How the raised bed soil calculator works
cubic feet = length (ft) × width (ft) × depth (in ÷ 12) × number of beds × (1 + settling %)
cubic yards = cubic feet ÷ 27
liters = cubic feet × 28.32
Bagged soil is sold by the cubic foot or liter, and bulk soil by the cubic yard or cubic meter, so the calculator gives every unit along with bag counts.
Worked example
Two 4 × 8 ft beds filled 12 inches deep hold 4 × 8 × 1 × 2 = 64 cu ft. Adding 10 percent for settling gives 70.4 cu ft, which is 2.61 cubic yards or about 1,994 liters. In bags that is 71 bags of 1 cu ft, 47 bags of 1.5 cu ft or 36 bags of 2 cu ft. With a 60/30/10 mix you need 42.2 cu ft of topsoil, 21.1 cu ft of compost and 7 cu ft of aeration material.
Soil volume for a 4 × 8 ft bed
| Fill depth | Cubic feet | Cubic yards | Liters | 2 cu ft bags |
|---|---|---|---|---|
| 6 in | 16 | 0.59 | 453 | 8 |
| 12 in | 32 | 1.19 | 906 | 16 |
| 18 in | 48 | 1.78 | 1,359 | 24 |
Figures exclude settling. Deep beds use a lot of soil; a common trick is to fill the bottom of a very tall bed with coarse material and use good soil in the top 12 inches or so where most roots grow.
Bags or bulk?
Bags are convenient for one or two small beds and for places a wheelbarrow cannot reach. Once you need more than about a cubic yard, bulk delivery is usually cheaper per cubic foot, but compare prices locally. One cubic yard equals 27 cubic feet, so it replaces 27 one-cubic-foot bags or about 13 to 14 two-cubic-foot bags.
| Unit | Equals |
|---|---|
| 1 cubic yard | 27 cu ft, about 765 liters |
| 1 cubic foot | about 28.3 liters |
| 1 cubic meter | 1,000 liters, about 35.3 cu ft |
Why add extra for settling?
Fresh mixes, especially those with a lot of compost, sink as they are watered and as organic matter breaks down. Adding about 10 percent is a common allowance, and you can top up beds each season with a layer of compost. Mixes heavy in bark or uncomposted material settle more.
Choosing a mix
The mix selector splits the total volume by percentage. A blend of topsoil for body, compost for nutrients and structure, and a coarse aeration material such as perlite, pumice or fine bark for drainage is a widely used starting point. Avoid filling raised beds with heavy clay dug from the yard on its own; it compacts and drains poorly in a contained bed. See the guide to the best soil mix for raised beds for options.
Filling tall beds for less
Beds 24 inches or taller hold a lot of soil, and most vegetables do not need all of it. Two common ways to cut the volume of bought soil:
- Fill the bottom third with coarse organic material such as small logs, branches and leaves. It breaks down over the years, so expect the surface to sink and top up with compost.
- Loosen the ground under a bed that sits on soil, so roots can grow down into it, and build a shallower bed on top.
To estimate the bought soil, enter only the depth you will fill with your mix in the calculator.
Tips
- Measure the inside of the frame. Boards 1.5 inches thick on both sides make a 4 ft bed only 3.75 ft wide inside.
- Leave an inch or two below the top edge so water and mulch stay in the bed.
- For mulch on paths between beds, use the mulch calculator. For a gravel base under paths, use the gravel calculator.
- Fall is a good time to build and fill beds. The mix has the winter to settle, and you can top it up before spring planting.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?
A 4 × 8 ft bed filled 12 inches deep holds 32 cubic feet, about 1.19 cubic yards or 906 liters. That is 16 bags of 2 cu ft before any allowance for settling.
How many bags of soil fill a raised bed?
Divide the bed volume in cubic feet by the bag size. A 32 cu ft bed needs 32 bags of 1 cu ft or 16 bags of 2 cu ft, plus a few extra for settling.
How many cubic feet are in a cubic yard?
27. A cubic yard is 3 ft × 3 ft × 3 ft.
How deep should a raised bed be?
Many vegetables grow well in 10 to 12 inches of good soil. Deep-rooted crops such as tomatoes and carrots appreciate 12 to 18 inches.