How to Estimate Interior Paint Requirements

Figuring out how much paint you’ll need for a job is straightforward. Spend a few minutes measuring the area to be painted at home. Typically, one gallon of paint covers about 450 square feet based on the manufacturer's guidance. To be safe, it's recommended to calculate around 400 square feet per gallon. If you’re purchasing two or more gallons of the same shade, it's best to combine them at home to prevent noticeable color differences on the wall.

To estimate the amount of paint for a wall, simply multiply the height by the length, and then divide by 400. This means a gallon will cover a 10 X 15-foot room (two 10-foot walls and two 15-foot walls, all 8 feet high) with one coat. For two coats, you'll need 2 gallons.

There are, however, additional factors to keep in mind when calculating coverage.

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Estimating Paint Coverage

Textured Surfaces

Textured or rough-troweled walls require more paint than smooth ones. This is because the added texture increases the surface area to be covered, even though it doesn’t change the overall size. The amount of extra surface area depends on how rough the texture is, but for medium-textured, porous, or previously unpainted walls, you can estimate that one gallon will cover about 300 to 350 square feet.

Windows and Doors

Many walls feature doors, windows, or other areas that won't be painted. If the unpainted area is just a single window or door, you can disregard it in your calculations. However, multiple windows, a door and window, sliding doors, or a fireplace will reduce the amount of paint you need. To calculate the reduction, multiply the length by the width of these unpainted sections to find the total square footage to subtract from your overall estimate. Alternatively, you can subtract approximately 15 square feet for each typical window and 21 square feet for each door. (These numbers also help if you plan to paint these areas in a different color or finish.) If you're painting the ceiling, calculate its area by multiplying its width by length as well.

Time Estimation

Estimating how long a painting project will take is a bit trickier. Since some people work faster than others, it’s impossible to predict individual speed differences. On average, you can cover around 120 square feet of a flat surface in an hour. For bare wood or plaster, estimate about 100 square feet per hour. In a typical 12 X 15-foot room, the first coat should take about four to five hours, including trim work. The second coat, if needed, will be quicker, but you’ll need to allow time for the first coat to dry, which can take anywhere from 2 to 36 hours.