SoilBenchRaised bed planning and fill math for real gardens.
Bed sizes

Raised bed garden soil calculator

The same footprint changes fast when you move from 6 inches of fill to 12 inches. Depth matters more than most first builds expect.

Footprint6 in depth8 in depth12 in depth
6 x 3 ft9 cu ft12 cu ft18 cu ft
8 x 4 ft16 cu ft21.3 cu ft32 cu ft
10 x 4 ft20 cu ft26.7 cu ft40 cu ft