Paint Square Footage Calculator
Calculate total wall area, subtract windows and doors, and find exactly how many gallons of paint you need for any room.
What is Paint Square Footage Calculator?
A paint square footage calculator is a tool that measures the total paintable wall area of a room and converts it into the number of gallons of paint you need to buy. It accounts for the perimeter of the room, wall height, and subtracts standard window and door openings from the total.
This tool exists because buying the wrong amount of paint is one of the most common DIY mistakes. Too little paint means an extra trip to the store and a potential color mismatch between batches. Too much paint wastes money — leftover cans often sit in garages for years. The paint calculator eliminates guesswork by using industry-standard window size (15 sq ft) and door size (21 sq ft) deductions.
Professional painters use this calculation method on every job. Enter your room dimensions, the number of openings, and the paint coverage rate (printed on every can) to get an accurate gallon count.
Paint Square Footage Calculator Formula
Step 1: Calculate the room perimeter: 2 × (Length + Width). A 15 × 12 ft room has a perimeter of 54 feet.
Step 2: Multiply perimeter by wall height: 54 × 8 = 432 sq ft of total wall area.
Step 3: Subtract openings. Standard window = 15 sq ft, standard door = 21 sq ft. With 2 windows and 1 door: 432 − 30 − 21 = 381 sq ft of paintable area.
Step 4: Multiply by coats and divide by coverage. For 2 coats at 350 sq ft/gal: (381 × 2) ÷ 350 = 2.18 gallons → round up to 3 gallons.
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