Raised Bed Soil Calculator
Find out exactly how much soil you need for your raised garden beds. Enter your bed dimensions and quantity to get results in cubic yards, cubic feet, and number of bags.
How to Calculate Soil for Raised Beds
Calculating the right amount of soil for your raised bed is straightforward: multiply the length by the width by the depth. The key is converting your measurements to the same units. Since bed length and width are typically in feet but depth is in inches, divide your depth by 12 to convert to feet before multiplying. The result is the volume in cubic feet, which you can then convert to cubic yards by dividing by 27.
Getting the right amount of soil saves money and avoids the hassle of making multiple trips to the garden center. Too little soil leaves plants with shallow root zones, while too much is wasted money.
Ideal Soil Mix for Raised Beds
The best soil for raised beds is a balanced blend that provides nutrients, retains moisture, and drains well. The most widely recommended mix follows the rule of thirds:
- 1/3 Quality Topsoil: Provides the mineral base and structure. Look for screened topsoil free of debris and weed seeds.
- 1/3 Compost: Supplies essential nutrients and beneficial microorganisms. A blend of mushroom compost, composted manure, and yard waste compost works best. This is the engine that feeds your plants.
- 1/3 Drainage Material: Perlite, vermiculite, or coarse builder's sand improves drainage and prevents soil compaction. Good drainage ensures roots get enough oxygen and prevents waterlogging.
Common Raised Bed Sizes
Raised beds come in many sizes, but a few dimensions are especially popular among home gardeners:
- 4 x 8 feet: The most popular size. Provides 32 square feet of growing space while allowing you to reach the center from either side. At 12 inches deep, it requires about 32 cubic feet (1.19 cubic yards) of soil.
- 4 x 4 feet: Ideal for small yards or square-foot gardening. At 12 inches deep, it needs about 16 cubic feet (0.59 cubic yards) of soil.
- 3 x 6 feet: A good compromise for narrow spaces. At 12 inches deep, it requires about 18 cubic feet (0.67 cubic yards) of soil.
- 2 x 8 feet: Narrow beds perfect for along fences or walls. At 12 inches deep, needs about 16 cubic feet (0.59 cubic yards).
Recommended Soil Depth
The ideal depth for your raised bed depends on what you plan to grow:
- 6 inches: Suitable for herbs, lettuce, and shallow-rooted flowers.
- 8 to 12 inches: Works for most vegetables including tomatoes, peppers, beans, and squash. This is the most versatile range for general gardening.
- 12 to 18 inches: Best for root vegetables like carrots, parsnips, and potatoes that need deep soil.
- 18 to 24 inches: Recommended when your raised bed sits on concrete, asphalt, or heavily compacted soil where roots cannot penetrate below.
Frequently Asked Questions
How deep should a raised bed be?
Most raised beds should be at least 8 to 12 inches deep for growing vegetables and flowers. A depth of 12 inches accommodates most plant root systems and provides good drainage. For root vegetables like carrots and potatoes, 12 to 18 inches is ideal. If your raised bed sits on concrete or heavily compacted soil, go deeper—at least 18 to 24 inches—since roots cannot penetrate the surface below.
What is the best soil mix for raised beds?
The ideal raised bed soil mix follows the rule of thirds: 1/3 quality topsoil, 1/3 compost (for nutrients and organic matter), and 1/3 drainage material such as perlite, vermiculite, or coarse sand. This blend provides an excellent balance of water retention, drainage, and nutrients. You can also use Mel's Mix, which is equal parts peat moss or coconut coir, vermiculite, and blended compost from multiple sources.
How much soil do I need for a 4x8 raised bed?
A 4x8 foot raised bed at 12 inches deep requires approximately 32 cubic feet of soil, which is about 1.19 cubic yards. At 8 inches deep, you will need about 21.3 cubic feet (0.79 cubic yards). This translates to roughly 16 bags of 2-cubic-foot soil at 12 inches deep, or about 11 bags at 8 inches deep. For multiple beds, buying soil in bulk by the cubic yard from a landscape supply company is usually more economical.
Should I fill the entire raised bed with soil?
For beds deeper than 18 inches, you can fill the bottom portion with a layer of organic filler material like logs, branches, leaves, or straw (a technique called Hugelkultur). This saves money on soil, improves drainage, and as the organic material decomposes it feeds your plants over time. Fill the top 12 to 18 inches with your quality soil mix for the best growing results.