How much soil does a raised bed need?

Multiply the bed's inside length by its inside width by the depth you plan to fill, in feet. That number is the cubic feet of soil you need. An 8 by 4 foot bed filled 12 inches deep needs 32 cubic feet, which is about 1.19 cubic yards or 16 bags of 2 cubic feet.

The raised bed soil calculator does this math for you and converts the result into the bag size you are buying.

Measure the bed correctly

Use the inside dimensions, not the outside. Timber walls add width that does not hold soil, and outside measurements slightly overstate the volume on beds with thick sides.

For depth, measure only the empty space you plan to fill. If the bed already has soil, compost, or drainage material in the bottom, subtract that from the wall height. A 12 inch tall bed that you plan to top up by 4 inches needs a 4 inch fill depth, not 12.

Bags or bulk

Under about 1 cubic yard, bags are usually the simpler choice. They are easy to move and let you mix products, for example topsoil plus compost.

At 1 cubic yard or more, bulk delivery starts to make sense on cost. One cubic yard is 27 cubic feet, which is 13 or 14 large bags. Bulk soil is sold by the cubic yard and delivered loose, so check that you have a place to drop it and a weekend to move it.

Common mistakes

• Entering the full bed height for a partial refill. Measure only the depth you are adding.
• Forgetting settling. Fresh fill can drop an inch or two after the first few waterings. Many gardeners order about 10 percent extra for a deep bed.
• Mixing up cubic feet and cubic yards. Bagged soil is sold in cubic feet. Bulk soil is sold in cubic yards. Multiply cubic feet by 27 to compare prices, or divide yards by 27 going the other way.
• Filling a deep bed entirely with topsoil. Beds deeper than about 12 inches can use cheaper fill such as logs, branches, or unfinished compost in the lower half, with quality soil only in the top 8 to 12 inches where roots actually grow.

What to do next

Once you know the volume, the soil calculator converts it to your bag size, and the plant spacing tools can tell you how much each bed will hold.