Estimate exactly how much paint you need for walls, ceiling, and trim — accurate to the gallon, with automatic deductions for doors and windows.
Quick Answer: To estimate paint, measure the room's perimeter and height to get the wall area. Subtract 20 sq ft per door and 15 sq ft per window. Multiply by the number of coats, then divide by the paint's coverage rate (typically 350 sq ft per gallon). Finally, add 10% for touch-ups.
Our free paint calculator takes the guesswork out of your next painting project. Simply enter your room's length, width, and wall height, then specify the number of doors and windows so we can accurately deduct those unpaintable areas. Choose how many coats you plan to apply — most interior paint jobs require two coats for even, professional coverage — and the calculator instantly shows you how many gallons (or liters) you need to purchase. Never make a second hardware store run mid-project again.
Use this quick reference table to see how much paint you'll need for standard room sizes (assuming 8 ft ceilings and 2 coats of paint):
| Room Size | Wall Area (approx.) | Net Area (after deductions) | Paint Needed (2 coats) | Recommended Purchase |
|---|---|---|---|---|
| 10 × 10 ft | 320 sq ft | 270 sq ft | 1.5 gallons | 2 gallons |
| 12 × 12 ft | 352 sq ft | 300 sq ft | 1.7 gallons | 2 gallons |
| 15 × 15 ft | 480 sq ft | 430 sq ft | 2.5 gallons | 3 gallons |
| 20 × 20 ft | 640 sq ft | 590 sq ft | 3.4 gallons | 4 gallons |
The industry standard for paint coverage per gallon is approximately 350 square feet on a smooth, primed surface, and around 250–300 square feet on textured, porous, or previously unprimed walls. Our calculator defaults to 350 sq ft/gallon, but you can adjust this value based on your specific paint brand's label. Premium paints often cover more area per gallon with better opacity, while budget paints and deep-toned colors may require an additional coat entirely — which you should factor into your quantity estimate.
Professional painters always deduct doors and windows from the total wall area before calculating paint. A standard interior door covers approximately 20 square feet, while a typical window occupies around 15 square feet. Our tool handles these deductions automatically, preventing you from over-purchasing paint and wasting money. Ceiling paint is calculated separately as its own product (always use a flat or matte finish to hide imperfections), while trim and baseboard paint uses a different formula based on linear feet of perimeter.
Material costs can vary significantly based on your location and the type of paint you choose. As a general rule of thumb in 2026:
If you're painting over a dark color, painting bare drywall, covering stains, or switching to a dramatically different color, a primer coat is essential. Primer seals the surface so your topcoat achieves full coverage in fewer coats, saving you money overall. A standard primer covers 300–350 sq ft per gallon. Self-priming paints (labeled "paint + primer") can skip this step on previously painted, light-colored surfaces in good condition, but they still require a separate primer when dealing with patchy repairs or bare spots.
Our calculator gives you the material estimate. Now connect with local professional painters for a precise quote.
Find Local Painters →Q: How many gallons of paint do I need for a 12×12 room?
A: A standard 12×12 ft room with 8 ft ceilings has approximately 352 sq ft of wall area. After deducting one door and two windows (~50 sq ft), you have ~300 sq ft. Two coats requires about 1.7 gallons — buy 2 gallons to be safe.
Q: Is two coats of paint always necessary?
A: For most color changes and full repaints, yes. One coat rarely provides uniform coverage or color depth. Exception: light refresher coats over an identical existing color may need only one coat with high-quality paint.
Q: How much extra paint should I buy for touch-ups?
A: Always buy 10% more than calculated. Store leftover paint in an airtight container labeled with the room, date, and paint code. Touch-up paint from the same batch matches perfectly — paint from a new can may have slight color variance due to batch differences.
Q: Can I use the same paint for walls and ceiling?
A: Technically yes, but it's not recommended. Ceiling paints are formulated with higher viscosity to prevent drips on vertical tool strokes and are optimized for flat/matte finishes that hide ceiling texture. Using wall paint on the ceiling often results in noticeable lap marks.
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