The formula
board feet = thickness″ × width″ × length″ ÷ 144
Thickness uses the rough (nominal) size: a 4/4 board counts as a full inch even though it surfaces to about 13/16″. Width and length are what you measure. For length in feet, multiply by 12 first — this calculator does that for you.
Planning more than one size? The planner keeps a whole parts list with species pricing, waste, and tax.