Calculating Paint: How Much Paint Should I Buy?

This is my pup, my shadow, my furry worker buddy, my fuzzy stalker, Percy. You probably should not trust Percy to do the math for this, but he does look very studious with his bow tie and sweater vest, doesn’t he?

One of the simpler things to do, but somewhat intimidating when calculating the budget, is to figure out how much paint you need to buy for your room or rooms that you want to paint. No one wants to over purchase and then have to find a good place to store the extra paint. Real estate in your home is valuable, don’t clutter it up with paint cans. Am I right?

Fortunately the formula for figuring this out is simple, there are just multiple steps. Simply find the surface area of the walls and ceiling, if you’re painting the ceiling the same color, that is. If you are painting the walls a different color than the ceiling, keep those measurements separate.

To Find the Surface Area:

Multiply the length of the wall by the height of the wall. Do this for all of the walls and/or ceiling you plan to paint in the same color and then add those measurements together. That is the total surface area. There are most likely some windows or doors in those walls that won’t be painted. Find the surface area of those features and total them up. Subtract the surface area of the doors and windows from the total surface area of the room.

A gallon of paint can cover approximately 400 square feet. Divide your newly calculated surface area by 400 and you will get the number of gallons of paint you will need for your project to cover one coat of paint. You should double this if you are going to do two coats.

Paint Calculation Worksheet

Example:

Say my room is 12 feet by 15 feet with walls that are 10 feet high. I also want to paint the ceiling in the same color. There is one door that is 3 feet by 8 feet and one window that is 6 feet by 4 feet.

First wall= 12×10= 120

Second wall= 12×10= 120

Third wall= 15×10=150

Fourth wall= 15×10=150

Ceiling= 12×15= 180

Add together= 120+120+150+150+180= 720 This is your total surface area in feet

Combine the window and door surface areas.

Door= 3×8= 24

Window= 6×4= 24

Add together= 24+24= 48

Now subtract this from the total surface area of 720.

720-48= 672

I want to do two coats so I will double that number.

672 x 2= 1,344 square feet.

Now I want to know how many gallons I will need so I divide 1,344 square feet by 400 square feet ( the approximate square footage one gallon of paint will cover).

1,344÷400= 3.36 gallons

Variables:

With this being calculated, I also need to mention there may be some variation to this number if you are painting over flat paint or unpainted drywall. These surfaces soak up more paint than surfaces previously painted with a higher sheen. So if this was the case in the above scenario, I would probably purchase 4 gallons of paint. That works out well anyway because paint is most often sold by the gallon as opposed to half gallons.

Also worthy to note is that the second coat of paint seems to not take as much paint as the first coat. So if my room calculations for two coats of paint called for slightly more than four gallons, I would probably risk it and just get the four gallons. I like to live dangerously at times. You can tell that I have a wild side by the fact that my dog wears bow ties. We’re crazy like that. Ha ha.

For Trim:

To calculate how much paint you need for trim, measure the length, in feet, of all of the trim in the room to be painted. This includes all baseboards, trim around doors and windows, and any crown moulding or chair rail that you may have. Multiply that total by .5 (this is the approximate width of the trim). Now divide that total by 400. To use our previous example, we’ll pretend that we will paint the baseboard and trim around the door and window.

12+15+12+15 (these are the baseboard measurements)+ 8+3+8 (this is the trim around the door)+ 6 (assuming it is just a window sill)= 79

Divide that in half, aka, multiply by .5 and you get 39.5

Divide that by 400 and you get 0.09875. So clearly a quart would be plenty of paint for the trim in one room. This would probably be more useful if you were figuring out the trim in an entire house, but you get the idea.

Paint Calculation Factors:

These are the factors to consider when determining how much paint to purchase.

Surface texture of wall

If you have a highly textured surface, it may require more paint to get into all of the nooks and crannies.

Unpainted surface

If you are painting a raw surface, you should prime first. With that said, painting a newly primed wall, or over flat paint will take more paint for the initial coat. The second coat usually takes less paint because the first coat basically sealed the porous surface.

Product coverage

Check the paint can label to identify the wall coverage for that product. This information is generally on the upper back part of the label and in bold type.

Paint technique

Different paint techniques will use more or less paint. For example, if you are glazing and thinning down the paint, you won’t need as much as if you were painting a solid base coat. In addition, rollers soak up quite a bit of paint compared to applying paint with a brush. I try to err on the side of purchasing a little too much paint because then I always have paint for future touch ups.

Paint color

The color of paint will have an effect on how much paint you need to use. I painted our media room with a dark redwood color and it took three coats of paint to get the tone even and rich. If painting a light color over a dark color, you will want either a paint with a primer in it or to prime it first to block the dark color from showing through the lighter paint. Just be aware that it may take more coats than you anticipated when figuring out your budget.

Still a little nervous about calculating your paint? I made a worksheet which you can get here to help you calculate how much paint is needed for your space. Paint Calculation Worksheet

You’ve got this. Enjoy ordering paint with confidence and painting up a storm in your house. I actually enjoy painting. I think it is because painting gives immediate results and is a relatively inexpensive way to transform a space. Painting a room(s) is an awesome start to making a house a home. 💙