Why Measure Soil Volume?

Building raised garden beds is a popular way to grow vegetables and flowers with fewer weeds and better drainage. However, filling those beds can get expensive if you miscalculate how much soil is needed. Our Raised Bed Soil Volume Calculator converts your bed's length, width, and depth into cubic feet so you can order the right amount of soil mix the first time. Knowing the volume also helps you blend your own mix of compost, topsoil, and amendments for healthy plant growth.

Soil volume isn't just about how much dirt you need. It determines how deep plant roots can grow and affects drainage. Too shallow and your plants may dry out quickly or become root bound. Too deep and you'll spend extra money filling space that may not benefit shallow-rooted crops. By carefully measuring bed dimensions and using this calculator, you can design a bed that suits the vegetables or flowers you want to grow while keeping costs under control.

How the Calculation Works

We first convert the bed depth from inches to feet because soil is typically sold by the cubic foot. Then we multiply length × width × depth to get total cubic feet. For example, an 8-foot by 4-foot bed that's 12 inches deep converts to 1 foot deep. Multiplying 8 × 4 × 1 gives 32 cubic feet of soil needed. If you buy soil in bags listed as quarts or liters, divide cubic feet by 0.035 to estimate the number of 1-liter units required.

Remember that many garden beds settle a bit once watered, especially if your mix is fluffy with compost or peat. Ordering an extra 5–10% can help compensate for settling. Some gardeners also add a coarse base layer of sticks or wood chips at the bottom for drainage, which reduces the soil volume slightly. Include or exclude that layer when entering bed depth depending on your design.

Recommended Soil Mixes

A common raised bed blend uses roughly one-third compost, one-third topsoil, and one-third coarse material like perlite or peat moss to ensure good drainage. Adjust these ratios based on what you plan to grow. Leafy greens thrive in nutrient-rich soil, while root vegetables prefer looser mixes so they can expand easily. Mix ingredients thoroughly before filling your bed to avoid dense layers that can impede root growth.

Some gardeners layer materials rather than mixing them, placing coarse twigs or straw at the bottom, followed by alternating layers of compost and soil. Over time, the layers break down and settle, adding organic matter to the bed. If you take this approach, include the total depth of all layers in the calculator so you know how much material to gather.

Example Scenario

Imagine you're planning two 6-foot by 3-foot beds, each 18 inches deep. First, convert 18 inches to 1.5 feet. Multiply 6 × 3 × 1.5 to get 27 cubic feet per bed. For both beds, you'll need about 54 cubic feet of soil mix. Buying in bulk is often cheaper than purchasing individual bags, so consider contacting a local landscape supplier if you have multiple beds to fill.

Tips for Filling Your Beds

Before adding soil, line wooden beds with landscaping fabric or cardboard to slow weed growth and prevent soil from washing out the sides. If your bed is deeper than 18 inches, you can save money by partially filling the bottom with coarse organic material that will break down over time. As you add soil, water lightly every few inches to help it settle and remove large air pockets. Level the surface with a rake when you're done.

Keep in mind that soil quality can vary widely. If using purchased topsoil or compost, look for products that have been screened and are free from weed seeds or contaminants. Many local garden centers sell blended raised bed mixes specifically formulated for vegetables. Investing in good soil up front leads to healthier plants and fewer headaches later in the season.

Conclusion

Raised beds offer excellent control over soil conditions, but filling them correctly is essential for plant success. With our calculator, you'll know precisely how much soil to purchase or prepare. This saves money, reduces waste, and ensures your plants have the depth they need to flourish. Use the calculation results along with the tips above, and your raised beds will be off to a productive start.