Paint Calculator

You’ve got a few days off and you’ve decided it’s time to paint your house’s interiors. A job like that requires precision and patience. Many people prefer to outsource the job, while others like to take some time out and get a little painting therapy into their day.

If you prefer to do it yourself, or if you’re a professional painter, you’ll agree that the best way to ensure a successful paint job is in the planning stage. You should first decide on what paint you want to use, what texture and how you want to go about applying it. Then you’ll need to carefully measure the dimensions of your project and quantify how much paint you’ll need to use.

Before actually painting your interior walls, you’ll have to do a little math to figure out how much paint is adequate for you. If you’re good at math, this will be a breeze. If you hate math, don’t worry! It’s a very easy calculation and we’ll show you how to do it.

To calculate how many gallons of paint you need, measure the following:

1.   Width

2.   Length

3.   Wall height

4.   Number of windows

5.   Number of doors

Taking measurements

You’ll need a stool and a measuring tape, a drawing book or blank sheet of paper, a pencil and a ruler. Feel free to ask friends or family to help you with this, since you’ll most likely be on the stool taking the measurements.

After you’ve noted down the measurements, make a drawing of all the walls you need to measure. This will help you avoid any mistakes in your calculation. Leave enough room in the drawing to include measurements. Include the doors and windows in your drawing.

  • Calculate the area of a single wall: Measure the width and height of each wall separately. To calculate the area of one wall, multiply these two. Record the measurements on your drawing.
  • Calculate the square footage of a single wall: By multiplying the length and width of a single wall, you will get its square footage.
  • Measure your doors, windows and fixtures separately. Note down the length and width of each using a measuring tape. By multiplying the length and width of individual doors and windows, you’ll be calculating the square footage of each. The same goes for any fixture on your wall. You’ll need these numbers because eventually, you’ll need to add up the total areas of the windows, doors or fixtures. (This only applies to walls that have more than one door, fixture, or window.)

Doing the calculations

1)  You’ll need to add together the length of all the walls.

Wall 1 + Wall 2 + Wall 3 + Wall 4 = A

2)  Then multiply this number (A) by the room’s height – from floor to ceiling. The resulting number is the room’s square footage.

Square footage of room/ total wall area (R) = A x height of room

3)  Now you’ll have to judge how much of the square foot is paintable surface. How do you do this? Calculate the combined areas of the doors and windows and subtract that number from the overall wall area. Those surfaces require a different paint from the one you’ll be using to cover the walls. The resulting number will be close to the actual wall area that needs to be covered with paint.

Let’s take 20 square feet for each door, and 15 square feet for each window. Let’s assume there are two doors and two windows.

Add the areas of the 2 doors and windows. That’s 20 x 2 = 40 and 15 x 2 = 30. 40 + 30 = 70.

Now subtract this (70 in our example) from the square footage of room/total wall area to find the actual amount of wall area you’re painting.

That’s R – 70 = Actual area to be painted (P) 

4)  But this doesn’t tell you how much paint you’ll need, does it? Here’s how you can figure that out. Finally, you’ll have to divide the paintable wall area by 350. Why 350? It’s the square-foot coverage in each gallon can.

So finally, it’s P – 350 = Gallons of paint you’ll need.

So finally, you’ll get the number of gallons of paint you’ll need for the walls. You can get a few quarts above that number just to be safe.

Things to keep in mind

Q) Which material are you going to use?

In general, you can expect a gallon of paint to cover 350 – 400 square feet. If the walls are made out of unpainted drywall, you’ll need more, since that material absorbs more paint.

Q) How many coats will you apply?

Keep in mind that you’ll have to apply more than one coat if the walls are patchy or if you’re using a dark color.

Q) What color do you plan on applying?

Dark colors usually take more than one coat to come out consistently. You can add a color tint to the white primer. Choose a tint in a color that is similar to the top coat. Light colors can be done with less paint.

General guideline

According to experts, the following is a general guideline of how much paint you’ll need according to the size of your project:

One gallon – A gallon of paint can typically cover 350 – 400 square feet – this is enough to cover a small room like a bathroom.

Two gallon – This amount can usually cover 800 square feet. You’ll be able to paint an average sized room. This much should also be enough to go for a second coat of paint.

Three gallon – You can cover 1200 square feet of area with this much paint. You should be able to cover a large room and a couple of average sized adjoining rooms. This is a good option if you’re painting a room with vaulted ceilings.

PRO TIP – If you want to make this process a lot easier, why don’t you check out this paint calculator? Just feed in your measurements and boom – you’ll get an estimate of how much paint you need.

Follow these steps and you’ll never be out of paint in the middle of refurbishing or glowing up your interior walls. Remember to get the right paint sprayer to spruce up your interiors. Don’t know which one to get? Then check out our comprehensive guide on which paint sprayer is the best for interior walls.