Computes the liters of paint needed to cover a rectangular room's walls, given openings, number of coats, and coverage per liter.
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Inputs
length (number, meters, required) min 0 : Room length.
width (number, meters, required) min 0 : Room width.
height (number, meters, required) min 0 : Wall height.
openings_area (number, square meters, required) min 0 : Total area of doors and windows to subtract.
coats (integer, coats, required) min 1 : Number of coats of paint to apply.
coverage_per_liter (number, square meters per liter, required) min 0 : Manufacturer-stated coverage of the paint, one coat.
Outputs
wall_area (square meters): Net paintable wall area, one coat.
paint_needed_liters (liters): Total paint needed for all coats.
Steps
Wall perimeter = 2 * (length + width).
Gross wall area = perimeter * height.
Net wall area = gross wall area - openings_area.
Paint needed (liters) = net wall area * coats / coverage_per_liter.
Round both outputs to 2 decimals.
Constants
None.
Sources
Paint Calculator: How Much Paint Do I Need? (This Old House, 2026-09-30T22:29:13Z) - Confirms the standard method: wall area (perimeter times height, minus openings) times number of coats, divided by the paint's stated coverage rate per gallon/liter.