Room Dimensions (feet)
Openings & Coats
How it's calculated
Wall area = perimeter × height − (21 sq ft × doors) − (15 sq ft × windows). Gallons = wall area × coats ÷ 350. One gallon ≈ 3.79 liters.
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gallons (round up from 1.78)
Frequently Asked Questions
How many square feet does a gallon of paint cover?
One gallon of paint typically covers about 350 square feet with a single coat on a primed or previously painted wall. Rough or unprimed surfaces can absorb more and cover closer to 250–300 sq ft per gallon, while smooth surfaces may stretch to 400 sq ft.
Do I need to count primer as a coat?
Primer is applied and covers like a coat, so if you are priming new drywall or covering a dark color, add one coat to the calculation and buy primer separately in the same quantity. Two finish coats over primer is the standard recipe for a durable, even finish.
Does this calculator include the ceiling?
No. The calculator measures walls only (perimeter × height minus doors and windows). Ceilings are usually painted with flat ceiling paint at the same coverage rate: ceiling area = length × width, then divide by 350 sq ft per gallon per coat.
How many coats of paint should I use?
Two coats is standard for most interior walls: one coat rarely hides the old color evenly. Use three coats when making a dramatic color change, painting over dark or bold colors, or switching between light and dark extremes.
How much paint do I need for a 12x12 room?
A 12×12 ft room with 8 ft ceilings, one door, and one window has (48 × 8) − 21 − 15 = 348 sq ft of wall. Two coats need 696 ÷ 350 ≈ 1.99 gallons, so buy 2 gallons.
Is this paint calculator free to use?
Yes. It is completely free with no signup, and every calculation runs locally in your browser — your room dimensions and prices are never sent anywhere or stored.
Quick Answer
One gallon of paint covers about 350 square feet with one coat. Wall area = (2 × (length + width) × height) − 21 sq ft per door − 15 sq ft per window. Gallons = wall area × coats ÷ 350. A 12×12 ft room with 8 ft ceilings, 2 doors, 2 windows, and 2 coats needs (48 × 8 − 42 − 30) × 2 ÷ 350 = 1.78, so buy 2 gallons. Most rooms need two coats for full coverage.
Key Facts
- One gallon of paint covers approximately 350 sq ft per coat
- Formula: gallons = wall area × coats ÷ 350
- Wall area = perimeter × height − 21 sq ft per door − 15 sq ft per window
- One gallon ≈ 3.785 liters
- Most interior projects need two coats; dark or bold colors may need three
- A standard door subtracts ~21 sq ft; a window subtracts ~15 sq ft
- Ceiling paint is usually bought separately at the same 350 sq ft per gallon rate
- Buy slightly more than calculated — touch-ups and waste are common
Frequently Asked Questions
How many square feet does a gallon of paint cover?
One gallon of paint typically covers about 350 square feet with a single coat on a primed or previously painted wall. Rough or unprimed surfaces can absorb more and cover closer to 250–300 sq ft per gallon, while smooth surfaces may stretch to 400 sq ft.
Do I need to count primer as a coat?
Primer is applied and covers like a coat, so if you are priming new drywall or covering a dark color, add one coat to the calculation and buy primer separately in the same quantity. Two finish coats over primer is the standard recipe for a durable, even finish.
Does this calculator include the ceiling?
No. The calculator measures walls only (perimeter × height minus doors and windows). Ceilings are usually painted with flat ceiling paint at the same coverage rate: ceiling area = length × width, then divide by 350 sq ft per gallon per coat.
How many coats of paint should I use?
Two coats is standard for most interior walls: one coat rarely hides the old color evenly. Use three coats when making a dramatic color change, painting over dark or bold colors, or switching between light and dark extremes.
How much paint do I need for a 12x12 room?
A 12×12 ft room with 8 ft ceilings, one door, and one window has (48 × 8) − 21 − 15 = 348 sq ft of wall. Two coats need 696 ÷ 350 ≈ 1.99 gallons, so buy 2 gallons.
Is this paint calculator free to use?
Yes. It is completely free with no signup, and every calculation runs locally in your browser — your room dimensions and prices are never sent anywhere or stored.