Estimate how many gallons of paint you need to cover a room's walls, based on room size, doors, windows and number of coats.
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This calculator finds the room's wall area by multiplying the perimeter (2 × (length + width)) by the wall height, then subtracts a standard estimated area for each door (about 21 sq ft) and window (about 15 sq ft) you specify. That net area is multiplied by your number of coats, then divided by your paint's coverage rate per gallon and rounded up to a whole gallon.
Most paint covers around 350-400 sq ft per gallon on a smooth, primed surface, but this varies by paint brand, sheen, and the porosity or texture of your walls. Check your specific paint can's label for its stated coverage rate for the most accurate estimate.
For a 12 ft × 10 ft room with 8 ft walls, 1 door and 1 window, 2 coats, at 350 sq ft coverage per gallon: perimeter = 2 × (12+10) = 44 ft, gross wall area = 44 × 8 = 352 sq ft. Subtracting openings (1 door at 21 sq ft, 1 window at 15 sq ft = 36 sq ft) gives a net wall area of 352 − 36 = 316 sq ft. For 2 coats, total paint area = 316 × 2 = 632 sq ft. Dividing by 350 sq ft per gallon: 632 ÷ 350 ≈ 1.81, rounded up to 2 gallons needed.
Forgetting to subtract doors and windows from the wall area, this calculator does it automatically using standard estimated sizes, but if your openings are unusually large or numerous, the standard 21 sq ft door / 15 sq ft window estimate may undercount how much area you're actually saving.
Using the wall area for a single coat when the room needs two, this calculator multiplies net wall area by the number of coats before converting to gallons, forgetting to double it (when estimating by hand) is a common way to under-order paint.
Assuming every paint's coverage rate is the same, coverage varies meaningfully by brand, sheen and wall texture, always check the specific can's label rather than assuming the 350 sq ft per gallon default.
No, this calculator estimates wall paint only. Ceilings are typically painted with a separate ceiling paint and would need to be calculated separately using the room's floor area.
Two coats is standard for most new paint jobs. A single coat may work for a minor touch-up in a similar color, while three coats might be needed for a light color covering a much darker existing wall.
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