Paint Calculator

How This Is Calculated

Wall area is calculated as the room's perimeter (2 × (length + width)) multiplied by wall height, giving the total surface area of all four walls. Standard door and window sizes (about 20 sq ft per door, 15 sq ft per window) are subtracted to get the actual paintable area. That area is then multiplied by your number of coats and divided by your paint's coverage rate to estimate gallons needed. Adjust any field and every downstream figure, from paintable area to gallons and liters, recalculates instantly.

Why Subtracting Openings Matters

Doors and windows don't get painted (or are painted with a different product, like trim paint or a different wall color for the door itself), so leaving their area in the total wall calculation would overstate how much wall paint is actually needed. For a typical room, subtracting doors and windows can reduce the paintable area by 10 to 15 percent compared to the raw wall area, a meaningful difference that would lead to buying noticeably more paint than necessary if openings weren't accounted for. This effect becomes even more pronounced in rooms with multiple large windows or sliding doors, where openings can account for a substantially larger share of the total wall footprint.

Why Coverage Rate Varies

The commonly cited 350-400 sq ft per gallon assumes a smooth, previously painted surface. Textured walls, unprimed drywall, or darker color changes typically reduce actual coverage, check your specific paint can's label for its stated coverage rate for the most accurate estimate.

Worked Example

For a room 12 ft long and 10 ft wide with 9 ft walls, the perimeter is 2 × (12 + 10) = 44 ft, and total wall area is 44 × 9 = 396 sq ft. With 1 door (20 sq ft) and 2 windows (15 sq ft each, 30 sq ft total), the openings subtract 20 + 30 = 50 sq ft, leaving a paintable area of 396 − 50 = 346 sq ft. Applying 2 coats at a coverage rate of 350 sq ft per gallon gives (346 × 2) ÷ 350 = 692 ÷ 350 ≈ 1.98 gallons. Converting to liters, 1.98 × 3.78541 ≈ 7.48 liters. Since paint is sold in whole can sizes, this project would realistically require purchasing 2 gallons (or the equivalent liter cans) to cover the calculated 1.98-gallon need.

Painting Multiple Rooms

For projects spanning several rooms, running this calculator once per room and summing the resulting gallon figures is more accurate than trying to estimate a whole-house total from an average room size, since ceiling height, door and window counts, and room dimensions typically vary between rooms. Keeping track of each room's individual paintable area is also useful for buying the right size cans per room, since a small bathroom might only need a quart while a large living room needs several gallons, and purchasing paint per room rather than in one bulk estimate avoids ending up with an awkward mix of partially used cans.

Why Wall Area Uses Perimeter Instead of Individual Wall Measurements

Rather than requiring four separate wall measurements, this calculator uses the room's overall perimeter, 2 × (length + width), multiplied by wall height, a shortcut that works because opposite walls in a rectangular room share the same length. The perimeter formula effectively adds up all four walls at once: two walls matching the room's length and two matching the room's width, doubled to account for both pairs. This is mathematically identical to measuring and summing each of the four walls individually, but requires only two room dimensions rather than four separate wall widths, making data entry faster for the most common rectangular room shape.

Estimating Paint for Non-Standard Situations

Rooms with vaulted ceilings, extensive trim work, multiple color zones, or significantly larger-than-standard windows and doors will need manual adjustment beyond this calculator's standard assumptions. For a vaulted ceiling, using an average wall height between the shortest and tallest points gives a reasonable approximation, though a more precise calculation would split the wall into a rectangular lower section and a triangular upper section calculated separately. For a room with an unusually large picture window or sliding glass door, subtracting that specific opening's actual measured area, rather than the standard 15 sq ft window assumption, produces a more accurate paintable area and avoids over-purchasing paint for a wall with less surface to actually cover.

Primer, Base Coat, and Finish Coat Considerations

The "number of coats" field is flexible enough to represent different painting scenarios beyond a simple two-coat topcoat application. Painting over a drastically different existing color, going from a dark wall to white, or vice versa, often needs a dedicated primer coat plus two finish coats, effectively three total coats, to achieve full, even coverage without the old color showing through. Painting a similar color over an existing coat of the same family might only need a single coat in good lighting conditions. New, unpainted drywall almost always needs a primer coat before the finish coats, since drywall and joint compound absorb paint unevenly without it, leading to a blotchy finish if primer is skipped. Adjusting the coats field to reflect the actual number of full passes planned, including any dedicated primer coat, keeps the gallon estimate aligned with the real project scope.

Common Mistakes When Estimating Paint Needs

A frequent mistake is using a generic coverage rate without checking the specific paint can's label, since coverage can vary meaningfully between paint brands, finishes (flat, eggshell, satin, semi-gloss), and formulations, sometimes by 50 sq ft per gallon or more from the commonly cited 350-400 range. Another common mistake is forgetting to account for trim, baseboards, and crown molding as separate painted surfaces requiring their own smaller quantity of trim paint, typically a different product from wall paint entirely. A third mistake is underestimating how much a color change affects real coverage, going from a light wall color to a significantly darker one (or vice versa) frequently requires an extra coat beyond what a same-family color change would need, which this calculator's coats field should be adjusted to reflect.

Frequently Asked Questions

Should I round up my paint purchase?

Yes, paint is usually sold in whole gallon or liter cans, and running short mid-job to find a color match can be frustrating. Round up to the next full can size.

Does this include the ceiling?

No, this calculates wall area only. If you're painting the ceiling too, calculate that area separately (room length × width) and add the paint needed for that as its own coverage calculation.

Why does the calculator use standard sizes for doors and windows instead of asking for their exact dimensions?

Standard interior door openings and typical residential windows fall within a fairly narrow, predictable size range, so using an average approximate size (about 20 sq ft per door, 15 sq ft per window) gives a reasonably accurate estimate without requiring you to measure every opening individually. For unusually large windows, sliding glass doors, or oversized openings, the actual paintable area will be somewhat smaller than this calculator estimates, since those openings remove more wall area than the standard assumption accounts for.

Why does paint coverage decrease on textured or unprimed surfaces?

Coverage rates like 350-400 sq ft per gallon are based on a smooth, previously painted, non-porous surface where paint spreads evenly across the surface area. Textured surfaces have more actual physical surface area within the same wall footprint (every bump and groove adds surface), and unprimed or porous drywall absorbs more paint into the material itself rather than leaving it all on the surface, both of which mean a gallon covers less effective wall area than the standard rate assumes.

Why do I need more paint for a second coat instead of just half as much?

Each coat covers the same total paintable area, so two coats require twice the paint of one coat, not half, since every coat is a full application across the entire surface rather than a partial touch-up. This calculator multiplies the paintable area by the number of coats specifically for this reason, ensuring the gallon estimate accounts for painting the full wall area as many times as the number of coats specified.

Why does the calculator show both gallons and liters?

Paint is sold in gallons in the United States and in liters in most metric countries, so showing both units means the same calculation is directly usable regardless of which measurement system your local paint supplier uses. The conversion simply multiplies the gallon figure by 3.78541, the standard number of liters in a US gallon.