A board foot is a volume: 144 cubic inches of wood. The formula is the whole game — thickness (in) × width (in) × length (in) ÷ 144 — and everything confusing about board feet is a footnote to it.
Construction lumber is sold by its nominal (pre-surfacing) name: a 2×6 is really 1.5″ × 5.5″. Hardwood lumber plays a different game: it is sold rough or surfaced by the board foot on its rough thickness in quarters — 4/4 ("four-quarter") is 1″ rough, surfacing to about 13/16″ — and the board footage is figured on the rough size and the board's actual width and length. Know which game your yard is playing before you compare prices: a cheap board foot of 4/4 rough is not the same wood as a cheap board foot of S2S.
Net board feet is the wood in the finished piece. What you must buy adds milling waste, defect culling, and the grain-matching tax. Shop rules of thumb, labeled as such: 15–20% for straight, paint-grade work from good stock; 25–30% for typical furniture from commercial hardwood; 40%+ for small parts, heavy figure matching, or rustic grades where knots get cut around. Wide, clear, single-board parts (a tabletop from one plank) break the model entirely — buy the board, not the percentage.
Run your own list in the parts-list calculator (waste factor is an input), or tally a stack of dimension lumber in the bare tally.