Paint Calculator
Gallons of paint needed for a wall area and number of coats.
Paint needed
2.29 gallons
2 coat(s)
Round up to buy
3 gal
Quarts
9.1
- Coat 150%
- Coat 250%
Each of the 2 coat(s) needs about 1.14 gallons to cover 400 ft² — 2.29 gallons in total.
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How the Paint Calculator works
The Paint Calculator estimates exactly how many gallons of paint you need based on your wall area and the number of coats you plan to apply, eliminating the guesswork that leads to costly over-purchasing or mid-project shortages. It's ideal for homeowners, contractors, and DIY painters tackling interior or exterior projects of any scale.
At its core, the calculator divides your total paintable surface area by the coverage rate of the paint you're using, then multiplies that result by the number of coats required. Most standard interior and exterior paints sold in the US cover approximately 350 to 400 square feet per gallon under ideal conditions—thin, even coats applied to a smooth, primed surface. The formula therefore gives you a raw gallon estimate, which is then rounded up to the nearest practical purchase unit (typically a quart or gallon) so you never run short mid-wall.
Wall area calculation is where many painters make their first mistake. To find paintable square footage, multiply the width of each wall by its height and sum all walls in the room. Critically, you should subtract the area of windows, doors, and other unpainted surfaces—a standard interior door is about 20 sq ft and a typical window around 15 sq ft. Skipping these subtractions on a room with four windows and two doors can inflate your estimate by nearly a full gallon, wasting money on paint that sits unopened in your garage.
The number of coats dramatically affects your total paint requirement and is more nuanced than simply doubling a single-coat figure. A first coat on raw drywall or a dramatically color-changed surface acts as a sealer and primer, absorbing far more paint than a second topcoat. Professional painters generally plan for two coats as a baseline, but switching from a deep color to white—or vice versa—may require three coats for a uniform, streak-free finish. If you use a separate primer beforehand, your finish coats will go on more evenly and you may genuinely achieve full coverage in two coats rather than three, often saving money overall.
Coverage rate, the single biggest variable in paint estimation, depends on paint quality, surface texture, and application method. Cheaper paints with lower pigment loads may only cover 300 sq ft per gallon, while premium paints with higher solids content can hit 400 sq ft or more. Textured walls such as orange peel or knockdown reduce effective coverage by 15–25% because the paint must fill surface cavities. Roller nap thickness also matters—a thicker nap (¾ inch) deposits more paint to reach texture peaks and valleys, effectively raising consumption. Always use the coverage figure printed on your specific paint can rather than a generic industry average for the most accurate estimate.
Formula
Gallons Needed = (Total Wall Area in sq ft ÷ Coverage Rate in sq ft/gallon) × Number of Coats
Pro tips
- Always add a 10% overage buffer to your calculated gallon total before purchasing—this accounts for touch-ups, roller waste, and slight measurement errors, and it ensures you have matching paint from the same dye lot for future repairs.
- Use the coverage rate printed on the specific paint can you plan to buy, not the industry default of 400 sq ft/gallon; premium paints and budget paints can differ by 100 sq ft/gallon, which adds up to nearly a full extra gallon on a standard bedroom.
- If you're changing from a dark wall color to a light one, select a tinted primer matched to your new paint color—this single step often reduces the required finish coats from three to two, saving both time and paint cost.
- Measure each wall individually and subtract openings accurately; for a room with multiple windows or an archway, even rough subtractions can reduce your estimate by a quart or more and prevent unnecessary waste.
- When buying paint for a large project, purchase all gallons at once and mix them together in a larger container (a process called 'boxing')—this eliminates subtle color variations between cans and produces a perfectly consistent finish across all walls.
Key terms
- Coverage Rate
- — The number of square feet one gallon of paint can coat in a single, even layer, typically printed on the paint can label and usually ranging from 300 to 400 sq ft/gallon.
- Coats
- — The number of individual paint layers applied to a surface; most projects require at least two coats for durable, uniform color.
- Paintable Surface Area
- — The net wall area that will actually receive paint, calculated as total wall area minus the square footage of windows, doors, and other unpainted features.
- Primer
- — A preparatory coating applied before paint to seal porous surfaces, improve adhesion, and reduce the number of finish coats needed for full coverage.
- Spread Rate
- — A practical synonym for coverage rate that accounts for real-world application losses due to roller waste, brush drag, and surface absorption, often 10–15% lower than the theoretical label figure.
- Wet Mil Thickness
- — The thickness of a single applied coat of paint in thousandths of an inch, which directly influences how many square feet a gallon covers and how well the dried film hides the substrate.
