Before you pick up a brush, it’s important to know exactly how much paint your room will require. Running out of a custom color halfway through a wall can derail the entire project. This simple paint-calculation guide walks you through measuring your space, estimating coverage, and determining how many gallons—or quarts—you’ll need for the walls, ceiling, doors, and trim.
Estimate your room size and paint needs before you go to the paint store. Running out of a custom color halfway through the job could mean disaster!
How to Calculate Paint for Walls
Follow the instructions below to calculate how much paint you’ll need. To help, we’ve added an example: A room that is 10 x 15 feet with an 8-foot ceiling. The room has two doors and two windows.
- Measure the total distance (perimeter) around the room. (10 ft. + 15 ft.) x 2 = 50 ft.
- Multiply the perimeter by the ceiling height to get the total wall area: 50 ft. x 8 ft. = 400 sq. ft.
- Doors are usually 21 square feet (there are two in this example): 21 sq. ft. x 2 = 42 sq. ft.
- Windows average 15 square feet (there are two in this example): 15 sq. ft. x 2 = 30 sq. ft.
- Take the total wall area and subtract the area for the doors and windows to get the wall surface to be painted: 400 sq. ft. (wall area) – 42 sq. ft. (doors) – 30 sq. ft. (windows) = 328 sq. ft. of walls that need to be painted.
As a rule of thumb, one gallon of quality paint will usually cover 400 square feet. One quart will cover 100 square feet. Because you need to cover 328 square feet in this example, one gallon will be adequate to give one coat of paint to the walls. (Coverage will be affected by the porosity and texture of the surface. In addition, bright colors may require a minimum of two coats.)
How to Calculate Paint for Ceilings
Using the rule of thumb for coverage above, you can calculate the quantity of paint needed for the ceiling by multiplying the width of the room by its length: 10 ft. x 15 ft. = 150 sq. ft. This ceiling will require approximately two quarts of paint. (A flat finish is recommended to minimize surface imperfections.)
How to Calculate Paint for Trim
The area for the doors and windows has been calculated above. (The windowpane area that does not get painted should allow for enough paint for any trim around doors and windows.) Determine the baseboard trim by taking the perimeter of the room, minus 3 feet per door (3 ft. x 2 = 6 ft.), and multiplying this by the average trim width of your baseboard, which in this example is 6 inches (or 0.5 feet). 50 ft. (perimeter) – 6 ft. = 44 ft. 44 ft. x 0.5 ft. = 22 sq. ft.
- Add the area for doors, windows, and baseboard trim. 42 sq. ft. (doors) + 30 sq. ft. (windows) + 22 sq. ft. (baseboard trim) = 94 sq. ft.
- One quart will be sufficient to cover the doors, windows, and trim in this example.
Tip: Before you paint, ventilate. Keep a window or outside door open in the room you are painting to avoid breathing noxious fumes.
General Paint Coverage Chart
| Surface | Coverage | Notes |
|---|
| Walls | 350–400 sq ft per gallon | Depends on texture & color |
| Ceiling | 350–400 sq ft per gallon | Flat finish recommended |
| Trim & Doors | 100 sq ft per quart | Usually two coats |
Frequently Asked Questions
How do I figure out how much paint I need for a room?
Measure the room’s perimeter, multiply by the ceiling height to get total wall area, then subtract the square footage of doors and windows. Divide the final number by the paint’s coverage rate (usually 400 sq. ft. per gallon).
Do I need more paint for two coats?
Yes. If your project requires two coats, simply double the amount of paint needed. Bold or bright colors and big color changes often require at least two coats.
Should I buy extra paint?
It’s smart to buy a little extra—especially with custom colors—to ensure consistent color and for future touch-ups. Many pros recommend buying 10% more than calculated.
Does primer affect how much paint I need?
Yes. Priming helps paint go on smoother and improves coverage, often reducing the total amount of paint needed—especially on new drywall or when covering dark colors.
How do I avoid running out of paint halfway?
Measure carefully, calculate based on coverage, and purchase slightly more than you need. Using the same brand and batch number also helps maintain color consistency.
What affects paint coverage?
Surface texture, wall porosity, paint quality, and color all impact coverage. Rough or unprimed surfaces will absorb more paint than smooth, sealed ones.
Now see how to choose and use a paintbrush, and you’ll be in business!
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