. Two coats is almost always the right answer — one coat leaves the previous color or bare wall visible through the paint film. For dramatic color changes (dark to light or vice versa), a primer coat is essential. The cost estimate uses an average of $35/gallon, which reflects mid-grade paint; premium paints run $55–$80/gallon but often provide better coverage and washability.
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Top PickBehr Interior Paint — Home Depotfrom $30/gal* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
A 12×12 room with 9-foot ceilings has roughly 360 sq ft of wall area (before subtracting doors and windows). At 350 sq ft per gallon with 2 coats, you need about 2 gallons for walls plus 1 gallon for the ceiling (144 sq ft). One standard door subtracts 21 sq ft; a standard window subtracts 15 sq ft.
Do I need primer before painting?
Prime if: you're painting new drywall (bare drywall absorbs paint unevenly without primer), making a drastic color change (especially dark to light), or covering stains, repairs, or nicotine. Skip primer if you're repainting with the same or similar color over clean, previously painted walls in good condition. Self-priming paints exist but are not a full substitute for a dedicated primer on bare surfaces.
How many gallons of paint does it take to paint a whole house interior?
A typical 2,000 sq ft house with 9-ft ceilings has about 2,000 sq ft of wall area and 1,200 sq ft of ceiling (after subtracting doors, windows, and trim). At 2 coats, budget 12–14 gallons for walls and 7–8 gallons for ceilings — roughly 20 gallons total for the main body. Add 1–2 gallons for trim and doors.
What paint finish should I use for walls vs. ceilings?
Use flat or matte for ceilings — it hides imperfections and doesn't reflect light harshly. For walls, eggshell is the most popular choice for living areas (slight sheen, washable). Use satin in kitchens and bathrooms (more moisture-resistant). Save semi-gloss for trim, doors, and window casings — it's durable and easy to wipe clean.