Nominal and actual sizes
Lumber is named by nominal size, but dressed boards measure less than their name. Whether you calculate with nominal or actual sizes changes the result, so use the same basis your supplier prices on – and actual measurements when you need the true volume of wood.
Buying by the board foot
Board feet measure volume, not length or coverage. To compare prices between suppliers, convert each offer to a price per board foot on the same size basis. For stacks of mixed sizes, calculate each size separately and add them up.