Calculate soil volume by bed size and depth
The amount of soil you need depends on three things: the length and width of your bed, and how deep you fill it. Most vegetable gardens work with 10 to 12 inches of soil depth. Herbs and shallow-rooted plants like lettuce can go as shallow as 6 inches. Root vegetables like carrots and parsnips need 12 inches minimum, and potatoes need 14 to 16 inches.
To find the volume in cubic feet, multiply length × width × depth (all in feet), then divide by 12 if your depth is in inches. A 4-by-8-foot bed filled 10 inches deep needs 26.7 cubic feet of soil. A 4-by-4-foot bed at the same depth needs 13.3 cubic feet. Soil is sold by the cubic yard (27 cubic feet), so a single yard covers roughly two 4-by-8 beds at 10 inches, or four 4-by-4 beds.
Key Takeaways
- Most vegetable beds need 10 to 12 inches of soil depth; root vegetables and potatoes need 14 to 16 inches.
- Calculate volume in cubic feet by multiplying length × width × depth in feet, then convert to cubic yards by dividing by 27.
- One cubic yard of soil covers roughly two 4-by-8-foot beds at 10 inches deep, or four 4-by-4-foot beds.
- Soil settles over time, so buy 10 to 15 percent more than your calculation to account for compaction in the first season.
- Bulk soil from a landscape supplier costs less per cubic yard than bagged soil, but requires delivery or pickup.
Why depth matters for what you grow
Shallow-rooted plants like lettuce, spinach, and herbs establish in 6 to 8 inches. If you are growing only greens and herbs, you can reduce your soil volume significantly. A 4-by-8 bed at 6 inches deep needs only 16 cubic feet — less than two-thirds of a cubic yard.
Medium-rooted crops — tomatoes, peppers, beans, squash — need 10 to 12 inches to develop a strong root system and hold moisture between waterings. This is the standard depth for most mixed vegetable gardens. Root vegetables like beets, carrots, and turnips need at least 12 inches, and longer varieties like parsnips benefit from 14 to 16 inches. Potatoes are the deepest feeders in a typical garden and need 14 to 16 inches to produce a full crop without the tubers pushing through the soil surface.
Soil settling and how to account for it
New soil compacts as it settles, especially in the first season after planting. You will lose roughly 10 to 15 percent of your volume to settling, depending on how much you water and how much organic matter breaks down. This means a bed that starts at 12 inches may drop to 10 or 10.5 inches by midsummer.
To avoid ending up short, buy 10 to 15 percent more soil than your calculation shows. For a bed that needs 26.7 cubic feet, purchase 30 cubic feet (just over 1.1 cubic yards) instead. This gives you a buffer and lets you top off the bed in late spring if settling is visible. You can also add compost or aged manure to the top layer in year two, which both adds volume and improves soil structure.
Bulk soil versus bagged soil: cost and logistics
Bagged soil costs more per cubic foot but requires no delivery and no equipment. A 2-cubic-foot bag of potting mix or garden soil costs between $3 and $8 depending on quality and location. For a 4-by-8 bed at 10 inches deep (26.7 cubic feet), you would need roughly 14 bags, which runs $42 to $112. Bagged soil is convenient for small beds or when you have no way to receive a bulk delivery.
Bulk soil from a landscape supplier or soil yard costs $30 to $60 per cubic yard delivered, depending on your region and the type of soil. The same 4-by-8 bed costs $35 to $70 for bulk delivery, plus any delivery fee (typically $50 to $150 for a single yard). Bulk soil is cheaper per cubic foot but requires a place to dump it and a way to move it into the bed — a wheelbarrow and shovel, or a helper. For beds larger than 4-by-4, bulk soil almost always costs less overall.
What type of soil to buy
Garden soil and topsoil are not the same thing. Topsoil is mostly mineral — sand, silt, and clay — with little organic matter. It is cheap and works as a base layer, but plants need organic matter to hold water and nutrients. Garden soil or garden mix contains topsoil blended with compost, aged bark, or other organic material. It costs more but performs better for vegetables and perennials.
Potting mix is lighter and fluffier than garden soil, designed for containers and seed starting. It drains faster and holds less water, which is useful for pots but can dry out too quickly in a raised bed. For a raised bed, use garden soil or a blend of topsoil and compost (roughly 60 percent topsoil, 40 percent compost). If you are buying bulk, ask the supplier what is in their garden mix — some are mostly topsoil with a thin layer of compost on top, while others are genuinely blended.
Filling a bed in layers to save money
You do not have to fill the entire depth with premium garden soil. A common strategy is to use cheaper material for the bottom and reserve better soil for the top 4 to 6 inches where roots are densest. Fill the bottom half with topsoil or even coarse compost, then top with 4 to 6 inches of garden soil or a compost-rich blend.
For a 4-by-8 bed at 10 inches deep, you could use 13 cubic feet of topsoil ($20 to $30) and 13 cubic feet of garden soil ($40 to $60), for a total of around $60 to $90. This costs less than filling the whole bed with premium mix and still gives you good growing conditions. The roots of most vegetables will reach into the topsoil layer by mid-season, so the quality of that bottom layer does matter — avoid pure clay or heavily compacted soil.
Quick reference table for common bed sizes
| Bed Size | 10 Inches Deep | 12 Inches Deep | 16 Inches Deep |
|---|---|---|---|
| 2 × 4 feet | 6.7 cubic feet (0.25 yd) | 8 cubic feet (0.3 yd) | 10.7 cubic feet (0.4 yd) |
| 4 × 4 feet | 13.3 cubic feet (0.5 yd) | 16 cubic feet (0.6 yd) | 21.3 cubic feet (0.8 yd) |
| 4 × 8 feet | 26.7 cubic feet (1 yd) | 32 cubic feet (1.2 yd) | 42.7 cubic feet (1.6 yd) |
| 4 × 12 feet | 40 cubic feet (1.5 yd) | 48 cubic feet (1.8 yd) | 64 cubic feet (2.4 yd) |
Frequently Asked Questions
Can I use less soil if I add compost on top each year?
Yes, but only after the first season. In year one, you need the full depth for roots to establish. Starting in year two, you can add 2 to 3 inches of compost to the top each spring, which both adds volume and improves soil structure. Over time this builds the bed deeper, but do not skimp on the initial fill.
What if my bed is an odd shape or not rectangular?
Measure the average length and width, then multiply as usual. For a triangular or curved bed, break it into rectangles, calculate each one, and add them together. If the shape is very irregular, fill it with water first, measure how many gallons it holds, then convert to cubic feet (1 cubic foot = 7.5 gallons).
How much does a cubic yard of soil weigh?
Dry topsoil weighs around 1,300 to 1,500 pounds per cubic yard. Garden soil with compost mixed in weighs 1,400 to 1,600 pounds. Wet soil is heavier — add 200 to 300 pounds if it has been rained on recently. This matters if you are loading it yourself or if your driveway has weight limits.
Should I line the bottom of my bed with cardboard or landscape fabric?
Cardboard breaks down in one to two seasons and suppresses weeds during that time, which is useful if your bed sits on grass. Landscape fabric lasts longer but can trap water and prevent drainage if it is too thick. Neither is necessary if your bed sits on bare soil or gravel. If you use either, it goes under the soil, not on top.
Can I reuse soil from last year?
Yes, but refresh it. Remove the top 2 inches (which may contain disease spores or pest eggs), then mix the remaining soil with 2 to 3 inches of fresh compost. This replaces nutrients and organic matter that broke down over the season. If the old soil is compacted, loosen it with a garden fork before adding compost.