How to Calculate Paint Needs
The standard rule of thumb in the US painting industry is that one gallon of high-quality interior paint covers roughly 400 square feet. To find out exactly how many gallons you need, follow these manual steps:
1. Measure the Perimeter
Add up the length of all the walls in the room you intend to paint. Multiply that total perimeter by the height of the walls from the floor to the ceiling to get your gross square footage.
2. Subtract the Voids
You don't paint over doors and windows, so you need to subtract them from your gross area. The industry standard deduction is 21 square feet for a standard door and 15 square feet for a standard window.
Multiply the resulting Net Area by the number of coats you plan to apply, and then divide by 400. Round up to the nearest whole gallon to ensure you have enough paint to finish the job and keep a small amount for future touch-ups.