Concrete and firewood are both sold in fixed units, a bag and a cord, while your project is a specific volume that rarely divides evenly into either. Concrete and firewood - the same confusion both times: 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 is 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.
Multiply the area by the depth you want. A 20 m² bed at 75 mm deep needs 1.5 cubic metres.
Divide the volume by the bag size. Garden bags are commonly 25, 40 or 50 litres, and there are 1,000 litres in a cubic metre.