Paint Calculator
Calculate the amount of paint needed based on wall area, number of coats, and paint coverage rate.
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Results
For a 15ft x 12ft room with 8ft ceilings, you need 3 gallon(s) of paint for 2 coat(s). This covers 381 sq ft of paintable wall area.
How to Use This Calculator
Enter the room dimensions: length, width, and wall height in feet. The calculator computes the total wall area by summing the perimeter multiplied by the height. Enter the number and size of doors and windows to subtract from the total surface area. A standard door is 21 square feet (3 by 7 feet) and a standard window is 15 square feet (3 by 5 feet). Select the surface type, such as drywall, textured wall, or bare wood, which affects coverage rate. The calculator divides the net paintable area by the coverage rate, typically 350 square feet per gallon for smooth drywall. For a 12 by 14 foot room with 8 foot ceilings, 2 doors, and 2 windows, the net wall area is approximately 344 square feet, requiring 1 gallon for one coat or 2 gallons for two coats.
Understanding the Concept
Paint estimation prevents wasted trips to the store and ensures color consistency by purchasing all paint from the same batch. A gallon of interior latex paint covers approximately 350 to 400 square feet on smooth, previously painted drywall. Coverage drops significantly on porous or textured surfaces: bare drywall absorbs more paint and may cover only 250 to 300 square feet per gallon on the first coat. Textured walls such as knockdown or orange peel have 15 to 25% more surface area than smooth walls, increasing paint consumption proportionally. Exterior surfaces vary even more, with rough stucco consuming up to twice as much paint as smooth siding. Professional painters typically apply two coats for color changes and touch ups. When switching from a dark color to a light one, a tinted primer coat reduces the number of finish coats needed. Always buy all paint at once from the same batch to avoid color variation between cans.
The Formula Explained
Total wall area equals the perimeter multiplied by the ceiling height: A = 2 multiplied by (L + W) multiplied by H, where L is room length, W is room width, and H is wall height, all in feet. Subtract openings: net area equals A minus (number of doors multiplied by 21) minus (number of windows multiplied by 15). Gallons required equals the net area divided by the coverage rate. Standard coverage rates are 350 to 400 square feet per gallon for smooth drywall, 300 to 350 for semi rough surfaces, and 250 to 300 for rough or porous surfaces. Multiply by the number of coats, typically 2 for a quality finish. For ceiling paint, the area is simply length multiplied by width. Trim paint is estimated at 1 quart per 100 linear feet of standard baseboard and casing.
Frequently Asked Questions
How much paint do I need for a 12 by 12 room?
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall area. After subtracting a door and a window, the net area is approximately 348 square feet. You need 1 gallon per coat, so 2 gallons for two coats of wall paint plus 1 quart for trim.
How many square feet does a gallon of paint cover?
One gallon of interior latex paint covers 350 to 400 square feet on smooth, primed drywall. Coverage is lower on textured, porous, or unpainted surfaces. Exterior paints vary from 250 to 400 square feet per gallon depending on the surface texture and porosity.
Do I need two coats of paint?
Two coats are recommended for most painting projects to ensure uniform color, full coverage, and durability. A single coat may suffice when repainting with the same or similar color over a well prepared surface. Always apply two coats when changing from dark to light colors.
How do I estimate paint for textured walls?
Textured surfaces such as knockdown, orange peel, or popcorn have 15 to 25% more surface area than smooth walls. Multiply the flat wall area by 1.15 to 1.25 before dividing by the coverage rate. Use a thicker nap roller (3/4 inch or more) to get paint into the texture.
Should I buy extra paint for touch ups?
Yes. Keep at least 1 quart of each color for future touch ups. Paint colors can be difficult to match exactly later because formulations and batches vary. Store leftover paint in a cool, dry location with the lid sealed tightly.