The Amount of Dirt a Raised Garden Bed Needs Explained

How Much Dirt Does a Raised Garden Bed Need

The amount of dirt needed for a raised garden bed depends primarily on the bed’s dimensions and the types of plants you intend to grow. To calculate the volume, multiply the length, width, and depth of your bed. For example, a 4×8 footbed that’s 12 inches deep would require 32 cubic feet of soil.

However, it’s important to account for settling, so adding 10-20% more soil is something I recommend. The depth of your bed should be determined by the root depth of your planned plants, with shallow-rooted plants needing 6-8 inches and deep-rooted plants requiring up to 24 inches or more.

Key Takeaways

  • Use the formula – Volume = Length x Width x Depth to calculate soil needed
  • Consider plant root depth when determining bed depth
  • A typical soil mix includes 55% topsoil, 30% compost, and 15% sand
  • Add 10-20% extra soil to account for settling and compaction
  • Convert cubic feet to cubic yards when ordering in bulk (1 cubic yard = 27 cubic feet)

Calculating Soil Volume

To determine how much dirt for a raised garden bed you need, this is a simple formula I use.

  • Volume = Length x Width x Depth
  • For example, a bed measuring 4 feet long, 2 feet wide, and 1 foot deep would require:
    4 x 2 x 1 = 8 cubic feet of soil

For circular or uniquely shaped beds, you may need to adjust your calculations. Many online soil calculators can help with these more complex shapes.

Remember to convert cubic feet to cubic yards if ordering in bulk. 1 cubic yard = 27 cubic feet.

Recommended Soil Depths for Different Plants

The depth of your raised bed will significantly impact how much garden soil you need.

  • Shallow-rooted plants (6-8 inches): Lettuce, herbs, radishes
  • Medium-rooted plants (12-18 inches): Tomatoes, peppers, cucumbers
  • I suggest using it for (18-24 inches or more): Carrots, parsnips, asparagus

Soil Mixture Considerations

Quality soil is crucial for raised bed success. I have found this to be a good mix for most plants.

  • 55% topsoil
  • 30% compost
  • 15% sand for drainage

You can purchase pre-mixed raised bed soil or create your blend. When calculating how much dirt for a raised garden bed you need, factor in these components separately if mixing your own.

Accounting for Settling and Compaction

Soil naturally settles over time, so it’s wise to overfill your bed initially. I have found that adding an extra 10-20% to your calculated volume to account for this settling.

Calculating Soil for a Typical Raised Bed

Let’s consider a common raised bed size: 4 feet wide, 8 feet long, and 12 inches deep.

  • Volume calculation: 4 x 8 x 1 = 32 cubic feet
  • Accounting for 15% settling: 32 x 1.15 = 36.8 cubic feet
  • Converting to cubic yards: 36.8 ÷ 27 = 1.36 cubic yards
  • For this bed, you would need approximately 1.4 cubic yards of soil mix.

Joel Simon

As a kid, Joel found enjoyment in caring for the many houseplants he grew up with, learning how to transplant them safely, cloning them, and more. At about the age of 10, he wanted to see if he could sprout an orange seed from a store-bought orange and ended up using it as a science experiment in a school project. Throughout the many years of gardening, he has helped many friends and family set up their food and botanical gardens. After years of caring for plants, he was talking with other gardeners and discovering old methods of farming and botanical gardening. Joel has decided to share his knowledge for others to enjoy as he has for many years.

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