Concrete, mulch, soil, firewood - the same confusion every time: the store sells it one way, your project needs it another. Enter your real dimensions and get a real number, not a guess rounded up "to be safe."
Concrete and mulch are sold in a fixed bag size, but your project is a specific volume that almost never divides evenly into that size - so the real question is never "how many bags," it's "how many cubic feet do I actually need," with the bag count as a second step. That's why this asks for your real dimensions first, converts to volume, and only then works out the bag count for whatever size you're buying.
Firewood is the more confusing case. A "cord" is a legally defined 128 cubic feet - but a "face cord" or "rick" is only exactly a third of that if the logs are 16 inches long, which is common but not universal. Buy the same face cord with 12-inch logs instead, and you get noticeably less wood for the same money. Entering your actual log length here, not assuming 16 inches, is the difference between an estimate and a guess.