Subtract approximately 20 to 21 square feet per door.

Multiply the width of each wall by the room's height (e.g., a

If painting the ceiling, multiply the room's length by its width. Step 2: Subtract "Unpaintable" Areas

Calculating exactly how much paint to buy prevents unnecessary costs and reduces waste. The general rule of thumb is that with a single coat . Step 1: Calculate Total Wall Surface Area