How to Calculate Exactly How Much Paint You Need for a Room

One of the most common mistakes in DIY home improvement is miscalculating paint. Buy too much, and you waste money. Buy too little, and you have to run back to the hardware store in the middle of a project (hoping the color mix matches perfectly).

Fortunately, calculating your paint needs is simple geometry.

The Standard Paint Coverage Rule

As a general rule of thumb, one gallon of standard interior paint covers roughly 350 to 400 square feet of wall space for a single coat.

Note: If you are painting highly textured walls, bare drywall, or using low-quality paint, you may get closer to 300 square feet per gallon.

Step-by-Step Paint Calculation

1. Calculate the Perimeter: Measure the length of all the walls in the room and add them together. For a 10x12 room, the perimeter is 44 feet (10+10+12+12).
2. Calculate Total Wall Area: Multiply the perimeter by the ceiling height. If the ceilings are 8 feet high, the total area is 352 square feet (44 x 8).
3. Subtract Doors and Windows: You don't paint over your doors and windows! Subtract 21 square feet for every standard door, and 15 square feet for every standard window.
4. Divide by Coverage: Divide your final square footage by 350 to find out how many gallons you need for one coat.
5. Multiply by Coats: Multiply that number by the amount of coats you intend to apply. Most professional jobs require two coats.

Use the Calculator

Don't want to do the math on a notepad? Use our free Paint Calculator. Just enter your room's dimensions, the number of doors and windows, and how many coats you want to do. It will instantly tell you exactly how many gallons to buy!