Paint Calculator: How Much Paint Do I Need?

Determine your paint needs with this simple calculator and room examples

Pouring a can of paint into a tray

The Spruce / Margot Cavin

This paint coverage calculator makes calculating the amount of paint you'll need for your project a nearly automatic task. Your own role in the process involves just a few elements:

  • Measuring the height and width of each wall to be painted.
  • Logging in the number of windows and doors in the space
  • Deciding if you want to paint the ceiling
  • Deciding how many coats of paint you'll apply
  • Pressing the "Calculate" button.

If that doesn't sound easy enough, let's go through each step individually.

Using the Paint Calculator

The paint calculator is useful if you're planning an interior or exterior home painting project. It automates calculations you could make yourself using fairly basic math skills. The calculator makes several basic assumptions in its computations:

  • One gallon of paint is assumed to cover approximately 350 square feet.
  • A standard door accounts for approximately 20 square feet.
  • An average window accounts for approximately 15 square feet.


Please make sure that you are using the most up-to-date version of your web browser in order to avoid potential errors.

Room Shape and Wall Dimensions

Your first entries involve picking the room shape, then entering the length and width of each wall to be painted. For basic rectangular or square rooms, there are four walls that can be painted, more complicated room shapes may have more walls. An L-shaped room, for example, will have six wall surfaces that can be painted.

Windows and Doors

On the right-hand side of the calculator, you can select how many windows and/or doors are in the room you’re trying to paint so that the calculator knows not to count that square footage in the estimate. 

Ceiling Paint

If you’re planning on painting the ceiling of your particular room as well as the walls, you can click the checkbox next to “Ceiling Paint” on the calculator. Ceiling paint will be calculated as a separate item.

Number of Coats

The calculator automatically defaults to two coats of paint. That's because it's typical to coat most walls twice, which does two things: covers bare spots left behind from the first coat and creates a seal or barrier that makes the wall easier to clean. And if you are painting dark over light or light over a dark color that's been primed over, two coats are best to cover and smooth over any bare or thin spots. But you can also change how many coats of paint you plan to apply. Here are some reasons you might change the default value:

  • High-quality paints with more pigments and resins may only require one coat.
  • If you are refreshing a wall with the same color of paint, one coat is all that's necessary.
  • If you want your paint job to last longer, use two to three coats of paint, which will last significantly longer than a single coat.

Editing Your Entries

You can edit your paint calculation at any point.

  • Editing square footage measurements: Update the width and/or height fields you would like to change, and press “calculate” again. To add extra walls and their measurements, press the plus sign next to “add wall.” To remove walls, press the trash can icon.
  • Starting over: To get a new estimate, select your desired room shape, and enter the new measurements. Then press “calculate” again. Note: Doing so will delete your current paint calculation, so be sure to make note of your results.

How the Calculator Works: Examples

Using the calculator is simply a matter of entering the length and height of the walls to be painted, then entering the number of windows and doors that interrupt the wall areas. All mathematical computations occur invisibly to produce a fairly accurate estimate of how much paint you'll need.

For example, let's imagine you have a 12 x 12 room with standard 8-foot-high ceilings. Using the calculator works like this:

  1. Pick a rooms shape (ours is a basic square).
  2. Enter the length and height of each wall, individually (each of our walls is 12 x 8).
  3. Decide if you want to paint the ceiling, as well (we choose not to).
  4. Enter the number of windows and doors that interrupt the wall expanses (ours has two windows and one door).
  5. Decide if you want to apply one, two, or three coats of paint (our example uses 2 coats).
  6. Hit the "Calculate" button.

Automatically, the paint calculator gives you an estimate of 2 gallons of paint for applying two coats to a 12 x 12 room with 8-foot ceilings with two windows, 1 door, where you have no intention of painting the ceiling.

Here's another example: You have a shed or garage with exterior walls that are 25 feet long and 10 feet tall, for a total of 1,000 square feet. Because the existing walls are weathered and in poor shape, they're likely to absorb a lot of paint and you estimate that three coats will be necessary. There are three windows and one door in your shed or garage. If you enter all these values into the paint calculator, you are given an estimate of 9 gallons to paint your structure.

Of course, you could do these calculations by hand. But why bother, when this convenient app does the work for you?


If you'd like to add this paint calculator to your website or blog, you can copy the provided embed code. Once you've finalized your measurements and other specifications, click or tap "Embed This Tool" to generate a code that you can insert into your website

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