How to Calculate Paint Coverage for Any Room or Surface
Buying too little paint means a second trip to the store mid-project. Buying too much wastes money. This post explains how to calculate paint coverage accurately for walls, ceilings, and trim.
Understanding Paint Coverage Rates
Most interior paints cover between 350 and 400 square feet per gallon when applied to a smooth, properly primed surface. The exact coverage depends on the paint's formulation, the color being applied, and the surface texture. Darker colors often require two coats, and rough or porous surfaces absorb more paint than smooth ones.
Manufacturers print the estimated coverage on the paint can, expressed as square feet per gallon or square meters per liter. Use this number as your starting point, but apply a realistic adjustment for your specific conditions. For textured walls or ceilings, reduce the estimated coverage by 10 to 20 percent.
Two-coat applications require doubling the paint quantity you calculate for a single coat. Most professional painters apply two coats for new construction, color changes, and full-room repaints. Touch-ups and same-color refreshes can often be done in one coat, reducing material needs significantly.
Measuring Wall Area
To measure wall area, multiply the perimeter of the room by the wall height. The perimeter is the total length of all the walls. For a rectangular room that is 12 feet by 14 feet, the perimeter is 2 times (12 plus 14), which equals 52 feet. If the ceiling height is 8 feet, the total wall area is 52 times 8, which equals 416 square feet.
Subtract the area of doors and windows from the total wall area. A standard interior door is approximately 20 square feet (2.5 feet wide by 8 feet tall). A standard window is approximately 15 square feet (3 feet wide by 5 feet tall). If the room has two doors and two windows, subtract 2 times 20 plus 2 times 15, or 70 square feet, giving a net wall area of 346 square feet.
For rooms with angled walls, vaulted ceilings, or other irregular shapes, measure each wall section individually and add the areas together. Break complex walls into rectangular sections, triangles, or other simple shapes, calculate each, and sum the totals.
Measuring Ceiling and Floor Area
The ceiling area of a rectangular room is simply the room's length times its width. For a 12-by-14-foot room, the ceiling area is 168 square feet. If the ceiling has a different texture or color treatment, calculate its area separately from the walls.
Vaulted or cathedral ceilings require a different approach. For a symmetrical gabled ceiling, calculate the area of two rectangles: the horizontal span of one side multiplied by the slant length of that side. For a ceiling that rises from 8 feet at the edges to 12 feet at the center over a 14-foot span, the slant can be calculated using the Pythagorean theorem.
Floors are rarely painted, but stained concrete, epoxy-coated garage floors, and deck paint projects all require similar area calculations. Measure the floor's length and width, subtract any fixed obstacles like islands or built-in cabinetry, and use the result as your paintable area.
Calculating the Number of Gallons
Once you have the total paintable area, divide by the coverage rate from the paint can to find the number of gallons needed for one coat. For a wall area of 346 square feet and a coverage rate of 350 square feet per gallon, divide 346 by 350 to get approximately 0.99 gallons — round up to 1 gallon.
For two coats, double the quantity: 2 gallons for this example. Add 10 percent for waste, drips, and touch-ups: 2 times 1.10 equals 2.2 gallons. Since paint is sold in quarts (0.25 gallons) and gallons, you would buy 2 gallons and 1 quart to have a small buffer.
For very large rooms or commercial applications, buy in bulk where possible. Larger containers (5-gallon buckets) cost less per gallon than individual quarts and gallons. The savings are significant on projects requiring 10 gallons or more.
Accounting for Primer and Multiple Surfaces
Primer coverage rates are typically higher than paint, often 200 to 300 square feet per gallon, because primer is absorbed more readily by bare surfaces. Calculate primer quantity separately using the same area calculation but with the primer's listed coverage rate.
When painting multiple surfaces with different colors — walls, ceiling, trim, and doors — calculate each surface area separately and determine the paint quantity for each. This prevents overbuying one color and underbuying another, which wastes money and causes delays.
Keep track of the paint batch number from the can label. Paint from the same batch is guaranteed to match. If you need to buy additional paint mid-project, buying from a different batch may result in a subtle color variation, especially noticeable on large wall surfaces in natural light.