How to Calculate Soil Volume for Raised Garden Beds (2024)

The most important part of a new raised garden bed is the soil that goes inside.

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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.

How to find soil volume for square or rectangular beds

The formula for soil volume in square or rectangular beds is the easiest to calculate. 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:

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.
Multiply all the measurements together: 6 x 3 x 1.4 = 25.2 cubic feet.
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!

FAQs

How to Calculate Soil Volume for Raised Garden Beds? ›

Add a mixture of compost and purchased topsoil in a 1:2 or 1:1 ratio, to the top of the bed. There are vendors who sell topsoil mixed with compost. Alternatively, fill the bed with compost and a soilless growing mix in a 1:1 ratio.

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What is the soil ratio for raised garden beds? ›

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How do you calculate soil volume? ›

Length in feet x Width in feet x Depth in feet (inches divided by 12). Take the total and divide by 27 (the amount of cubic feet in a yard). The final figure will be the estimated amount of cubic yards required.

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How much soil for a 100 square foot garden? ›

One yard of topsoil covers 324 square feet of soil with a depth of 1 inch, or 100 square feet with a depth of 3 inches.

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How do you calculate the area of a raised garden bed? ›

Multiply the length by the width to determine the square footage—or area—of a square or rectangle. Find the square footage by multiplying the length and width of the area in question. Make sure to keep your units the same (feet or inches).

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How do I calculate how much soil I need for a raised bed? ›

To calculate:

Measure the length, width and height of your bed. (Unless the boards are very thick, don't worry about inside/outside measurements.) ...
Multiply all the measurements together: 6 x 3 x 1.4 = 25.2 cubic feet.
Divide the answer by 27 to get the number of cubic yards.

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How deep should soil be in raised bed for vegetables? ›

Vegetable Beds: On the other hand, when it comes to vegetable beds, the bed must be approximately 12 to 18 inches deep to ensure adequate depth for the roots of your plants. This is especially important if your raised bed is placed on cement or the patio, which will inhibit roots from growing deeper into the ground.

How high off the ground should a raised garden bed be? ›

Ideally, the beds should be around 24-30 inches tall to allow for comfortable gardening without bending or reaching too much.

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How to fill in raised garden beds? ›

To start, lay down sheets of cardboard or newspaper for weed suppression and then fill the raised bed structure halfway up with alternating layers of nitrogen-rich materials (like kitchen scraps and grass clippings) and carbon-rich materials (like wood chips and dried autumn leaves).

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What is the best soil mixture for a vegetable garden? ›

Gardening magazine's trial of the best soil mixture for raised beds found that a blend of 50% topsoil, 25% coarse sand and 25% compost was best.

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What should I fill my raised garden bed with? ›

Layers of yard debris, such as grass clippings, sticks, branches, trimmings, and leaves, can be beneficial when filling a raised bed for several reasons: Organic Matter: Yard debris is rich in organic matter, which helps to improve soil structure and fertility over time as it decomposes.

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What is the best soil mix for raised bed tomatoes? ›

The soil is the key ingredient to a successful raised vegetable bed. Get enough light topsoil or raised bed soil to fill the estimated depth of the raised bed. Since tomatoes are heavy feeders and prefer a rich, organic soil, mix in two- to three-inch layers of compost or cow manure to the top one-third of the topsoil.

Is there a difference between garden soil and raised bed soil? ›

Raised bed soil is nutrient rich and contains a higher percentage of organic material than garden soil. However, you should also add additional organic fertilizer throughout the growing season to ensure a productive harvest. Organic and natural fertilizers include compost, algae and worms.

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