A paint estimate starts with geometry, not a can size. Walk around the project and list each rectangular surface receiving the same paint. Keep separate walls as separate rows because a visible source is easier to correct than a remembered total. Then list only rectangular openings you deliberately want to subtract. PaintCalc adds positive surfaces and subtracts positive openings; it does not identify doors, windows, cabinets, trim, or unpainted zones by itself.
Map non-overlapping rectangles
Sketch and name every row: north wall, window wall, alcove return, or ceiling. Measure width and height in one unit system selected before entry. Split an L-shaped boundary into non-overlapping rectangles; overlapping rows count shared area twice. Curves, triangles, slopes, mouldings, cabinets, radiators, and trim are outside v1. Calculate them separately with an appropriate method. Switching unit systems changes labels and starter values but never reinterprets an existing number, so choose the system first and read each row back against the sketch.
Subtract only measured openings
Measure an opening instead of selecting a generic door or window allowance. A partly painted door, deep reveal, or trim needs a separate decision recorded beside the sketch. Openings must total less than surfaces; otherwise no positive paintable area remains and PaintCalc rejects the project. Small interruptions can still consume paint on edges, so subtracting every switch is not automatically more accurate. You decide which measured rectangles form the subtraction. Review net area before coats and use coverage later only from the exact paint label or datasheet.