When you’re planning a project, one of the first questions is simple: how to measure for paint. Without a good estimate, you risk buying too little and running out mid-project, or buying too much and overspending. Fortunately, the math is easy, and once you know the steps, you can use them on any surface—from siding to bedroom walls.

Step 1: Measure the walls
Grab a tape measure and find the length and height of each wall. Multiply those numbers to get the square footage of that wall. Repeat this for each wall in the space.

Step 2: Subtract windows and doors
Windows and doors don’t need paint, so subtract their square footage from your total. This gives you the actual paintable area.

Step 3: Do the paint math
On average, one gallon of paint covers about 400 square feet for a single coat on a sealed surface. Divide your total square footage by 400 to estimate how many gallons you’ll need.

And that’s it. How to measure for paint really comes down to length × height, minus doors and windows, divided by coverage. With this simple formula, you’ll know exactly how much paint to buy and save yourself a lot of stress on project day.