How the ceiling paint calculator works
Ceiling math and finish choice
Ceiling area is length × width — no openings, no fussy subtraction, the easiest paint math in the house. Flat (or matte ceiling-specific) finish is the standard: it hides taping imperfections and texture, and spatters less during roll-out. Ceilings painted wall-white look gray next to the walls; dedicated ceiling white is worth the second can.
Trim math, in linear feet
Baseboards, casings, and doors present roughly one square foot of paintable surface per linear foot (two faces of ~6" trim). Ninety linear feet — a typical bedroom — needs about a quart per coat. Semi-gloss or enamel is the standard: trim takes hands, vacuums, and mop handles, and sheen is what washability looks like.
The order of operations
Ceiling first (spatter lands on walls), then walls, then trim last (it covers the wall-paint edge mistakes). This sequence is why professionals cut in ceilings into walls and never the reverse — and why your gallon count for walls assumes the ceiling already happened.