Paint Calculator: How Much Paint You Need?

Starting an interior painting project? One of the first questions you’ll probably ask is, how much paint do I actually need?

That’s exactly what a paint calculator is for. It takes the guesswork out of the process and gives you a solid starting point so you don’t overbuy or run out mid-project.

If you’ve ever stood in the paint aisle unsure whether to grab another gallon or not, this tool is for you. Just a few basic room measurements, and you’ll get an instant paint estimate.

This blog explains how the paint calculator works, what information you’ll need, and how it adjusts for things like windows, doors, and coats. 

Let’s break it down so you can get to the fun part—choosing your color and painting.

Key Takeaways:

• A paint calculator helps estimate how many gallons or quarts you need per room.
• Subtracting windows and doors gives a more accurate square footage.
• Two coats are usually best for full coverage and better results.
• Ceiling paint needs to be measured separately for best planning.
• The calculator saves you from overbuying or making last-minute store runs.

How the Paint Calculator Works

The paint calculator uses a basic formula based on square footage to help you estimate how many gallons of interior paint you’ll need. The idea is simple: measure the size of your room, subtract any spaces you won’t paint (like windows and doors), and then decide how many coats you’re applying.

For reference, one gallon of paint typically covers around 350 square feet. That’s the industry average, though the final coat coverage can vary depending on things like the type of surface, the primer, and whether you’re painting over dark colors or fresh drywall.

You’ll need to factor in things like:

• Wall length and height
• Number and size of windows and doors
• Whether you’re painting the ceiling
• How many coats you plan to apply
• Whether the surface needs primer

It’s a straightforward tool once you know how to use it.

How to Use the Paint Calculator

Here’s how to get started. Just follow these simple steps and plug in your room measurements:

1. Choose your room shape (rectangle, square, or custom).
2. Enter each wall’s length and height in feet or inches.
3. Decide if you’re painting the ceiling and add the ceiling dimensions.
4. Add the number of windows and doors in the space.
5. Select how many paint coats you’ll apply.
6. Click calculate, and your paint quantity appears in gallons and quarts.

Once you have that number, you’re ready to plan your paint purchase.

Room Shape and Wall Dimensions

This is where it all starts. Every wall you want to paint needs to be measured for its height and length. Multiply those together to get the square footage of each wall. Then add them up.

For example, in a rectangular kitchen, you’d likely have four walls—two long, two short. A typical room might measure 12 feet by 10 feet, with an 8-foot ceiling. Multiply and add each to get your total wall area.

If your room isn’t a perfect shape, don’t worry. Use the “custom” setting in the calculator to enter each wall’s measurement manually.

Subtracting Windows and Doors

You won’t be painting over windows and doors, so those areas should be removed from your total square footage. The calculator does that for you.

Generally, a standard window is about 15 square feet, and a door is roughly 20 square feet. Just input the number of each, and the calculator will deduct the right amount.

This matters more than you think. If your room has large windows or double doors, skipping this step can throw off your paint estimate and lead to buying more than you need.

Ceiling Paint

Painting the ceiling? You’ll need to calculate it separately. Just multiply the length by the width of the ceiling to get your total square feet.

Ceilings usually require the same amount of paint coverage per gallon, but because most people apply only one coat, the total amount needed might be less. However, if you’re covering stain, old wood stain, or water damage, or switching from a dark color, you might need two coats or a separate primer.

You can learn more about preparing a surface in our blog, which discusses the number of coats of primer you need.

Number of Coats

The default setting in the paint calculator is two coats, and for most projects, that’s the right move.

One coat might work if you’re just freshening up a wall with the same color. But when covering old or bold interior paint, applying over brick, concrete, or wood, or making a major color change, two coats offer better adhesion and coverage.

Planning to use a paint sprayer? Keep in mind that sprayers can use up to 30% more paint than a brush or roller, especially for exterior painting.

For tips on what tools to use, you can check out different paint sheens for insight on finish and application.

Example Room: How It All Adds Up

Let’s say you’re painting a 12×12 bedroom with 8-foot ceilings, two windows, and one door. You’re using two coats and painting the ceiling as well.

• Wall area: (12+12+12+12) x 8 = 384 square feet
• Ceiling: 12 x 12 = 144 square feet
• Windows: 2 x 15 = 30 square feet
• Door: 1 x 20 = 20 square feet

Total to paint: 384 + 144 – 50 = 478 square feet

Two coats? That’s 956 total square feet.

At 350 square feet per gallon, you’ll need just under 3 gallons. Always round up—better to have a little extra than not enough, especially if you need to touch up later.

Final Thoughts on Using the Paint Calculator

Using a paint calculator takes the guesswork out of estimating your next home improvement project. It saves time, money, and trips to the hardware store. Instead of guessing how many gallons you’ll need for your room, you can focus on what really matters—choosing the right color, getting the right primer, and enjoying the process.

Whether you’re tackling your kitchen, bathroom, or an entire interior repaint, the paint calculator helps you make informed choices from the start. And if your project includes surfaces like wood, epoxy, or concrete, having an accurate square footage count means less waste and fewer delays.

Our Painting Process: What You Can Expect

If you’re ready to move forward, here’s how FR Contractors and Investors, Inc. can help:

• We offer free estimates based on accurate square footage measurements
• Every project starts with clear communication and expert prep
• Our painters are certified and trained for both interior and exterior painting
• We use high-quality paints from trusted brands
• We protect your floor, windows, and furniture every step of the way

Planning a painting project in Parkton, Cockeysville, Timonium, MD, or the surrounding areas?

Call us at 443-775-2994 for a FREE estimate and let’s make your space look and feel brand new!