Raised Bed Soil Volume Calculator

Why Soil Volume Matters for Raised Beds

Accurately estimating raised bed soil volume helps you avoid both running short and overspending on bagged mixes or bulk deliveries. Raised beds typically use a lighter, well-draining mix than native soil, and that material is often more expensive. A simple volume calculation lets you plan how much to buy and how to blend compost, topsoil, and other ingredients for healthy plants.

Soil volume affects root depth, drainage, and water retention. Too little depth can stress plants, especially deep-rooted crops like tomatoes or carrots. Extremely deep beds can be costly to fill and may not provide extra benefit for shallow-rooted plants like lettuce or many herbs. Using the calculator on this page, you can match bed depth and volume to the plants you intend to grow and budget accordingly.

How the Raised Bed Soil Volume Calculator Works

The calculator assumes a straight-sided, rectangular raised bed with a uniform depth. You enter:

• Bed length in feet
• Bed width in feet
• Bed depth in inches
• Number of identical beds (optional, defaults to one)
• Optional bag volume in cubic feet to estimate bag counts

The tool first converts the depth from inches to feet, then calculates soil volume per bed and for all beds combined. Because soil is commonly sold in cubic feet or cubic yards, the calculator focuses on these units and also uses your bag size (if entered) to approximate how many bags you should purchase.

Formula for Raised Bed Soil Volume

Step 1: Convert the bed depth from inches to feet:

depth_feet = depth_inches ÷ 12

Step 2: Compute cubic feet for one raised bed:

volume_per_bed_cu_ft = length_ft × width_ft × depth_feet

Step 3: Multiply by the number of identical beds:

total_volume_cu_ft = volume_per_bed_cu_ft × bed_count

The calculator can also show cubic yards and approximate bag counts. A cubic yard is 27 cubic feet, so:

total_volume_cu_yd = total_volume_cu_ft ÷ 27

Formula in MathML

The total volume for all beds, in cubic feet, can be represented as:

Where:

• = total soil volume (cubic feet)
• = bed length (feet)
• = bed width (feet)
• = bed depth (inches)
• = number of identical beds

Interpreting Your Soil Volume Results

When you use the calculator, you will typically see:

• Cubic feet per bed – the volume needed to fill one raised bed to the depth you entered.
• Total cubic feet – the volume needed for all identical beds combined.
• Cubic yards (optional) – useful when comparing with bulk soil or compost delivered by truck.
• Approximate bag count – if you enter a bag volume in cubic feet, the calculator divides the total volume by that number and suggests how many bags to buy.

Because soil settles after watering and as organic matter decomposes, it is often wise to round up to the next full bag or to add 5–10% to the total volume when ordering. This helps you avoid a bed that ends up visibly low after the first few waterings.

Common Unit Conversions

Here are typical conversions related to raised bed soil volume:

• Cubic feet to cubic yards: divide cubic feet by 27.
• Cubic yards to cubic feet: multiply cubic yards by 27.
• Cubic feet to liters: multiply cubic feet by approximately 28.3.
• Cubic feet to 1-liter units: multiply cubic feet by 28.3, or equivalently, divide cubic feet by about 0.035 to estimate the number of 1-liter units.

If your soil or compost is sold in quarts, remember that 1 cubic foot is about 29.9 liters, and 1 liter is roughly 1.06 US quarts. These are approximations but are sufficient for garden planning.

Worked Example: 4×8 Raised Bed

Suppose you are filling a common vegetable garden bed that is 8 feet long, 4 feet wide, and 12 inches deep. You have three identical beds, and soil is sold in 1.5 cubic foot bags.

1. Convert depth to feet

   Depth = 12 inches. Convert to feet: 12 ÷ 12 = 1 foot.

2. Calculate volume per bed (cubic feet)

   Volume per bed = 8 ft × 4 ft × 1 ft = 32 cubic feet.

3. Calculate total volume for all beds

   Total volume = 32 cubic feet × 3 beds = 96 cubic feet.

4. Convert to cubic yards

   96 cubic feet ÷ 27 ≈ 3.56 cubic yards. If ordering in bulk, you would typically order about 3.5 to 4 cubic yards, depending on how much settling margin you want.

5. Estimate bag count

   Each bag holds 1.5 cubic feet. Bag count = 96 ÷ 1.5 ≈ 64 bags. In practice you might round up to 65 or 68 bags to allow for settling and any spillage.

The calculator automates these steps: you simply enter the dimensions, bed count, and bag size, and it outputs both the total volume and the approximate number of bags you need.

Reference Table: Typical Raised Bed Volumes

The table below shows approximate soil volumes for a few common raised bed sizes at different depths. These estimates assume straight-sided beds with inner dimensions as listed.

Bag counts are rounded up to the nearest whole bag and include a small buffer for settling. Use the calculator for exact values matching your specific dimensions and bag size.

Recommended Soil Depths for Raised Beds

Not all crops require the same soil depth. When planning your raised beds, you can often save money by choosing a depth that matches what you want to grow:

• 6–8 inches: Suitable for shallow-rooted greens and herbs (lettuce, spinach, basil) when placed on top of existing soil that roots can enter.
• 10–12 inches: A good general-purpose depth for most vegetables, including peppers, bush beans, and many flowers.
• 12–18 inches: Better for deep-rooted plants like tomatoes, squash, and many perennials, especially where native soil is poor or compacted.
• 18+ inches: Typically reserved for special situations, such as very deep-rooted shrubs, or when the bed is on top of hard surfaces where roots cannot grow downward.

If your beds are open to native soil underneath and that soil is reasonably healthy, roots will often grow beyond the raised bed soil into the ground below. In that case you may not need extreme raised bed depth; 8–12 inches of improved soil on top is often sufficient.

Choosing and Mixing Soil for Raised Beds

A common starting point for raised bed soil is a mix that balances drainage, water retention, and nutrients. One widely used guideline is:

• About 1/3 compost (finished compost, well-rotted manure, or a blend).
• About 1/3 topsoil (screened, weed-free, not overly clayey).
• About 1/3 coarse material for structure (such as peat moss, coconut coir, perlite, or pine bark fines).

You can modify these proportions based on your climate, irrigation, and crops:

• Leafy greens and heavy feeders benefit from higher compost content, but avoid going to 100% compost, which can drain and dry inconsistently.
• Root crops like carrots and parsnips prefer loose, stone-free soil with moderate fertility and good structure so roots can grow straight.
• Dry climates might justify more moisture-retentive ingredients like coconut coir, while very wet climates may require extra coarse material to prevent waterlogging.

Thoroughly mix the ingredients before filling the bed to avoid distinct layers that could impede root movement and water flow. Some gardeners choose to layer materials (for example, sticks and coarse debris at the bottom, then alternating layers of compost and topsoil). Over time, these layers tend to blend as materials break down, but initial mixing still improves consistency.

Assumptions, Limitations, and Practical Tips

The calculator is designed as a planning tool. While it provides a solid estimate of how much soil you need, real-world factors can cause actual requirements to differ. Keep the following points in mind:

• Rectangular, straight-sided beds: The formula assumes your bed is a perfect rectangle with vertical sides. Irregular shapes, tapered sides, or curved beds will have different volumes.
• Inner dimensions: For best accuracy, measure the inside length and width of the bed, not the outside edge of the boards. Lumber thickness reduces internal volume.
• Uniform depth: The calculation expects the depth to be the same across the entire bed. If the bed sits on a slope or you plan a sloped soil profile, the true volume may vary.
• Settling and compaction: Freshly filled beds are often fluffy and can settle 5–10% (or more) after watering and over the first season. Order a little extra material to compensate.
• Layered or hugelkultur-style beds: If you fill the bottom with logs, large branches, or thick layers of coarse wood chips, those materials displace soil. In that case, either reduce the depth you enter in the calculator or understand that less soil will be needed than the calculated value.
• Moisture content and bulk density: Wet or compacted soil weighs more and packs more tightly than dry, fluffy mixes. Volume-based calculations do not account for weight or density changes.

If you are on the fence between two order sizes, consider how difficult it is to store leftover soil, and whether you have other garden projects (like filling containers or low spots in the yard) that could use surplus material.

Frequently Asked Questions

How deep should a raised bed be for vegetables?

For most vegetables, 10–12 inches of good-quality soil works well. If your beds sit directly on native soil that roots can enter, 8 inches may be enough for shallow-rooted crops. For deep-rooted plants like tomatoes, or if you are gardening on top of concrete or very poor soil, 12–18 inches is a safer target.

How much extra soil should I order for settling?

A common rule of thumb is to add 5–10% to the calculated volume. Very fluffy mixes high in compost or bark may settle even more during the first season. If you are filling deep beds or using coarse organic material that will break down, err toward the higher end of that range.

Can I fill the bottom of a raised bed with logs, branches, or other wood?

Yes. Many gardeners use a hugelkultur-inspired approach, placing logs, branches, or coarse woody material at the bottom to save on soil and improve long-term moisture retention. Wood will gradually decompose and reduce the soil level over time, so you may need to top up your beds in future seasons. If you use a thick wood layer, you can either subtract that depth from what you enter in the calculator or simply expect to use less soil than the calculator suggests.

What is the best soil mix ratio for raised beds?

There is no single perfect recipe, but a starting point of roughly one-third compost, one-third topsoil, and one-third coarse material (such as coir, peat, perlite, or bark fines) works well for many gardens. Adjust based on your crops and climate: more compost for heavy feeders and poor native soil, more coarse material for heavy clay areas that need drainage, and more moisture-retentive ingredients in hot, dry regions.