Raised Bed Soil Calculator
Beds
Your mix
Loose soil settles after spreading; add a % to over-order.
Check the size printed on your bag.
Optional. Loads the soil USDA maps at a US address to prefill and tailor the result.
How deep should a raised bed be?
University of Illinois Extension: at least 6 to 12 inches deep for good root growth, and beds built higher than 18 to 24 inches need extra reinforcement[3]. University of Tennessee Extension says beds are generally 6 to 12 inches tall but can be deeper, and leafy crops such as lettuce, spinach and kale can grow in beds only 4 to 6 inches deep[4].
Soil for common bed sizes
Calculated as length × width × depth, with 27 cubic feet to the cubic yard and bags rounded up[2]:
| Bed (ft) | Depth (in) | Cubic feet | Cubic yards | Bags (1.5 cu ft) |
|---|---|---|---|---|
| 4 × 4 | 6 | 8 | 0.3 | 6 |
| 4 × 4 | 8 | 10.7 | 0.4 | 8 |
| 4 × 4 | 10 | 13.3 | 0.5 | 9 |
| 4 × 4 | 12 | 16 | 0.6 | 11 |
| 4 × 8 | 6 | 16 | 0.6 | 11 |
| 4 × 8 | 8 | 21.3 | 0.8 | 15 |
| 4 × 8 | 10 | 26.7 | 1 | 18 |
| 4 × 8 | 12 | 32 | 1.2 | 22 |
| 3 × 6 | 6 | 9 | 0.3 | 6 |
| 3 × 6 | 8 | 12 | 0.4 | 8 |
| 3 × 6 | 10 | 15 | 0.6 | 10 |
| 3 × 6 | 12 | 18 | 0.7 | 12 |
| 4 × 12 | 6 | 24 | 0.9 | 16 |
| 4 × 12 | 8 | 32 | 1.2 | 22 |
| 4 × 12 | 10 | 40 | 1.5 | 27 |
| 4 × 12 | 12 | 48 | 1.8 | 32 |
32 cubic feet fills one 4 × 8 ft bed 12 inches deep[2].
Why gardeners build raised beds
Raised beds drain better than the ground around them and their soil warms up earlier in spring[1]; because they drain faster than nearby level soil, a deeper bed needs watering less often[4]. Filling them can be expensive, especially large beds, and as the compost in them breaks down they settle, so more soil and compost may be needed each year[1]. That’s why it pays to calculate before you buy. Enter an address in the calculator to see how USDA maps the soil there.
Planning your mix
Set your own recipe and the calculator splits the total into each ingredient. If you’re building on and improving your existing soil instead, Colorado State University Extension’s guidance for a new garden is to work 3 to 4 inches of compost into the top 8 to 12 inches[5]. Test before you add anything: a soil test shows whether a raised bed’s soil needs lime or fertilizer, and compost in the mix should be tested too[4]. See the compost calculator.
Worked example
Two 4 × 8 ft beds and one 4 × 4 ft bed, 10 inches deep: 66.7 cu ft in total = 2.5 cubic yards, or 45 bags of 1.5 cu ft[2].
Ways people use this calculator
- Filling new beds before the season.
- Topping up beds that have settled as their compost broke down[1].
- Deciding between bagged mix and a bulk delivery.
- Planning several beds of different sizes at once.
How we calculate this
The arithmetic, with unit factors from NIST[2]:
- Rectangles: length × width × depth. Circles: π r² × depth.
- Each bed’s quantity multiplies its volume.
- Your mix splits the total by the percentages you enter (they must total 100).
- Bags are rounded up per ingredient.
- Settling is optional and your choice.
- Conversions: 1 yd³ = 27 ft³ = 0.764555 m³; 1 ft³ = 28.3168 L.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?
At 6 inches it needs 16 cu ft, at 8 inches 21.3, at 10 inches 26.7, and at 12 inches 32[2].
How deep should a raised bed be?
University of Illinois Extension recommends at least 6 to 12 inches; beds higher than 18 to 24 inches need extra reinforcement[3].
How many bags of soil fill a raised bed?
Divide the bed’s volume by your bag size and round up: a 4 × 8 ft bed 12 in deep takes 22 bags of 1.5 cu ft or 16 of 2 cu ft[2].
Do raised beds need fertilizer?
Test first: University of Tennessee Extension says testing is how you find out whether a raised bed needs lime or fertilizer[4].
Why does my raised bed keep sinking?
The compost and other organic matter in it decompose, so the bed settles, and more soil and compost may be needed each year[1].
Sources
- University of Illinois Extension, Raised Bed Gardening (K. Johnson, 2023)
- NIST, Guide for the Use of the International System of Units (SP 811), Appendix B.8: Factors for Units Listed Alphabetically
- University of Illinois Extension in Cook County, Raised Bed Checklist (C. Miller, 2016)
- UT Extension W 346-E, The Tennessee Vegetable Garden: Building and Using Raised Beds (N. Bumgarner, 2016)
- Colorado State University Extension, Soil Amendments (GardenNotes #241; reviewed Sept 2024)
Marko Visic · MPharm, BSc Physics · Last reviewed November 13, 2026