Free Guide to Calculating Paint Coverage and Amount

Understanding Paint Coverage Basics

Paint coverage refers to how much surface area a gallon of paint can cover when applied at the proper thickness. This measurement is typically expressed in square feet per gallon. Most standard interior latex paints cover between 350 and 400 square feet per gallon when applied in a single coat at the recommended thickness of 3 to 4 mils (a mil equals one one-thousandth of an inch). Understanding this baseline helps you calculate how much paint you'll need for your specific project.

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The coverage rate on paint cans reflects ideal conditions: smooth, non-porous surfaces that have been properly prepared. Real-world projects rarely meet all these ideal conditions, which is why many professionals use 300 to 350 square feet per gallon as a more realistic estimate. Factors that reduce coverage include textured surfaces, porous materials like drywall that hasn't been primed, and multiple color changes that require additional coats.

Manufacturers conduct coverage tests using standardized methods. They typically apply paint to a smooth test surface and measure how far the paint spreads. The numbers on the can label come from these controlled tests, making them useful as reference points even though your actual coverage may differ. A gallon of paint covers approximately 350 square feet at standard thickness, but textured surfaces might only cover 250 to 300 square feet, while ultra-smooth surfaces could stretch coverage to 400 to 450 square feet.

When reading paint labels, look for the coverage information in the technical specifications section. Paint manufacturers must list this information, though it may be located differently on various cans. Some list coverage per quart as well as per gallon, which is useful when calculating smaller projects. Remember that coverage rates apply specifically to the paint type—primer, exterior paint, and specialty finishes all have different coverage rates.

Practical Takeaway: Check your paint can's label for the stated coverage rate in square feet per gallon. Use 300 to 350 square feet per gallon as a conservative estimate for planning purposes, accounting for the difference between laboratory conditions and actual application.

Measuring Your Painting Space Accurately

Calculating the area you need to paint is the foundation of determining paint quantity. For interior walls, measure the length and height of each wall, then multiply these dimensions to find square footage. For example, a wall that measures 12 feet wide and 10 feet tall equals 120 square feet. If your room has four walls measuring 12 feet by 10 feet, the total wall area is 480 square feet. However, you'll need to subtract areas that won't be painted, such as windows, doors, and any permanent fixtures.

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A standard interior door measures approximately 20 square feet, while average windows range from 10 to 15 square feet each. Most rooms have closet doors that are similar in size to regular doors. When calculating, measure or estimate these areas and subtract them from your total. For a room with four walls totaling 480 square feet, two regular doors (40 square feet), and two average windows (30 square feet), your actual paintable area would be approximately 410 square feet.

For ceiling projects, measure the length and width of the room and multiply them together. A 12-foot by 14-foot room has a ceiling area of 168 square feet. Ceiling paint typically has lower coverage rates than wall paint—often 250 to 300 square feet per gallon—because textured ceilings and the application method itself reduce efficiency. If your ceiling is textured (popcorn or acoustic texture), expect even lower coverage rates.

Exterior painting requires measuring different components. For a house exterior, calculate wall area by measuring the height from the foundation to the roofline and multiplying by the width of each wall side. Add the triangular gable areas separately by multiplying the height of the triangle by its width and dividing by two. Include measurements for soffits (the underside of roof overhang), which are typically 1 to 2 feet wide and wrap around the house perimeter. A one-story house with 1,500 square feet of floor space typically has 1,200 to 1,500 square feet of exterior wall area to paint.

Practical Takeaway: Measure each wall's length and height, multiply to find square footage, then subtract doors and windows. Create a simple chart listing each surface and its dimensions, then add all areas together to get your total paintable square footage.

Accounting for Surface Type and Condition

Different surfaces absorb paint differently, which significantly affects how much paint you'll need. Drywall and plaster are porous materials that absorb more paint than glossy or sealed surfaces. New drywall that hasn't been primed will absorb paint like a sponge, potentially reducing your coverage rate by 25 to 50 percent. This is why primer is essential on new drywall—it seals the porous surface and allows the topcoat paint to spread more efficiently. Using primer dramatically improves paint coverage and often reduces the total amount of paint needed despite requiring an extra step.

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Textured surfaces—including textured drywall, popcorn ceilings, brick, stucco, and rough wood—require significantly more paint than smooth surfaces. Textured walls might only receive 250 to 300 square feet of coverage per gallon instead of 350 to 400. The texture creates peaks and valleys that the paint must fill, increasing consumption. Heavily textured surfaces like brick or stucco can reduce coverage to as low as 150 to 200 square feet per gallon because so much paint fills the irregular surface.

Glossy or semi-gloss painted surfaces that are in good condition provide excellent coverage rates because the paint doesn't soak in. These surfaces can sometimes achieve 400 to 450 square feet per gallon coverage. However, glossy surfaces require proper preparation—they must be cleaned and lightly sanded so the new paint adheres properly. If you're painting over high-gloss surfaces without proper preparation, the paint may not adhere well regardless of coverage calculations.

Exterior surfaces have their own considerations. Wood siding that's weathered or unfinished absorbs more paint than painted wood. Raw wood might require a primer coat in addition to finish coats, increasing total paint volume needed. Fiber cement board and composite materials fall somewhere between wood and masonry in terms of porosity. Metal surfaces like aluminum gutters or steel doors create a different challenge—they reflect paint and have minimal absorption, so coverage rates are typically high, but adhesion requires specific metal primers.

Practical Takeaway: Reduce your estimated coverage rate by 25 to 50 percent for porous surfaces, new drywall, or heavy texture. Use primer on new surfaces or when making dramatic color changes, as primer improves the final paint coverage and appearance.

Calculating Paint Needs for Different Numbers of Coats

Most interior painting projects require two coats of paint to achieve adequate color coverage and durability. The first coat covers the existing surface, while the second coat provides uniform color and builds the protective finish. However, the number of coats needed depends on several factors: the existing color, the new color you're applying, surface condition, and the type of paint used. When calculating total paint volume, multiply your base coverage calculation by the number of coats required.

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If you're painting a 400-square-foot room and your paint covers 350 square feet per gallon with two coats needed, your calculation is: 400 square feet ÷ 350 square feet per gallon × 2 coats = 2.3 gallons. In this case, you would purchase three gallons of paint to ensure adequate coverage. Rounding up is always safer than rounding down, as you might need additional paint for touch-ups or if coverage is lower than expected.

Color changes affect coat requirements. Painting a dark wall a light color typically requires three coats minimum. The first coat may show the old color showing through, and the second coat might still have visible undertones of the previous color. A third coat ensures solid, uniform coverage. When making this type of dramatic change, add 50 percent more paint to your calculation than you would for a simple color refresh. For example, if a room normally needs 2 gallons for two coats, plan for 3 gallons for three coats when covering dark colors with light paint.

Primer is often considered a separate "coat" in coverage calculations. When you apply primer, that counts