How Much Soil Do I Need for a Raised Garden Bed?

A standard 4 × 8 ft raised bed filled to a depth of 12 inches needs 32 cubic feet of soil, or about 1.2 cubic yards. Allow 10% more for settling and you are at about 35 cubic feet. Filling raised beds is the most expensive part of building them, so it pays to measure properly and to choose between bagged and bulk soil with your eyes open.

The formula

Remember to convert the depth from inches to feet (divide by 12). Then divide by 27 to get cubic yards, or by the bag size (typically 1 to 2 cubic feet) to count bags. For a 4 × 8 ft bed 12 in deep: 4 × 8 × 1 = 32 ft³, which is 1.19 yd³.

Soil needed by bed size

Cubic feet of soil to fill the bed completely, and cubic yards including 10% for settling:

Use the raised bed soil calculator for any other size, or for several beds at once.

How deep should a raised bed be?

• 6 to 8 inches: enough for lettuce, herbs, radishes and other shallow-rooted crops, if the ground underneath is loosened.
• 10 to 12 inches: a good general depth for most vegetables.
• 15 to 18 inches or more: best for deep-rooted crops such as tomatoes, carrots and parsnips, and for beds on concrete or compacted ground, where roots cannot go further down.

The bed's walls can be taller than the soil depth. Fill to about an inch below the rim.

What to fill it with

There is no single correct recipe, but these are the common approaches:

• Equal parts mix. A popular recipe is one third compost (ideally from several sources), one third peat moss or coco coir, and one third vermiculite. It is light, holds water well and drains well, but is the most expensive. Our calculator splits the total into thirds.
• Topsoil and compost. A mix of roughly 50 to 60% screened topsoil and 30 to 40% compost, with some aeration material (such as perlite or coarse sand) is a cheaper alternative, and works well in larger beds.
• Bagged raised bed mix. Convenient and consistent, but the highest cost per cubic foot. It is a good choice for one or two small beds.

Look for a mix that is dark and crumbly, and avoids anything with lots of sticks, stones or an unpleasant smell. If you buy in bulk, ask the supplier for a screened mix that suits vegetable gardens, and check that it was made from clean ingredients.

Bulk or bags?

Bagged soil is easy to move and fits a car boot, but it is expensive per cubic foot. Once you need more than about 20 to 30 cubic feet, bulk soil delivered by the yard is usually far cheaper. A 4 × 8 ft bed 12 in deep needs 1.3 cubic yards, which is about 18 bags of 2 ft³, or one small delivery. If you have several beds, bulk wins easily. Check the delivery minimum and fee, and where the truck can tip it.

Ways to use less soil

• Hügelkultur. Put logs, branches and woody debris in the bottom of deep beds, and cover them with soil. It cuts the amount of soil needed, and the wood slowly breaks down and holds water. Keep out rot-resistant woods such as black locust, and avoid black walnut, which can harm other plants.
• Shallower beds built over loosened ground, where roots can go down into the native soil.
• Use compost and leaves from your own yard for part of the volume.

Settling and topping up

Fresh soil settles by an inch or two after the first few waterings, as air pockets collapse. That is why we add a 10% allowance. Top up with compost each spring and fall to replace what the plants have used and the soil has lost, rather than replacing all the soil.