Nothing is more frustrating than running out of paint before you finish painting a room. By the same token, you don’t want to finish the room and have an extra two cans of paint. There are several factors that will help you determine how much paint you need, including the shape of the room, the square footage of the walls, and how many coats you plan to paint.

**Make Your Own Calculations or Use an Online Calculator**

You can make your own calculations or use an online calculator. There are websites that allow you to enter the shape of the room, the size of the room, the number of doors and windows, and other information, and the calculator will tell you how much paint you need.

However, you can do the calculations pretty easily yourself. You need to measure the width and the height of a wall and then multiply them together to get the area of that wall. For example, if the width is 15 feet, and the height is 10 feet, the wall is 150 square feet. You need to repeat the measure of each wall in the room.

If you have two walls that are 150 square feet, and two more that are 200 square feet, you need to add the four walls together. You will have a total of 700 square feet.

The next step is to subtract the area of doors and windows. A standard door is around 20 square feet, and an average window is around 15 square feet. You can also measure them if you want a more accurate number.

If you had one door and two windows, you would subtract 50 from your original number, which takes you to 650 square feet.

One gallon of paint covers approximately 350 square feet, so you divide your final number by 350. In this example, you will need two gallons of paint for one coat. If you are painting two coats, you will double it to four gallons of paint.

**Doors and Windows**

You can either measure your doors, or, if your doors are standard size, you can calculate 20 square feet for each door. Take the number of doors, multiply it by 20, and you will know how much paint you need. If you have four doors, you will need 80 square feet, and one gallon of paint will more than cover it.

Standard windows are around 15 square feet each, but much of the window is glass. You can take the number of windows and multiply it by 7.5 to determine how much paint you need for your windows. If your windows are odd sizes, you may want to measure them to determine the square footage, and then divide that number in half. So, if you have a window that is four feet by five feet, you will multiply four by five, which is 20. Now divide it in half, and you will need to cover about 10 square feet. If you are only painting a few doors or windows, you may only need a quart of paint.