Raised Bed Soil Mix Calculator
How much soil a raised bed actually takes, by the bag and by the cubic yard, split into the compost, peat or coir, vermiculite, or topsoil you need for a Mel’s Mix or a plain topsoil-and-compost fill, with a first-year settling allowance built in.
Set your place for the best week to fill and plant it.
What goes in the bed
Mel’s Mix, the square-foot-gardening recipe, is equal parts by volume of blended compost, peat moss or coconut coir, and coarse vermiculite, one third each. The topsoil and compost fill is the usual university-extension recommendation for a deeper bed, around sixty percent topsoil to forty percent compost. Both are good; they answer different questions, a light soilless bed you build once versus a deeper bed you top up.
How the math works
Volume is length times width times depth. We take each bed in square feet, multiply by the depth in feet, and convert to cubic yards, since one cubic yard is twenty-seven cubic feet and bulk soil is sold and delivered in yards. Bag counts use the typical retail bag size for each material. Peat moss and coconut coir are sold compressed, so we count their expanded, fluffed volume, not the smaller number on the bale. Beds settle hard the first season, so we add a fifteen percent buffer by default and you should still plan to top-dress an inch or two of compost each year.
Sources
Mel’s Mix ratio: Square Foot Gardening Foundation. Topsoil-to-compost range (50:50 to 75:25, ~60:40 used here): Penn State, Iowa State, South Dakota State, and University of Maryland Extension. Settling and yearly top-dressing: University of Illinois Extension. Peat bogs store roughly a third of the world’s soil carbon, which is why coir is offered as an alternative: IUCN. Bag sizes vary by brand, so treat them as typical and confirm your own.