The Paint Can Problem

The wall-area formula has one step beginners skip: subtracting openings. A 12 × 14 room with 8-foot ceilings has a perimeter of 52 feet, so wall area = 52 × 8 = 416 sq ft. Subtract a door (21 sq ft) and two windows (24 sq ft) and you're down to 371 sq ft — roughly one gallon per coat, or 2 gallons for two coats.

Skip that subtraction and you overbuy. At $30-50 per gallon for quality paint, getting the count right saves real money on every room.