How the paint calculator works
The coverage number stores do not advertise
Paint cans claim 400 sq ft per gallon; experienced painters plan on 350 or less. Real walls absorb, roller texture adds surface area, and application thickness varies by elbow. Planning at 350 turns "one gallon should do it" into math that survives contact with the second coat.
Measuring once, correctly
Perimeter × height minus openings is the whole formula — the discipline is measuring the perimeter honestly (including closets and alcoves) and counting openings at their standard values (21 sq ft per door, 15 per window). Run the tape once, write it down, and every room in the house can be priced from the same note.
The 10% overage rule
Pros order ~10% extra for touch-ups, mis-tints, and the future wall scuff — and dye lots mean a gallon bought next year will not match this year. Buying the extra gallon now is cheaper than repainting a panel later; store it, shake-shaken, labeled by room.