Bed, soil and plant calculator
How much soil fills your beds, in cubic feet, cubic yards and bags, how much compost a top-dressing takes, and how many plants fit at a given spacing.
Soil to fill your beds
Assumptions behind the numbers
- Volume is geometric: length x width x depth. Nothing is added for settling. Ask your supplier how much their product settles and buy that much extra.
- 1 cubic yard = 27 cubic feet (NIST Handbook 44, Appendix C; Oregon State EM 9308).
- Bag sizes vary. 1.5 cu ft is one maker's bag, used as a default only. Read the bag you are buying.
- Do not fill a bed with compost alone (Oregon State EM 9308). The University of Maryland suggests compost to topsoil at 1:2 or 1:1, and compost and soilless mix 1:1 on hard surfaces.
The Okafor garden: two beds, 8 x 4 ft, 12 in deep. Each holds 32 cu ft; together 64 cu ft, which is 2.37 cu yd. In 1.5 cu ft bags that is 43 bags before settling, so they priced a bulk delivery of 2 parts screened topsoil to 1 part compost instead and asked the supplier for its test results and how much the mix settles. The supplier, not the calculator, settles both answers.
Plants per bed
How many plants fit on a square grid at one equidistant spacing: the same distance between plants in every direction. Spacings for 21 crops are in the crop guide (from Virginia Cooperative Extension 426-335). A staggered layout, where every other row shifts by half a space, fits a few more; Module 5 explains both.
Plants per bed = floor(length x 12 / spacing) x floor(width x 12 / spacing). At 24 in, an 8 x 4 ft bed holds 4 x 2 = 8 plants; the Okafor garden (fictional) planted 4 tomatoes in the east bed, at the full 24 in.
The soil numbers are geometry; they do not say whether a fill is clean or good for vegetables. Ask the supplier for test results, and see Module 4 for fill ratios. The plant count says what fits, not what a bed will produce; we give no yield figures.