Hardwood is priced by the board foot, and a board foot is a volume, not a length or an area. That trips people up at the lumberyard counter, especially when the price list is written in quarters — 4/4, 5/4, 8/4 — and the board in your hand has already been planed thinner than that.
96 inches is 8 feet. Board footage is always calculated on the rough dimensions you are charged for, not the planed size you take home.
Quarters, and why your 4/4 board is three-quarters of an inch
Hardwood is sold in quarter-inch increments of its rough sawn thickness. 4/4 means one inch, 8/4 means two. You are charged on that rough dimension — but if you buy it surfaced on two sides (S2S), the yard has already planed it, and a 4/4 board typically arrives at 13/16 or ¾ of an inch. That is normal and it is not a shortchange: you pay for the wood that was there before the planer took the twist out of it.
The practical consequence is that you cannot design a project around a full inch of thickness and then buy 4/4. If you need a finished inch, you buy 5/4.
Board feet versus lineal feet versus square feet
Three different measurements, three different uses, and confusing them is how people end up short. Board feet measure volume and are how hardwood is priced. Lineal feet measure length only and are how trim and dimensional softwood are usually sold. Square feet measure area and are how sheet goods and flooring are sold. A 1×6 and a 2×6 of the same length are the same lineal footage and the same square footage, but the 2×6 is twice the board footage and roughly twice the price.
Buy more than you calculated
Rough lumber is not ready to use. It has to be jointed flat on one face and one edge, then planed to thickness, and every one of those operations removes material. On top of that you are cutting around knots, checks, wane along the edges, and whatever cupping developed while it dried. Twenty to thirty percent over your calculated need is the normal allowance, and on figured or highly variable stock it can be more.
Related: what plywood grades actually mean and OSB versus plywood for the sheet-goods side of the same problem.
Method
Board feet = (thickness in inches × width in inches × length in inches) ÷ 144. This is the standard industry formula and is calculated on nominal rough dimensions, which is what lumberyards price against. Lineal feet is length ÷ 12 × quantity. Surface area is one face only. The 20–30% project allowance is a widely used woodworking rule of thumb rather than a published standard — adjust it up for figured stock, live edge, or anything you need to match for grain.