Paint Calculator - How Much Paint Do I Need?

This Page Was Last Updated: August 29, 2022
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You need 1.11 gallons of paint and 1.78 gallons of primer to cover this room’s surface area of 445 square feet.

The estimated time is 2.37 hours and the estimated cost is $100.00.

About the Paint Calculator

This paint calculator estimates how much paint and primer you will need for your painting project based on the size of the room being painted and the number of doors and windows in the room. For a more accurate paint estimate, you can break down the amount of paint required for walls, ceilings, moulding, trims, and doors.

This paint calculator estimates both the amount of paint required and the amount of primer required. You can customize the paint coverage of both the paint and the primer, as well as select a custom number of coats. You will also get an estimated total cost and an estimated time it will take for your painting project, as well as the number of cans of paint you will need.

How Many Square Feet Does a Gallon of Paint Cover

The paint estimate that this paint calculator provides is based on one gallon of paint covering 400 square feet and one gallon of primer covering 250 square feet. This paint calculator provides an estimate based on the following assumptions:

  • One gallon of paint covers 400 square feet
  • One gallon of primer covers 250 square feet

You can customize the paint coverage used in the estimator to match your own painting project. That’s because different painting surfaces may require a different amount of paint to cover. Using a paint primer can also reduce the number of coats of paint that you’ll need, which means that you may need to buy less paint if you apply primer. Primer is also cheaper than paint.

Factors Affecting Paint Coverage

The three main factors that affect the paint coverage per gallon for your project are the wall type, the wall texture, and the color change.

Paint Roller

Wall Type

Fresh drywall is porous and may require multiple coats of primer to soak in before the surface is suitable for painting. Drywall primer is specifically used to seal new drywall and can also be much cheaper than regular interior paint primer. Stucco and brick walls will also require more paint.

Wall Texture

Flat and smooth surfaces require less paint compared to walls with a rough and coarse texture. While a gallon of paint can typically cover 400 square feet, textured walls may only have a paint coverage of 300 square feet or less per gallon of paint.

Color Change

Significant color changes can require more paint or primer, especially if you are changing from a dark wall color to a lighter wall color.

How to Calculate Paint Needed

Paint Calculation for Walls

To estimate the amount of paint that you will need for a room, you will first need to calculate the surface area that will be painted. To do this, measure the width of each wall in the room. You'll also need to measure the height of the walls, from the floor to the ceiling. In the case of sloped ceilings, the wall heights might not be the same.

Once you have measured the dimensions of each wall, you will need to find the total square footage of your walls. If your walls are rectangles, you can simply multiply the width by the height of each individual wall, and then add them all up to get the total square footage. Triangular-shaped or irregular-shaped walls can be more difficult to calculate. A square footage calculator can be used to find the surface area of odd-shaped walls.

For example, let’s say that a room has 4 walls, two of which have a width of 20 feet and two of which have a width of 15 feet. The ceiling height of the room is 10 feet. To calculate the square footage:

Painting a Wall

Now let’s assume that the paint coverage is 400 square feet per gallon of paint.

In this example, this room will require 1.75 gallons of paint.

A room will most likely have a doorway or opening to the room that will not be painted. Windows will also not be painted either. The paint calculator on this page assumes that a door is 20 square feet and that a window is 15 square feet. In the advanced calculation, you can input the exact size of the windows and doors that will not be painted. You can also select if the doors will be painted.

If the doors and windows will not be painted, you will need to subtract the surface area of these non-paintable surfaces from the total wall surface area. For example, if the room has one door of 20 square feet and three windows of 15 square feet each, the calculation would be:

By excluding the doorway and windows, you will now only need 1.5875 gallons of paint.

Paint Calculation for Moulding & Trims

Wall moulding and trims, as well as baseboards, reduce the total surface area of the walls that need to be painted. Instead, the wall moulding, trims, and baseboards will be painted, while the wall that is being covered would not need to be painted.

Moulding, trims, and baseboards are usually measured in inches. To determine how much paint is needed to paint moulding, trims, and baseboards, you will need to know the height as well as the length. The length will be based on the width of the walls.

For example, let’s say that the room has a 3-inch tall baseboard that runs along all four walls from the previous example. To calculate, you will first need to use the same units of measurement. This means converting inches into feet. Using a unit converter, 3 inches is 0.25 feet.

Painting a Wall

Now let’s assume that the paint coverage is 400 square feet per gallon of paint.

On its own, the baseboards will only require 0.04375 gallons of paint. That’s the same as 6.72 ounces of paint. Since the baseboards cover part of the wall, this means that the wall will require 0.04375 gallons less of paint. However, the total paint required for the room remains unchanged.

Paint Calculation for Ceilings

The paint calculation for the ceiling is similar to the calculation for walls. For a rectangular room, you can multiply the ceiling length by the ceiling width to get the ceiling’s surface area. Then, divide by the paint coverage to get the amount of paint required. If your room is not rectangular, you can easily calculate the surface area by using a square footage calculator.

Calculating Time to Paint

Painting Can

Priming is faster than painting. It takes about one hour to paint 300 square feet, while it takes one hour to prime 500 square feet. The more coats of paint, the longer it will take. This doesn’t include time to prep the room or to wait for coats of paint or primer to dry.

For example, if the total wall surface area that you are painting is 1,500 square feet, then it will take three hours to apply one coat of primer and five hours to paint one coat of paint.

The time it takes to paint can also depend on the surface being painted. It can take longer to paint trims and ceilings compared to flat walls.

Any calculators or content on this page is provided for general information purposes only. Casaplorer does not guarantee the accuracy of information shown and is not responsible for any consequences of its use.