How Much Soil Does a Raised Bed Need?
The amount of soil needed for a raised garden bed is based on volume: the inside length of the bed multiplied by its inside width and the depth you plan to fill. Measuring the interior of the bed gives a more accurate estimate than using its outside dimensions, especially when the walls are made from thick lumber, blocks, or other substantial materials.
The soil depth does not necessarily have to equal the full height of the raised bed. If the bed is 18 inches tall but you only intend to add 14 inches of soil, for example, use 14 inches as the fill depth.
Cubic Feet vs. Cubic Yards
Bagged garden soil, raised-bed mix, compost, and similar products are often sold by volume. Smaller quantities may be labeled in cubic feet, while bulk material from a landscape supplier is commonly sold by the cubic yard.
There are 27 cubic feet in one cubic yard. For a larger raised-bed project, converting the total to cubic yards can make it easier to compare the cost of bagged material with bulk delivery.
Estimating Bags of Soil
Bag sizes vary, so check the volume printed on the particular product you're considering. Once you know the total volume your beds require, that package volume can be used to estimate the number of whole bags needed.
Because you generally can't purchase part of a bag, the number of packages must be rounded up whenever the calculated amount doesn't divide evenly into the package size.
Should You Buy Extra Soil?
The exact calculated volume represents the space you're trying to fill, but real projects don't always use material quite that neatly. Soil can settle, material can be spilled during filling, and you may want some left over for topping off the bed later.
The calculator lets you add your own extra percentage rather than automatically assuming that everyone needs the same amount. If you don't want an allowance added, simply leave it at zero.
Soil Volume Is Not the Same as Soil Type
This calculator determines how much material will fit in the raised bed; it does not determine what kind of soil or soil mixture you should use. Different plants and gardening methods may call for different combinations of soil, compost, and other materials.
Use the calculator to determine the volume you need, then choose a soil or growing mix appropriate for what you plan to grow.