The most common reason people run out of paint mid-project isn’t measuring the wall wrong — it’s forgetting that a second coat means covering the same area again, in full. Our Paint Cost Calculator makes that multiplication explicit instead of leaving it as a mental math step you might skip.
The core calculation
Total paintable area is multiplied by the number of coats, since each coat covers the full wall area again: a 400 sq ft wall with 2 coats needs paint for 800 sq ft of total coverage, not 400. That total is then divided by your paint’s coverage rate (how many square feet one gallon covers) to get gallons needed.
Why coverage rate is an input, not a fixed assumption
Paint coverage isn’t a universal constant — most standard interior paints cover somewhere in the 350-400 sq ft per gallon range, but the actual number varies by product, and by surface: textured walls, porous new drywall, or premium thick-coverage formulas can all shift that number meaningfully. Using the coverage figure printed on your specific paint can, rather than a generic assumption, gives a materially more accurate estimate.
A worked example
For a 400 sq ft wall area, 2 coats, 350 sq ft/gallon coverage, and $35/gallon: total area to cover is 800 sq ft, requiring 800 ÷ 350 = 2.29 gallons — rounded up to the nearest quarter gallon (since paint is typically sold in gallon and quart increments) gives 2.50 gallons, at a total estimated cost of $87.50.
Rounding up, on purpose
The gallon result always rounds up rather than down, and to the nearest quarter gallon rather than a whole number, since that matches how paint is actually purchased in stores — better to buy a manageable, precisely-sized extra amount than to guess low and make a second trip mid-project.
What’s not included
This calculator expects your net paintable wall area — gross wall area minus windows, doors, and any other unpainted openings. It doesn’t subtract those automatically, since opening sizes vary too much from room to room to assume a sensible default; measure or estimate your net area before entering it for the most accurate result.
Try it yourself: Paint Cost Calculator.