Raised bed soil calculator: how much soil do you need?

How the calculation works

Soil is sold by volume. Length times width gives the area in square feet; times the depth in inches and divided by 12, that gives cubic feet. Divide by 27 for cubic yards, the unit bulk suppliers use. Bag counts are rounded up to whole bags.

One number is worth remembering: a 4 by 8 foot bed filled 10 inches deep holds about 27 cubic feet, or one cubic yard.

How deep should a raised bed be?

Most plants need a rooting zone of at least 6 to 12 inches, and deeper is better. What matters is whether the bed sits on soil or on a hard surface.

• On soil, roots grow through the fill into the ground below, so loosen that soil before you fill. About 8 inches of mix over loosened soil is enough for most vegetables.
• On a patio, driveway, or rooftop, the bed is all the soil there is. Build at least 8 inches deep for leafy greens, beans, and cucumbers, and 12 to 24 inches for peppers, tomatoes, and squash.

The vegetable garden guide covers bed size, depth, and materials in more detail.

What to fill it with

Blend mineral topsoil, for body, with compost, for organic matter. Compost alone slumps and dries unevenly. Extension recommendations on the compost share range widely, from 10 to 20 percent of the volume up to half. Maryland Extension suggests compost and topsoil at 1:2 or 1:1, which is 33 or 50 percent compost. A compost-heavy bed can build up excess phosphorus and salts, so rather than guessing at a ratio, the guide on what to fill a raised bed with suggests blending for body and organic matter, then running a soil test.

Read bag labels. Bags sold as "organic soil" typically contain no mineral soil, but combinations of wood waste, bark fines, compost, peat moss, and other organic materials. In bulk, ask the supplier what is actually in a "garden blend" or "three-way mix", because the names mean different things in different places.

Filling a very deep bed? A hügelkultur bed fills the bottom with logs and branches instead of soil.

Sources

• University of Missouri Extension: Raised-bed gardening — a 6- to 12-inch rooting zone, and rooting depth from soil below the bed.
• University of Maryland Extension: Growing vegetables in raised beds — depth by crop for beds on hard surfaces.
• University of Maryland Extension: Soil to fill raised beds — compost and topsoil blends, and what bagged "organic soil" contains.
• Oregon State University Extension: Raised bed gardening — about 8 inches of mix for most vegetables, and loosening the soil underneath.