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? ›

How to measure your area. To calculate how much topsoil you need, simply measure the dimensions of the area you wish to cover and multiply this by the depth required. Measuring in metres is simplest, as this will give you the volume of topsoil you need in cubic metres.

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How do you calculate the volume of soil needed? ›

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How to calculate cubic feet of soil? ›

To calculate cubic feet, you need to first determine the square feet of your garden. This is simply the length times the width of your garden bed. Then, to get the total cubic feet of each raised bed, simply multiply the square feet of your garden's footprint by the height of your raised bed garden.

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What is the soil ratio 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 formula for soil volume? ›

Explained: Calculate the volume of the soil by multiplying Pi by the Radius Square by the Hole Height. Most likely the soil desired amending depth with be between 12″-18″ so we'll need to convert this to feet. We do so by dividing the depth in inches by 12. For example 18″ soil depth = 18/12 = 1.5 ft.

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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 many cubic feet are in a 4x8 raised bed? ›

If you're planning a 12-inch deep raised bed, you'll need approximately 24 cubic feet of soil to fill a 4x8 bed. A depth of 18 inches will require approximately 36 cubic feet of soil. The type of soil you use will also affect the volume required to fill your raised bed.

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How to fill a raised garden bed cheaply? ›

Use the Lasagna Garden Method

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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How many cubic feet does a 40lb bag of soil cover? ›

A 40 pound bag of topsoil usually contains about 0.75 Cubic Feet of soil. There are 25.71404638 Dry Quarts in a Cubic Foot, so a 25 quart bag of potting soil would equal approximately 1 Cubic Foot.

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How do I calculate how much dirt I need? ›

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 deep should a raised garden bed be planted? ›

A 1-foot raised bed is more than enough to accommodate a salad garden box plan. Carrots, radishes and peppers thrive with 12 inches or more. Medium-rooting vegetables like kale, cucumbers, and zucchini love at least 18 inches.

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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.

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How high should you fill a raised bed with soil? ›

Soil and compost are then filled into the raised beds to a depth just a few inches (8 cm.) below its rim. A few flaws with this are that while many vegetable plants need a depth of 12 to 24 inches (31-61 cm.) for good root development, rabbits can still get into beds that are less than 2 feet (61 cm.)

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How do you calculate volume? ›

Height × width × length= volume

If the height, width and length are measured in cm, the answer will be cm³.

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How do you calculate soil capacity? ›

Researchers can calculate the bearing capacity of soil by determining the limit of contact pressure between the soil and the material placed on it. These calculations and measurements are performed on projects involving bridge foundations, retaining walls, dams and pipelines that run underground.

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How to figure cubic feet? ›

For instance, to find the volume of a cube, you would calculate the following: length x width x height. The product of this formula will return the volume of an object in total cubic feet (CFT).

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How much topsoil do I need for 1000 square feet? ›

So, for a standard-size lawn of 1,000 square feet, you would need approximately 13 cubic yards (18 tons) of topsoil to cover the entire lawn at 4 inches deep.

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How do you determine the capacity of soil? ›

Estimating Soil Capacity

Soil capacity is measured in pounds per square foot (psf). The load-bearing capacity of soils range from 4,000 psf for sedimentary rock to as little as 1,500 psf for clay and silt. Soils that are a predominantly sand or gravel typically fall in the middle, from 2,000-3,000 psf.

How do you calculate the volume of soil to be excavated? ›

General Excavation = Length x Breadth X Depth

The excavation calculator helps project managers estimate soil quantity quickly and plan logistics effectively.