The decision is about scope, not the object name. A painted door belongs in a separate door calculation; an open doorway may be excluded from wall area; a small fixture normally does not need a guessed deduction.
- No default door or window area is built into the calculator.
- Measure in the same area unit as the walls.
- Do not subtract trim when trim will receive a separate product.
Formula or decision boundary
net wall area = gross wall area − sum of measured unpainted opening areasOpening decision examples
| Feature | Wall calculation | Separate calculation |
|---|---|---|
| Unpainted window | subtract measured opening | none |
| Painted door | subtract from wall | calculate door faces |
| Open doorway | subtract measured opening | none |
| Painted trim | do not treat as wall coating | measure trim separately |
Use the answer
Mark coating boundaries
Use a sketch or photo to identify where the wall coating stops.
Measure exclusions
Record actual opening width and height instead of a catalog assumption.
Reconcile separate coatings
Make sure doors, trim, cabinets, and panels are either excluded or calculated elsewhere, never lost.
Safety and scope
- Do not disturb suspect old coating while measuring.
- Use safe access for tall windows and openings.
Sources and scope
Source links reviewed July 16, 2026. A review date is not the document's publication date.
- Sherwin-Williams: Paint CalculatorNorth America · manufacturer calculator
Actual coverage varies with surface condition, color change, application, and product label.
- National Institute of Standards and Technology: NIST Guide to the SI, Appendix B — Conversion FactorsUnited States · government standard
Code retains exact defining constants where NIST identifies an exact relationship.