A worked example: 5m × 3m wall, 3 m² of openings
A wall 5m long and 3m high has a gross area of 15 m² — but with 3 m² of combined door and window openings, the net area actually needing paint or plaster is only 12 m².
The formula: net area = (length × height) − openings
Gross area is the full rectangle the wall occupies; subtracting every door and window's area gives the net area — the material quantity actually required, since paint and plaster don't go over openings.
How to find the combined openings area
Multiply each door or window's width by its height individually, then add all of them together for this single wall before entering the total — a wall with one door and two windows needs all three openings summed into one figure.
Why using gross area instead would overstate material needed
Skipping the openings subtraction and using the full 15 m² gross figure would mean buying paint or plaster for 3 m² of wall that doesn't actually exist as paintable surface — a real, avoidable overestimate on every wall with doors or windows.
Where this net area feeds into next
Net wall area is exactly the figure a paint calculator needs to compute paint volume directly — this calculator stops at area, deliberately leaving the paint-volume conversion to a dedicated tool.