Wondering just how much paint you need for your project? It’s an age-old question for the DIYer! How many gallons should you buy? We can help you out with this, so read on!

How to Calculate How Much Paint You Need 

You can take some steps to help you estimate the quantity of paint you need, taking out the guessing. Here is how to calculate how much paint you need to purchase:

• • • Add the length, end to end, of all the walls to be painted. Let’s say that there are four walls, and each is 12 feet long. So, 12′ x 4 walls = 48. That number 48 is the perimeter of the room.
    • Measure the height of the walls from floor to ceiling. Let’s say the wall height is 10 feet.
    • Multiply the perimeter (48) number by the walls’ height (10′). This gives you the square footage of the room to be painted. In this case, it’s 480 square feet.
    • Next, divide that total number (480) by the approximate coverage of one gallon of paint (about 400 sq. ft.). So, 400 divided by 480 = 1.2 gallons of paint. That’s how many gallons of paint you need for one coat of paint on the walls. Double that amount if you need to do two paint coats – in most cases, you will need two coats.

How Much Paint for Ceilings? 

You need to calculate the ceiling’s square footage, just like walls. Multiply the ceiling’s length by its width. Then, to determine the gallons of paint needed, divide that number by 400.

Window and Doors 

When you’re painting a room but not the windows and doors in that same color, you want to ensure you don’t include their square footage in the overall calculation for how much paint to buy. When you calculated the length and height of the walls, that had the area of the windows and doors. You’ll need to calculate the square footage of the windows and doors to subtract them from the original calculation. Here’s how:

• • • Measure the windows and doors to calculate the square footage of each one.
    • Then add the number of each window and door in the room. For example, four windows and two doors.
    • Then subtract that square footage from the walls’ square footage you calculated earlier.
    • This helps you avoid buying unneeded paint.

How Much Paint Is Needed for Trim?  

If you use a different color for trim than the walls, you’ll need to calculate that separately.

• • • Calculate the length of the trim to be painted.
    •  Calculate the width of all the trim.
    • Multiply length x width to obtain the square footage of the trim you want to paint.
    • Next, divide that square footage total by 400 to determine the number of gallons of paint needed for one coat.

About Primers

Primers are a critical first step before you paint, mainly if it’s an unpainted surface, a highly porous one, or you’re painting over a dark color. Primers cover about 200-300 square feet per gallon, less than paint covers. You’ll need more primer than paint for your package.

To calculate how much to buy, take the total area you calculated earlier and divide it by 300 to arrive at the minimum number of gallons you’ll need for a primer. If you’re doing more than one primer coat, you’ll need more primer.

If you think that your paint project is more than you want to tackle yourself, we can take a look and provide you with a free, no-obligation estimate.

Mike Katounas

Mike Katounas is the owner of Home Works Painting, a painting business in Northern Virginia. He has over 15 years of experience in residential interior and exterior painting, drywall installation/repair, carpentry, wallpaper removal, power washing, commercial painting, color consultation, and staining/sealing. Their service areas include Chantilly, Fairfax, Herndon, Oakton, Reston. Mike takes pride in his work, and he always follows a strict code of conduct that includes the use of quality paint, a clean workspace, and an honest, respectful approach to his customers.