How to Use This Soil Calculator
This calculator takes the guesswork out of buying soil for your raised garden beds. Instead of eyeballing it at the garden center, you'll know exactly how many bags to load in your car.
- Select your bed shape. Most raised beds are rectangles, but we also support L-shaped, U-shaped, and circular beds for more complex garden layouts.
- Enter your dimensions. Measure your bed's length and width in feet, and depth in inches. For L and U shapes, you'll enter dimensions for each section.
- Adjust the number of beds. Building multiple identical beds? Just change this number instead of calculating separately.
- Use partial fill for existing beds. If you're just topping off beds that have settled, check the partial fill box and enter how many inches you need to add.
- Check your results. You'll see total volume in cubic feet and yards, bags needed in common sizes, a recommended soil mix breakdown, and cost estimates.
The calculator recommends a 40/40/20 mix of topsoil, compost, and drainage material—an ideal blend for most vegetables and flowers.
Understanding Soil Volume and Mix
Calculating soil volume is straightforward geometry. For a rectangular bed, multiply length × width × depth (in the same units). The tricky part is that beds are typically measured in feet for length and width, but inches for depth—so we convert depth to feet first (divide by 12).
The Volume Formula
Rectangle: Length (ft) × Width (ft) × Depth (ft) = Cubic Feet
Circle: π × radius² × Depth (ft) = Cubic Feet
L-Shape: Calculate two rectangles and add them
U-Shape: Calculate outer rectangle minus inner cutout
Why Soil Mix Matters
Raised beds drain differently than in-ground gardens. Pure topsoil compacts too densely and holds too much water. Pure compost is too light and dries out quickly. The 40/40/20 blend provides:
- Topsoil (40%): Structure and mineral content
- Compost (40%): Nutrients and beneficial microbes
- Drainage material (20%): Perlite or vermiculite prevents compaction and improves aeration
Buying Bagged vs Bulk
For beds under 16 cubic feet, bagged soil from a garden center is convenient. Above that, consider bulk delivery—it's often cheaper per cubic yard and you avoid lifting dozens of heavy bags. One cubic yard equals 27 cubic feet, enough to fill a 4×8 bed about 10 inches deep.
Examples
Example 1: Standard 4×8 Raised Bed at 12" Deep
The most popular raised bed size for vegetable gardens.
- Dimensions: 4 ft × 8 ft × 12 inches (1 ft) = 32 cubic feet
- Bags needed: 22 bags (1.5 cu ft) or 16 bags (2 cu ft)
- Mix: 12.8 cu ft topsoil + 12.8 cu ft compost + 6.4 cu ft perlite
- Cost: ~$88-121 in bags, or about $60-80 for 1.2 cubic yards bulk
Example 2: Two 4×4 Beds at 10" Deep
A pair of square beds, common for square foot gardening.
- Each bed: 4 ft × 4 ft × 0.83 ft = 13.3 cubic feet
- Total for both: 26.6 cubic feet
- Bags needed: 18 bags (1.5 cu ft) or 14 bags (2 cu ft)
- Mix: 10.6 cu ft topsoil + 10.6 cu ft compost + 5.3 cu ft drainage
Example 3: L-Shaped Bed for a Corner
An L-shaped bed with an 8×4 long section and a 4×4 short section at 12" deep.
- Long section: 8 × 4 × 1 = 32 cu ft
- Short section: 4 × 4 × 1 = 16 cu ft
- Total: 48 cubic feet (1.78 cubic yards)
- Bags needed: 32 bags (1.5 cu ft) or 24 bags (2 cu ft)
- Consider bulk delivery at this volume—it's around 2 cubic yards
Frequently Asked Questions
How deep should a raised garden bed be?
For most vegetables and flowers, 12 inches (30 cm) of soil depth is ideal. Shallow-rooted plants like lettuce and herbs can grow in 6-8 inches. Root vegetables like carrots, parsnips, and potatoes need 18-24 inches. If your raised bed sits on concrete or compacted soil, add extra depth since roots can't penetrate below.
What is the best soil mix for raised beds?
The classic recipe is 40% topsoil, 40% compost, and 20% drainage material (perlite, vermiculite, or coarse sand). This creates a nutrient-rich, well-draining mix. Some gardeners use "Mel's Mix" from Square Foot Gardening: equal parts peat moss (or coco coir), vermiculite, and blended compost. Avoid using only garden soil—it compacts too much in raised beds.
How do I refresh existing raised bed soil?
Each spring, add 2-4 inches of fresh compost to the top of your beds. Soil naturally compacts and decomposes over the season. Every 2-3 years, consider adding a few inches of new soil mix. Use the "fill only top inches" option in the calculator to determine how much you need for refreshing rather than filling an empty bed.
Do raised garden beds need drainage?
Yes, drainage is critical. Raised beds should have no bottom or a bottom with drainage holes. The 20% drainage material (perlite, vermiculite) in your soil mix helps water flow through. Avoid lining the bottom with plastic. If drainage is a concern, add a 2-inch layer of gravel below your soil mix.
Should I line my raised garden bed?
It depends. Hardware cloth on the bottom prevents burrowing pests like gophers and moles. Landscape fabric can prevent weeds from below but may impede drainage—use it sparingly. Never use plastic sheeting as it traps water and causes root rot. If using treated lumber, a food-safe liner can prevent chemicals from leaching into soil.
When is the best time to add soil to raised beds?
Fall is ideal—the soil has winter to settle and microbes can establish. Spring works too, but add soil 2-3 weeks before planting so it settles. Avoid adding new soil mid-season as it can disturb roots. For refreshing existing beds with compost, late fall after harvest or early spring before planting works best.