How to use
- Pick US or metric units.
- For each bed, choose rectangle or round and enter the length and width (or the diameter).
- Enter the bed height. If you won't fill it to the top, enter the space you'll leave empty, such as 1 or 2 inches.
- If you have more than one bed, select "Add another bed". If several beds are the same size, enter that number under "Number of identical beds".
- Select "Calculate" to see the total soil and bag counts. Use the optional mix to split the total among topsoil, compost, and anything else.
How the soil is calculated
Volume = area × soil depth. For a rectangle, the area is length × width. For a round bed, it is π × (diameter ÷ 2)². Convert the depth from inches to feet by dividing by 12. One cubic yard is 27 cubic feet, and one cubic foot is about 28.3 liters.
Example: a 4 ft × 8 ft bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet, or 32 ÷ 27 = 1.19 cubic yards. That is 22 bags of 1.5 cubic feet (21.33 rounded up) or 16 bags of 2 cubic feet. A round bed 4 ft across and 10 inches deep is π × 2² × (10 ÷ 12) = 10.47 cubic feet. In metric, a bed 1.2 m × 2.4 m × 30 cm is 0.864 cubic meters, which is 864 liters, or 18 bags of 50 liters (17.28 rounded up).
Soil for a 4 × 8 ft bed
These figures use the formula above for a 4 by 8 foot bed filled to the depth shown.
| Soil depth | Cubic feet | Cubic yards | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|
| 6 in | 16.0 | 0.59 | 11 | 8 |
| 8 in | 21.3 | 0.79 | 15 | 11 |
| 10 in | 26.7 | 0.99 | 18 | 14 |
| 12 in | 32.0 | 1.19 | 22 | 16 |
How deep should a raised bed be?
It depends on what you grow and what is under the bed. The University of Maryland Extension says beds placed on a hard surface such as a driveway are fine if they are at least 8 inches deep for leafy greens, beans, and cucumbers, and 12 to 24 inches deep for peppers, tomatoes, and squash. Rutgers Extension recommends a minimum of 12 inches of imported topsoil for proper root-zone volume and drainage. University of Maryland Extension notes that plant roots can grow through the bed soil into the ground below, so ask your local extension office what fits your site.
What to fill the bed with
Extension services give a few common mixes. These are examples from their published pages, not rules, and this calculator does not choose one for you.
- One common mix from University of Maryland Extension: compost and purchased topsoil at a ratio of 1:2 or 1:1, added to a bed over compacted, high-clay soil. The same page also describes compost and a soilless growing mix at 1:1.
- One common mix from Penn State Extension: 70% soil and 30% compost.
Rutgers Extension notes that a commercially blended "manufactured topsoil" has no legal specifications, and that you should check the label to see what is in a bag. Ask your supplier what is in the load before you buy in bulk. Enter the shares you decide on under "Split the total into a mix" and the calculator gives the volume of each material.
Settling
Soil can settle after you fill the bed. Rutgers Extension mentions initial settling, but the extension pages used for this tool give no percentage, so the calculator does not assume one. It gives the exact volume by default. If you want a cushion, enter an extra percentage. You can also fill the bed, let it settle, and top it off.
Bags or bulk?
Bags are easy to carry and fit small beds. For a lot of soil, bulk by the cubic yard can cost less per cubic foot, but you need a place for the delivery and a wheelbarrow. Use the cubic yard result to compare prices, and ask your supplier about delivery fees and minimum orders.
Common mistakes
- Using the outside height of the frame. Measure the depth of soil you will actually put in. If you leave 2 inches at the top, enter 2 in the empty-space field.
- Guessing the bag size. Read the volume printed on the bag. If it is not 1, 1.5, or 2 cubic feet, or 50 liters, divide the cubic feet result by your own bag size.
- Forgetting round beds. A round bed holds less than a square bed with the same width. Use the diameter, not the length of a side.
- Not allowing for other beds. Add every bed before you shop so you can compare bulk and bag prices on the full amount.
- Mix shares that don't add up. The mix percentages must total 100, or the calculator will ask you to fix them.
Sources
Bed depth on hard surfaces, and the compost and topsoil mix ratios: University of Maryland Extension, "Soil to Fill Raised Beds." Soil and compost mix: Penn State Extension, "Soil Health in Raised Beds." Topsoil specifications, label advice, 12 inch topsoil depth, and settling: Rutgers NJAES fact sheet FS1328, "Soil for Raised Beds." Cubic foot to cubic meter (0.028316846592 m³) and inch to centimeter (2.54): NIST SP 811, Appendix B.8 and the NIST Guide to the SI. A cubic yard is 27 cubic feet because a yard is 3 feet.
Related calculators
- Topsoil Calculator - soil for lawns and garden beds
- Mulch Calculator - mulch for beds and borders
- Cubic Yard Calculator - turn cubic feet into cubic yards
- Square Footage Calculator - area of a space
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
Multiply 4 × 8 × the soil depth in feet. At 12 inches deep, a 4 by 8 foot bed takes 32 cubic feet, which is about 1.19 cubic yards. At 8 inches deep it takes about 21.3 cubic feet.
How many bags of soil do I need for a raised bed?
Divide the cubic feet you need by the bag size, then round up. A 32 cubic foot bed takes 22 bags of 1.5 cubic feet (21.3 rounded up) or 16 bags of 2 cubic feet. Check the volume printed on your own bag, and use the cubic feet result if your bag size is different.
How do I figure out the soil for a round raised bed?
Use the area of a circle, which is pi times the radius squared, then multiply by the soil depth. A round bed 4 feet across and filled 10 inches deep holds about 10.5 cubic feet.
Do I fill the bed all the way to the top?
Not necessarily. Many gardeners leave an inch or two at the top so water and mulch stay in the bed. Enter that space under "Space left empty at the top" and the calculator subtracts it from the bed height.
What should I mix in a raised bed?
Extension services give different answers. The University of Maryland Extension suggests compost and purchased topsoil at a ratio of 1:2 or 1:1 for beds over compacted soil, or compost and a soilless mix at 1:1. Penn State Extension recommends 70% soil and 30% compost. Pick the mix that fits your soil and your local extension office, then enter your percentages in the calculator.
Will the soil settle after I fill the bed?
It can. Rutgers Extension mentions an initial settling after a bed is filled. Many gardeners top the bed off later. The calculator has an optional extra percentage if you want to order a little more, but no standard figure comes with it, so leave it at 0 if you prefer to top off.
How many cubic yards is 27 cubic feet?
One cubic yard. A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. For bulk delivery, divide your cubic feet by 27 to get cubic yards.