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hortyard

Raised bed soil calculator

Enter the inside length, width and height of your bed. You get the soil in cubic feet, cubic yards and bags, and how much of each part of the mix.

You need 32 cubic feet (1.19 cubic yards), or 22 bags of 1.5 cu ft or 16 bags of 2 cu ft. Example: one bed 8 by 4 ft, 12 in deep.

Estimates. Bag sizes, material density and settling vary; confirm quantities with your supplier.

Good to know

Vegetables grow well in 8 to 12 inches of soil; 6 inches is about the least for most crops. A bed built on good ground can be shallower because roots go on down.

Oregon State: about 8 inches of soil and organic matter is enough rooting depth for most vegetables; do not fill framed beds with compost alone; beds sink as compost decomposes, so top them up.

UC Marin: prepare beds at least 6 inches deep; 12 inches is better for drainage and roots.

Utah State: boxes at least 6 to 12 inches high; beds shallower than 12 inches should have no bottom so roots reach native soil.

Soil mixes from university guides

MixParts
Topsoil and compost 2:12 parts topsoil, 1 part compost
Topsoil and compost 1:11 part topsoil, 1 part compost
Soil 70 percent, compost 30 percent70 parts topsoil, 30 parts compost
Equal thirds1 part topsoil, 1 part compost, 1 part drainageMaterial
Compost and soilless mix 1:1 (beds on hard surfaces)1 part compost, 1 part soillessMix

The arithmetic

Volume is area times depth. With the area in square feet and the depth in inches, multiply them and divide by 12 for cubic feet, then by 27 for cubic yards. A bag count is the cubic feet divided by the bag size, rounded up.

Questions

How much soil for a 4 by 8 foot bed, 12 inches deep?

4 times 8 times 1 foot is 32 cubic feet, about 1.2 cubic yards: 16 bags of 2 cubic feet or 22 bags of 1.5 cubic feet.

How much for a 3 by 6 foot bed, 10 inches deep?

3 times 6 times 10 divided by 12 is 15 cubic feet: 8 bags of 2 cubic feet.

Do I fill the bed with compost only?

No. University guides warn against beds of compost alone; they shrink as the compost breaks down. Use a mix of topsoil and compost, like those in the table.

Typical values from these university extension guides: