How Much Paint Do I Need? Indoor Paint Calculator

A young Caucasian woman with light hair sits on the floor of her living room as she prepares to paint the walls. She is one her phone, looking up how much paint to use.

Whether you’ve just bought a house or you’ve lived in your home sweet home for years, at some point its walls and surfaces will be due for a fresh coat of paint. Repainting can breathe new life into an interior and help you personalize the space, whether you’re working within the latest interior design trends or blazing your own trail. But there’s one fundamental question facing every homeowner as they begin their painting project: How much paint do I need?

How much paint do I need?

Every project has a budget, and with the right planning you can execute the project to its full potential without going over budget. Painting is the ultimate DIY project and can be quite therapeutic, but still requires some calculation to determine how much you should expect to spend. With the right amount of paint, you’ll avoid overspending and getting saddled with the sunk cost of unused paint after you’ve completed your project.

The amount of paint required varies by project, but as a general rule of thumb, one gallon of paint covers about 400 square feet. So, it only takes a few simple measurements to calculate the amount of paint you’ll need for your walls.

How to Calculate How Much Paint You Need:

Start by measuring the length of each wall
Multiply the wall length by the wall height
Total length x total height = total square footage
Total square footage ÷ 400 = number of gallons
Subtract windows and doors square footage

Following this formula will give you the number of gallons you need to purchase for one coat of paint. Depending on your color scheme and the texture of your walls, your painting project may require multiple coats to have it looking just right.

If the walls you’re painting have windows and/or doors, simply perform the same basic calculation to determine their square footage and subtract that number from the total square footage value before calculating how many gallons you’ll need. When painting your ceilings, remember to account for the square footage of any skylights you may have in your home.

A middle-aged Caucasian man with dark hair paints bedroom walls sky blue. He wears painting overalls and uses a long-handled roller to apply the blue paint from the baseboard to the trim bordering the ceiling.

Image Source: Getty Images – Image Credit: aydinmutlu

Primer and Trim

It’s often the case that a paint job is only as good as its base coat. A solid layer of primer can really make your painting project shine. But the same query with your topcoat applies to your primer: how much do you need? A gallon of primer will cover up to 300 square feet, so you’ll need more primer than topcoat for your project. Perform the same calculations as above and divide your paintable square footage by 300 to determine how many gallons of primer you’ll need to pick up.

How to Calculate How Much Primer You Need:

Start by measuring the length of each wall
Multiply the wall length by the wall height
Total length x total height = total square footage
Total square footage ÷ 300 = number of gallons
Subtract windows and doors square footage

Calculating square footage for trim isn’t as straightforward as it is for a square or rectangular wall. When preparing to paint your baseboards and crown molding throughout your home, think in quarts rather than gallons. Trim paint may go on smoother depending on the wood finish, and you’ll be using a brush rather than a roller. If you end up with extra trim paint at the completion of your project, it never hurts to keep it around for future touchups.