Draw the surfaces
List each wall separately with its width and height in the same unit. Mark which walls are included. Decide whether trim, doors and ceilings belong in this calculation or need separate products. The planner multiplies one rectangular surface at a time; for a room, add the measured wall areas before entering the total dimensions or calculate surfaces separately.
Subtract only the openings you intend to exclude
Measure door and window openings and total their areas. Small recesses and reveals may themselves need painting, so keep the scope explicit. An opening allowance greater than the gross wall area is a measurement error; the planner rejects it.
Read the label
Enter the coverage per coat in square metres per litre, or square feet per US gallon, according to the selected units. The calculator does not supply a universal paint coverage. Porosity, texture, application and the specified coating system can change the amount needed. Use a separate estimate for primer.
Sherwin-Williams: paint calculator ↗ — An approximate quantity tool; take actual coverage from the selected product.
Round the purchase, not the measurements
The tool calculates net area times coats divided by coverage, then adds your chosen spare allowance. It rounds up to the container size you enter. A result is a purchasing estimate, not confirmation that the paint is compatible or that the surface is ready.
Keep substrate identification, product instructions, measurements, preparation photographs and observed results together. We do not turn a generic checklist into a structural or installation specification.
