Your bed's dimensions in — shopping list out. Assumes the hugelkultur split: ~50% wood below, ~25% bulk soil, ~25% quality top mix.
Approximations — logs don't pack like bricks, and the gaps get tucked with soil (counted in your bulk soil). Castings at 5–10% of the top mix, per the guide. 1 ft³ ≈ 7.48 gallons.