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.