What formula does the calculator use?
For a rectangular raised bed, multiply length by width by fill depth, using feet for all three measurements. A 4×8 bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet. Divide cubic feet by 27 to convert to cubic yards.
The calculator then applies your settling buffer, splits the volume by your soil mix percentages, and rounds each component up to whole bags. This is why the shopping list is more practical than a raw volume answer.
Example: 4 ft × 8 ft × 1 ft = 32 cu ft. With a 10% buffer, plan for 35.2 cu ft, or about 1.30 cubic yards.
Why do raised beds need so much soil?
Most retail bags are only 1 or 1.5 cubic feet. A single 4×8 bed at 12 inches deep is more than one cubic yard before buffer, so buying everything in bags can become expensive quickly. For multiple beds, bulk raised bed mix from a local landscape supplier is often worth pricing.
What soil mix should you use?
A conservative planning mix is 60% topsoil, 30% compost, and 10% aeration or soilless growing mix. University and garden references vary: some support 70/30 topsoil and compost, while others use compost with soilless mix when topsoil quality is uncertain. Treat the calculator ratios as a shopping planner, not a universal soil prescription.
Can filler material reduce the cost?
In tall beds, logs, branches, leaves, straw, cardboard, and other organic filler can reduce the amount of soil mix you buy. The tradeoff is settling. As the filler decomposes, the bed can sink and need topping up with compost or soil in later seasons.
Keep enough actual growing mix above filler. Maryland Extension notes that beds on hard surfaces need at least 8 inches for leafy greens, beans, and cucumbers, and 12 to 24 inches for peppers, tomatoes, and squash.