Quick Answer
For a 4×8 foot raised garden bed, you will need approximately 8-10 bags of soil, depending on the depth you want to fill the bed. A standard bag of soil contains 1 cubic foot of soil. To fill a 4×8 foot bed 6 inches deep, you would need around 8 bags. For a 12 inch deep bed, plan for 16-20 bags. The exact amount can vary slightly depending on soil type and bed construction.
Calculating Soil Needs for a Raised Bed
Figuring out soil needs for raised garden beds is a simple matter of math, but you need to know a few key details first:
1. Bed Dimensions
The length, width, and height of your raised bed determine its total volume. For a 4×8 foot bed, the volume would be:
Length x Width x Height
4 feet x 8 feet x (height in feet)
If doing a bed that is 6 inches (0.5 feet) deep, the math would be:
4 x 8 x 0.5 = 16 cubic feet
For a 1 foot deep bed, it would be:
4 x 8 x 1 = 32 cubic feet
The deeper the bed, the more soil volume there will be.
2. Soil Density and Air Space
Bagged soil is usually sold by the cubic foot. However, a cubic foot of lightweight, fluffy soil will be less dense and have more air space than a cubic foot of compacted clay soil. The air pockets take up volume.
As a result, you need 10-20% more soil than the calculated volume to account for settling and air space. Aim for adding an extra 1-2 cubic feet for every 10 cubic feet of calculated volume.
For a bed with 32 cubic feet of volume, getting 35-38 cubic feet of bagged soil allows for proper filling.
3. Bed Construction and Depth
Raised beds are commonly 6-12 inches deep, sometimes up to 18 inches. The framing boards around the edges take up some inside space as well.
For example, a bed framed with 2×8 lumber would have an internal width of around 7 1⁄2 inches less than the outer dimensions. An 8 foot long bed would only have an interior width of about 92 inches after subtracting the widths of the boards.
This needs to be accounted for in the soil calculations, especially for smaller beds. Going with a shallower depth or using thinner lumber reduces this disparity.
Soil Volume for a 4×8 Foot Raised Bed
Putting all this information together, here are some common examples for filling a 4×8 foot raised bed:
6 inch (0.5 foot) deep bed:
– Outer dimensions: 4ft x 8ft = 32 sq ft surface area
– Inner dimensions: Approximately 3.5ft x 7.5ft = 26 sq ft surface area
– Volume: 26 sq ft x 0.5ft deep = 13 cu ft of soil needed
– With 10-20% extra for air space/settling: 15-16 cu ft of soil
– Bags needed (1 cu ft per bag): 15-16 bags
12 inch (1 foot) deep bed:
– Outer dimensions: 4ft x 8ft = 32 sq ft surface area
– Inner dimensions: Approximately 3.5ft x 7.5ft = 26 sq ft surface area
– Volume: 26 sq ft x 1ft deep = 26 cu ft of soil needed
– With 10-20% extra for air space/settling: 29-31 cu ft of soil
– Bags needed (1 cu ft per bag): 29-31 bags
18 inch (1.5 foot) deep bed:
– Outer dimensions: 4ft x 8ft = 32 sq ft surface area
– Inner dimensions: Approximately 3.5ft x 7.5ft = 26 sq ft surface area
– Volume: 26 sq ft x 1.5ft deep = 39 cu ft of soil needed
– With 10-20% extra for air space/settling: 43-47 cu ft of soil
– Bags needed (1 cu ft per bag): 43-47 bags
As you can see, the deeper the bed, the more soil required!
Choosing a Soil Depth
For most gardens, a 6-12 inch soil depth is sufficient. Here are some factors to help choose the best depth:
- Root crops like carrots and potatoes need deeper soil, around 12-18 inches.
- Shallow-rooted crops like lettuce, spinach, and herbs do fine in just 6 inches of soil.
- Tomatoes, peppers, and other veggies are happy with 8-12 inches of soil depth.
- Deeper beds hold more moisture and nutrients.
- Shallower beds are easier to reach across and cost less to fill.
Match the soil depth to the crop types you want to grow. For a mix of plants, 12 inches is usually ideal and provides flexibility.
Saving Money on Garden Soil
Bagged topsoil, compost, manure, and other pre-mixed soils can get expensive, especially for large beds. Here are some tips for reducing costs:
- Check bulk pricing at soil yards – bulk delivery is cheaper per cubic foot than bags.
- Use square foot gardening techniques to reduce pathways and make beds smaller.
- Amend native soil from the bed area rather than bringing in 100% new material.
- Make your own compost and mix with native soil.
- Apply compost as a thin top layer rather than mixing it throughout.
- Replace a percentage of soil mix with cheaper amendments like coir, peat moss, or clean sand.
With some planning and creativity, you can cut down the number of bags needed and create an excellent growing medium.
Raised Bed Soil Recipe Ideas
Rather than buying expensive pre-mixed soil, many gardeners prefer to amend their native soil or create their own custom soil blend. Here are some raised bed soil recipes to try:
Simple Native Soil Blend
– 1 part compost
– 1 part coarse sand or perlite
– 2 parts native topsoil from the bed area
Lightweight Mix for Roof Gardens
– 1 part compost
– 1 part coconut coir or peat moss
– 1 part vermiculite or perlite
Soilless Mix for Containers
– 1 part compost or worm castings
– 1 part peat or coco coir
– 1 part perlite or vermiculite
Rich Organic Vegetable Soil
– 2 parts topsoil
– 2 parts compost
– 1 part manure
– 1 part peat
Simple Biointensive Soil
– 1 part compost
– 2 parts topsoil
– 1 part shredded leaves or straw
Experiment to find a soil recipe that works for your garden!
Finding the Right Soil Products
With so many soil products available, it can be tricky choosing what to use when building a new raised bed. Follow these tips:
- For the base, use topsoil or a 50/50 blend of compost and topsoil.
- Add compost, manure, or leaf mold to improve fertility and moisture holding capacity.
- Incorporate perlite or vermiculite for drainage and aeration.
- Use coir or peat to help retain moisture and add organic matter.
- Sand can add weight and improve heavy clay soils but isn’t necessary in most cases.
- Don’t use subsoil, straight sand, or constructon fill dirt as the main component.
- Avoid soil with persistent weeds, irritating woody debris, or chemical contamination.
- Read labels carefully and ask questions at the soil yard to understand what you are getting.
With a blend of topsoil and 1-2 good organic amendments, you can create an ideal raised bed soil. Determining how many bags you need is then simply a matter of calculating the bed size and desired depth.
Filling Raised Beds from On-Site Sources
In some cases, you may be able to fill or partially fill garden beds with on-site soil sources:
- Excavated soil from leveling the bed area
- Amended native surface soil from the installation zone
- Upcycled soil from renovating a lawn or old garden
- Nutrient-rich soil from a compost pile or bin
- Leaf mold, shredded leaves, or straw from onsite trees and plants
This can drastically reduce or even eliminate the need to import bagged soil.
However, on-site soil may still need amendments to optimize texture and fertility. Test the native soil and add organic matter like compost as needed to create an ideal growing medium.
Considerations for In-Ground Beds vs. Raised Beds
When creating new garden beds, you can choose to build them as raised beds or at ground level:
Benefits of Raised Beds
- Can be constructed on top of poor native soil
- Allow for improved drainage
- Soil warming occurs earlier in spring
- Easy to amend and enrich soil mix
- Clear separation from lawn areas
- Clean defined edges for a neat appearance
- Enable accessible gardening at waist height
Benefits of In-Ground Beds
- Less work to dig down vs. build up
- Don’t require added infrastructure and costs
- Allows plants to access deeper subsoil minerals and moisture
- Can utilize beneficial native soil organisms
- Natural gradual transitions at edges of beds
In most cases, the extra effort of raised beds is worthwhile for easier gardening, improved drainage, and healthier plants. But assess your own site conditions and gardening goals when deciding which approach is right.
If building up in-ground beds, many of the same soil volume calculations would apply. Just focus on the finished width, length, and depth rather than working with the height of a frame.
Improving Drainage in Raised Beds
While raised beds allow you to engineer the perfect soil environment for plants, drainage issues can still occur under some conditions:
- Compaction below beds prevents percolation
- Slow draining native clay soils
- Excessive rainfall or overwatering
- Bed height greater than width blocks surface evaporation
- Lack of drainage holes if using solid-walled planter boxes
To improve drainage for raised beds:
- Add 10-20% sand or perlite to soil mix
- Install drainage holes in walled planter boxes
- Include subsurface drain pipes and gravel
- Elevate beds over gravel, wood chips, or landscape fabric
- Loosens and amend native soil below beds
- Shape beds so the middle is slightly raised for drainage
With a well-draining foundation and soil mix, raised beds should avoid excess water issues.
Conclusion
When starting a new raised bed garden, calculating soil needs is an important early step. For a typical 4 foot by 8 foot bed, plan on buying around 8-10 cubic foot bags of soil for every 6 inches of planting depth. Deeper beds will need twice as many bags. Factor in the inner dimensions of your framed bed and aim for an extra 10-20% soil to account for settling. With these simple volume calculations and soil amending tips in mind, you’ll be ready to successfully fill your new raised garden beds!
