"How many gallons do I need?" seems like it should have one clean answer, but the real figure depends on more than just the size of the room — wall openings, coat count, and the paint's own coverage rating all move the number, sometimes by a lot.
How Paint Coverage Numbers Work
The "350 square feet per gallon" figure printed on most paint cans (or the roughly 10 square meters per liter equivalent) is an average for one smooth, primed coat under normal application. It's not a guarantee — textured or unprimed drywall, porous surfaces, and rollers with thicker naps all soak up more paint per square foot than that number assumes. Darker or more saturated new colors can also need extra coverage to fully hide what's underneath. Treat the printed coverage rate as a solid starting estimate, then check the specific can for its own stated number, since formulas vary between brands and even between finishes (matte vs. satin vs. semi-gloss) from the same brand.
Why Doors and Windows Get Subtracted
Calculating paint needs starts from the room's total wall area (perimeter × height), but a chunk of that area isn't wall at all — it's door and window openings, which get painted differently (or not at all, if they're trimmed rather than wall-painted). Subtracting a standard estimate for each door and window before dividing by the coverage rate keeps the calculation from overestimating how much wall surface actually needs paint. Skip this step and you'll consistently buy more paint than the room requires, which is a wasted cost rather than a helpful buffer.
Why You Usually Need Two Coats
A single coat of paint frequently lets the underlying color show through slightly, especially with a dramatic shift like painting over a dark wall with a light color, or vice versa. Two coats is the standard recommendation for even, opaque coverage that doesn't show streaks or patchiness once it dries. This is why coat count is a direct multiplier in the total paint needed — going from one coat to two doesn't just add a small buffer, it roughly doubles the amount of paint the job requires, which is often the single biggest factor separating an underestimate from an accurate one.
Buying a Little Extra vs. Buying Short
Paint calculations are estimates, and real-world painting rarely goes exactly to plan — a second coat that needs a touch-up, a slightly thicker application than expected, or future wall repairs that need color-matched paint down the line. Rounding up to the next full can (rather than buying the bare calculated minimum) avoids a mid-project store run, and keeping a small amount of leftover paint, properly sealed and labeled with the color name, makes future touch-ups far easier than trying to color-match years later.
Try It Instantly
Enter your room's dimensions, door and window count, and number of coats into the free Paint Calculator to get an instant gallon or liter estimate — or, if you're also flooring the room, the Tile & Flooring Calculator handles the same kind of area-based math for tile and boxes needed.
FAQ
How much wall area does a gallon of paint cover? This calculator assumes about 350 square feet per gallon (roughly 32.5 square meters, or about 10 square meters per liter), a commonly cited average for one coat of standard interior paint. Actual coverage varies by paint brand, wall texture, and application method — check your specific paint can for its stated coverage.
Why does it subtract doors and windows? Doors and windows aren't painted the same as walls, so their area is subtracted from the total wall area before calculating how much paint you need — otherwise you'd overestimate and could overbuy.
Why would I need more than one coat? A single coat often doesn't fully hide the previous wall color, especially with a big color change (like going from dark to light) — two coats is standard practice for full, even coverage, which is why the tool multiplies your paint needed by the number of coats you select.
Is my data sent anywhere? No — the calculation happens entirely in your browser using JavaScript, so nothing you enter is ever sent to a server.