Key takeaways
- Using outside measurements: Thick boards can reduce the interior growing area. Measure between the inside faces of the frame.
- Forgetting the depth: A bed’s footprint alone does not tell you how much soil it needs. Doubling the depth doubles the soil volume.
- Confusing cubic feet with square feet: Square feet measure surface area; cubic feet measure the three-dimensional volume needed for soil.
- Not allowing for settling: Soil often drops after watering. Buy 10% to 15% extra, particularly when filling a new bed.
- Filling to the rim: Leave 1 to 2 inches below the top edge to limit erosion and overflow.
- Relying on bag weight: Compare cubic-foot volume, not pounds. Moisture content changes the weight of packaged soil.
- Ignoring access: Beds wider than about 4 feet can be difficult to reach without stepping into them. A narrower design may use less soil and be easier to maintain.
What's inside
The Short Answer
To determine how much soil for raised beds, multiply the bed’s length by its width and by the soil depth, then convert the result to cubic feet:
Length (ft) × width (ft) × depth (ft) = cubic feet of soil
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For example, a 4-by-8-foot raised bed filled 12 inches deep needs:
4 × 8 × 1 = 32 cubic feet
Because soil settles after watering and may compact during transport, buy about 10% to 15% more than the calculated volume. That means a 4-by-8-foot bed at 12 inches deep usually requires approximately 35 to 37 cubic feet of soil.
For bags sold by the cubic foot, divide the total volume by the bag size. A 1-cubic-foot bag covers about 1 square foot at 12 inches deep, or about 2 square feet at 6 inches deep. A 1.5-cubic-foot bag covers 1.5 square feet at a 12-inch depth.
Standard Sizes
Raised beds come in many dimensions, but these are among the most practical sizes for home gardens. The figures below show the approximate soil needed at three common depths. Add 10% to 15% when planning your purchase.
| Bed size | Dimensions in centimeters | At 6 in. deep | At 12 in. deep | At 18 in. deep |
|---|---|---|---|---|
| 2 × 4 ft | 61 × 122 cm | 4 cu. ft. | 8 cu. ft. | 12 cu. ft. |
| 2 × 8 ft | 61 × 244 cm | 8 cu. ft. | 16 cu. ft. | 24 cu. ft. |
| 3 × 6 ft | 91 × 183 cm | 9 cu. ft. | 18 cu. ft. | 27 cu. ft. |
| 4 × 4 ft | 122 × 122 cm | 8 cu. ft. | 16 cu. ft. | 24 cu. ft. |
| 4 × 8 ft | 122 × 244 cm | 16 cu. ft. | 32 cu. ft. | 48 cu. ft. |
| 4 × 12 ft | 122 × 366 cm | 24 cu. ft. | 48 cu. ft. | 72 cu. ft. |
A 6-inch depth is adequate for many leafy greens, herbs, radishes and other shallow-rooted crops. Most vegetables perform better with 12 inches of soil. Tomatoes, carrots, parsnips, potatoes and other deep-rooted plants may need 18 inches or more, depending on the variety.
When comparing products, check the bag’s stated volume rather than its weight. Soil weight varies with moisture and ingredients, so a 1-cubic-foot bag may weigh substantially more or less than another brand. For example, a 1.5-cubic-foot bag of raised-bed mix may cost roughly $25 to $32, while a 1-cubic-foot organic product may cost about $20 to $25. Large delivered quantities can cost several hundred dollars; one 60-cubic-foot option is listed around $498.
How to Measure Your Space
- Measure the inside length. Use the interior dimensions of the frame, not the outside dimensions. Measure in feet for the simplest calculation, or record inches and convert them later.
- Measure the inside width. For irregular or L-shaped beds, divide the space into rectangles and calculate each section separately.
- Choose the soil depth. Measure from the bottom of the bed to the intended finished soil level. Leave about 1 to 2 inches below the top edge to reduce runoff and keep soil from washing over the sides.
- Convert inches to feet. Divide the depth in inches by 12. A 6-inch depth is 0.5 feet; 12 inches is 1 foot; 18 inches is 1.5 feet.
- Multiply the dimensions. Multiply length by width by depth. A bed measuring 3 by 6 feet and 12 inches deep needs 18 cubic feet before allowing for settling.
- Add extra soil. Multiply the result by 1.10 or 1.15. The 18-cubic-foot example becomes approximately 20 to 21 cubic feet to purchase.
- Divide by the bag size. If using 1.5-cubic-foot bags, divide the adjusted volume by 1.5. For 21 cubic feet, you would need 14 bags.
If you measure in inches, use this alternative formula: length × width × depth ÷ 1,728 = cubic feet. For a 48-by-96-inch bed filled 12 inches deep, the calculation is 48 × 96 × 12 ÷ 1,728, or 32 cubic feet.
By Room or Use Case
Patio and Balcony Beds
Compact beds measuring 2 by 4 feet or 3 by 3 feet are easier to fit on patios and balconies. A 2-by-4-foot bed filled 12 inches deep needs 8 cubic feet of soil, or about 9 cubic feet after adding a settling allowance. Before filling, confirm the surface can support the combined weight of the frame, wet soil, plants and water. A smaller 6-inch bed reduces the soil requirement to 4 cubic feet, but it limits crop choices.
Vegetable Gardens
A 4-by-8-foot bed is a popular vegetable-garden size because it provides 32 square feet of growing area while remaining reachable from both sides. At 12 inches deep, it needs 32 cubic feet of mix before settling, or approximately 35 to 37 cubic feet to purchase. Divide the bed into rows or square-foot sections so you can avoid stepping on the soil.
Root Crops
Carrots, parsnips and potatoes benefit from deeper soil, especially long-rooted varieties. An 18-inch depth in a 4-by-4-foot bed requires 24 cubic feet before the extra allowance. If only part of the bed will contain root crops, you can use 12 inches across the entire bed and mound an additional 6 inches in that section.
Herbs and Shallow-Rooted Plants
Many herbs, lettuce, spinach and radishes can grow in 6 to 8 inches of soil. A 4-by-4-foot bed at 6 inches deep takes 8 cubic feet, or roughly 9 cubic feet with extra. Perennial herbs such as rosemary and some larger plants may need more depth and excellent drainage.
Accessible and Elevated Beds
Raised beds designed for seated or standing gardening may be 24 to 36 inches tall. They do not always need to be filled completely with premium planting mix. Use a structurally appropriate, clean filler in the lower portion only if the design allows it, then reserve quality soil for the root zone. Avoid materials that can rot, leach chemicals or restrict drainage. A deeper bed also requires careful planning because 4-by-8 feet at 24 inches deep requires 64 cubic feet of soil before settling.
Common Sizing Mistakes
- Using outside measurements: Thick boards can reduce the interior growing area. Measure between the inside faces of the frame.
- Forgetting the depth: A bed’s footprint alone does not tell you how much soil it needs. Doubling the depth doubles the soil volume.
- Confusing cubic feet with square feet: Square feet measure surface area; cubic feet measure the three-dimensional volume needed for soil.
- Not allowing for settling: Soil often drops after watering. Buy 10% to 15% extra, particularly when filling a new bed.
- Filling to the rim: Leave 1 to 2 inches below the top edge to limit erosion and overflow.
- Relying on bag weight: Compare cubic-foot volume, not pounds. Moisture content changes the weight of packaged soil.
- Ignoring access: Beds wider than about 4 feet can be difficult to reach without stepping into them. A narrower design may use less soil and be easier to maintain.
FAQ
How many bags of soil do I need for a 4-by-8-foot raised bed?
At 12 inches deep, the bed requires 32 cubic feet of soil. With a 10% to 15% allowance, plan on 35 to 37 cubic feet. That equals about 35 to 37 one-cubic-foot bags, 24 to 25 bags measuring 1.5 cubic feet, or 12 to 13 bags measuring 3 cubic feet.
How deep should soil be in a raised bed?
Use at least 6 inches for shallow-rooted greens and herbs, 12 inches for most vegetables, and 18 inches or more for many root crops and large plants. Check the mature plant’s root requirements before choosing the depth.
Can I use regular garden soil in a raised bed?
Pure garden soil can become heavy and compacted in a contained bed, reducing drainage and air space. A raised-bed blend or a mixture of quality topsoil, compost and an aerating material is generally more suitable. Follow the product label, particularly when growing food crops.
How much does raised-bed soil cost?
Packaged mixes commonly range from about $20 to $32 per 1 to 1.5 cubic feet, depending on ingredients and certification. Large quantities may be more economical when delivered, but delivery fees can change the total substantially. Compare the final cost per cubic foot before buying; prices available in 2026 can vary by retailer, region and season.
