How to Calculate Soil Volume for Raised Garden Beds

Since few of us have extra garden soil hanging around our backyards, many of us need to buy enough to fill raised beds before planting. But how do you know how much soil to order to avoid waste?

Thankfully there are formulas for calculating soil volume that can help you know exactly what you need based on the size and shape of your bed. If you are looking to calculate the amount of soil needed for a raised bed, here’s what you need to know.

How to calculate soil volume for square or rectangular beds

The formula for soil volume in square or rectangular beds is the easiest to calculate–no instant garden soil volume calculator needed. It involves only three measurements: the length, width and height of your bed.

Use the formula below:

V = L x W x H

V stands for soil volume
L stands for your bed’s length
W stands for your bed’s width
H stands for the height of your bed

To calculate:

1. Measure the length, width and height of your bed. (Unless the boards are very thick, don’t worry about inside/outside measurements.) Convert all measurements to the same unit. For example, if your bed is 6 feet long, 3 feet wide and 16.5 inches high, convert the inches to feet so your new measurements are 6 x 3 x 1.4 feet.
2. Multiply all the measurements together: 6 x 3 x 1.4 = 25.2 cubic feet.
3. Divide the answer by 27 to get the number of cubic yards. In the example above, your bed would need 0.9 cubic yards of soil.

Calculating soil volume for L-shaped beds

Finding the soil volume for an L-shaped bed involves another step, but it’s still relatively simple. Start by dividing your bed into two figures with four sides each. For example:

Once you have the measurements for these two figures, use the formula above for rectangles to calculate each figure separately. Add the totals together.

V = (L x W x H) + (L x W x H)
V = (6 x 3 x 1.4) + (3 x 3 x 1.4)
V = 25.2 + 12.6
V = 37.8 ft³

To convert to cubic yards, divide by 27.

The total is 1.4 cubic yards.

How much soil for U-shaped garden beds?

Using the same premise as above, divide your U-shaped bed into three figures and calculate each one using the formula for squares and rectangles. For example:

V = (L x W x H) + (L x W x H) + (L x W x H)
V = (9 x 4 x 1.4) + (3 x 4 x 1.4) + (3 x 4 x 1.4)
V = 50.4 + 16.8 + 16.8
V = 84 ft ³

To convert to cubic yards, divide by 27.

The total is 3.1 cubic yards.

Calculating soil volume for hexagonal beds

Hexagonal raised beds form a shape called a hexagonal prism. These beds come as regular hexagons (where all the sides are the same size) and irregular hexagons (where all the sides aren’t the same size). To calculate the volume of soil needed to fill a regular hexagonal prism, use the following formula.

V= 3/2 (√3) s²h

V equals the total soil volume
S equals the length of one side of the regular hexagon
H equals the height of the bed

Here’s an example for the image above:

V= 3/2 (√3)(3²)(2)
V = 46.77 ft³

Divide by 27 for 1.73 cubic yards.

For a shortcut you can try this online calculator for regular hexagonal prisms.

Calculating soil volume for octagonal beds

Octagonal raised beds form a shape called an octagonal prism. As above, these beds come as regular octagons (where all the sides are the same size) and irregular octagons (where all the sides aren’t the same size). To calculate the volume of soil needed to fill a regular octagonal prism, use the following formula.

V = 2 (1 + √2) S²H

V equals the total soil volume
S equals the length of one side of the regular octagon
H equals the height of the bed

Here’s an example for the bed pictured above:

V = 2 (1 + √2)(3²)(1.4)
V = 60.84 ft³

Divide the total by 27 to get 2.3 cubic yards.

To instantly calculate regular octagonal volume, try this online calculator.

Finding soil volume for irregular shapes

If you’re comfortable with a volume estimate, you can treat your long octagon (or hexagon) as a rectangle measured from its longest point and use the formula above to gauge how much soil you’ll need to fill a corresponding four-sided figure.

One way to calculate soil volume for irregular shapes is to treat them like the closest sized rectangle and use the formula for rectangular beds.

If you want to be more exact, you have another option. A straightforward way to find the volume of any irregular prism is to first calculate the area of the base and then multiply that measurement by the height. If you have a long hexagon or octagon, you can separate the base into triangles and a rectangle and calculate the area for each. After adding them together for the base area, multiply this total by the height. This will give you the cubic measurement you’ll need for soil volume. (To calculate area of a triangle, use ½ the base multiplied by the height).

Measuring oddly shaped beds accurately

While soil volume can usually be calculated using the outside dimensions of a bed, some beds vary a lot between inside and outside measurements. In these cases, it’s best to take your measurements from inside or you may end up with extra garden soil.

Farmstead beds, for example, are constructed using a mortis and tenon construction. This makes the ends of the board quite a bit larger than the interior dimensions.

Getting it right

Knowing you have the right soil volume can save you time and money. By filling your beds adequately, you’ll not only make the best use of your space, you’ll give your plants the space they need to thrive.

Do you have pictures or stories about your raised beds to share? Leave a comment with us below!