Calculator
Raised Bed Soil Calculator
Enter the inside length, width, and filled soil depth — not outside frame dimensions, and not board height if the box is only partly filled. The calculator converts mixed units, multiplies by the number of identical beds, and estimates bag counts (rounded up). Optionally, it splits that volume into screened topsoil and finished plant-based compost using two practical points inside University of Minnesota Extension’s raised-bed range. Turn on hugelkultur to replace the bottom with twigs, leaves, or logs — a 12-inch bed can take about 3 inches of packed brush. Open-bottom beds can use loosened native soil as extra rooting depth; closed-bottom planters cannot.
Your result
Bags to buy
22bags1.5 cu ft each
- Soil volume
- 32 cu ft
- Cubic yards
- 1.185 cu yd
- Bags needed
- 221.5 cu ft bags, rounded up
Balanced raised-bed mix
- Screened topsoil19.2 cu ft
- Finished plant-based compost12.8 cu ft
Save soil, save money
Hugelkultur in a raised bed
Hugelkultur (from the German Hügelkultur, “hill culture”) means burying woody scraps under soil so you buy less mix. It does not have to be fat logs. In a typical 12-inch bed, pack 2–3 inches of thin branches, then stuff the gaps with leaves. That layer still counts — it is soil you do not purchase. Taller frames can take a thicker woody pile and save more.
Keep enough real mix on top for roots — about 9 inches in a 12-inch box, 10–12 inches when the frame is deeper. Turn on the checkbox above; the calculator starts you at a 3-inch brush-and-leaf layer.
| 4×8 bed | All soil | Mix + woody layer | You save |
|---|---|---|---|
| 12 in · 3 in twigs and leaves | 22 bags · $176 | 16 bags · $128 | 6 bags · $48 |
| 18 in · 6 in wood | 32 bags · $256 | 22 bags · $176 | 10 bags · $80 |
| 24 in · 12 in wood | 43 bags · $344 | 22 bags · $176 | 21 bags · $168 |
Money column uses $8 per 1.5 cu ft bag as a planning number — edit the bag price to match your store. It is not a quote. Pack twigs tight and fill gaps with leaves so the woody layer does not collapse into empty air the first week.
- Shallow bed: thin branches criss-crossed, then leaves packed in. Skip logs that eat the root zone.
- Deep bed: largest untreated wood first, then brush, then leaves. Water it before adding soil.
- Fresh wood can tie up nitrogen for a season. Use a compost-rich mix on top, or expect a lighter first year.
- Skip treated lumber, black walnut, and painted scraps. The bed will settle as wood shrinks — top off later.
Full hugelkultur guide — layering twigs and leaves in a 12-inch bed, and how much a taller frame saves.
Ingredient plan
Optional soil mix
Bagged raised-bed mix works. If you blend your own, these presets stay inside University of Minnesota Extension’s practical range of about one-half to two-thirds screened topsoil and one-third to one-half finished plant-based compost. They are starting recipes, not universal prescriptions. Ordinary pots should stay soilless.
A practical point inside University of Minnesota Extension’s raised-bed range: mostly topsoil with a large share of finished plant-based compost. Starting recipe only — local topsoil texture and drainage differ, and bagged garden soil is not the same as native yard soil.
| Ingredient | Share | Cubic feet | Cubic yards | 2 cu ft bags |
|---|---|---|---|---|
| Screened topsoilBulk topsoil is often practical for large volumes; bagged garden soil varies and is not identical to native soil | 60% | 19.2 | 0.711 | 10 |
| Finished plant-based compostDo not fill the whole bed with compost alone | 40% | 12.8 | 0.474 | 7 |
- Topsoil belongs in raised beds, not ordinary pots. Open-bottom frames can also connect to loosened native soil underneath; closed-bottom planters depend entirely on what you put in the box.
- Do not fill the bed with compost alone. Finished plant-based compost is a share of the mix. Newly filled beds may sink as organic matter decomposes — water the fill, then top off.
- Bagged garden soil and native yard soil are not identical. Local topsoil texture and drainage differ; screen rocks when you can and prefer plant-based finished compost.
- These two presets are practical points inside University of Minnesota Extension’s raised-bed range, not universal prescriptions. See best soil mix for raised beds.
How this calculation works
Convert every measurement to feet, then volume (cu ft) = inside length × inside width × soil depth × number of beds. Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ bag size, rounded up, because a partial bag is not a store unit. Purchased volume can differ from this number because bag fill, settling, compaction, and headspace vary. Frame height is not automatically the fill depth. Optional mix recipes multiply that total volume by each ingredient’s share (Balanced raised-bed mix: 60% screened topsoil and 40% finished plant-based compost; More-topsoil mix: about 67% topsoil and 33% compost). Both sit inside a documented Extension range and are starting recipes, not universal prescriptions. Hugelkultur soil volume = full volume × (soil on top ÷ bed depth). Bags saved = bags for a full soil fill minus bags for the soil layer. Dollar savings use the bag price you enter.
Example: one 4×8 bed, 12 inches deep
Inside 4 ft × 8 ft × 1 ft of fill = 32 cubic feet, which is about 1.19 cubic yards (32 ÷ 27) before you round up any bag count. At 1.5 cu ft per bag, that is 22 bags (32 ÷ 1.5, rounded up). Two identical beds would need 64 cubic feet. Using the Balanced raised-bed mix, that 32 cu ft is about 19.2 cu ft screened topsoil and 12.8 cu ft finished plant-based compost. With hugelkultur, 3 inches of packed twigs and fallen leaves means 24 cu ft of mix — 16 bags, about $48 less at $8 a bag. The same 4×8 frame 24 inches deep filled only with mix needs 43 bags; 12 inches of wood underneath stays at 22 bags and skips about $168.
Frequently asked questions
Do I measure the outside or inside of the bed?
Measure the inside planting area. Frame thickness does not hold soil. For a 4×8 bed built from 1.5-inch boards, the inside is slightly smaller than the advertised size.
Should I buy extra soil?
Fresh mix often settles after watering, especially when it is compost-heavy, and bags are not always filled to the printed volume. There is no one percentage that fits every mix. Top off after the first deep watering instead of treating a single overage as a rule.
Is bagged soil the same as bulk soil?
Not always. Bagged “raised bed mix,” garden soil, and topsoil have different textures and compost content. For larger fills, bulk topsoil or a delivered blend is generally more economical than buying the same volume as individual bags. Compare ingredients, not just the bag name. This calculator does not set prices.
What soil mix should I use?
There is no single correct blend. University of Minnesota Extension describes a practical raised-bed range of about one-half to two-thirds topsoil and one-third to one-half plant-based compost. This calculator offers two points in that range: Balanced raised-bed mix (60% screened topsoil, 40% compost) and More-topsoil mix (two-thirds topsoil, one-third compost). Do not fill the bed with compost alone. Ordinary pots should stay soilless — use the potting mix calculator for containers.
Why only topsoil and compost in the mix presets?
Those two ingredients match the University of Minnesota Extension range this tool uses. Other Extension examples mention sand, vermiculite, or lava rock in some blends, but those are not extra presets here. Local topsoil texture and drainage still matter, and bagged garden soil is not identical to native yard soil.
How deep should I fill the bed?
Extension guidance notes that about 8–10 inches of usable rooting depth can be enough for many vegetables, and that a tall accessibility frame is not the same as required soil depth. HarvestBench still uses 10–12 inches of fill as a practical default for mixed beds, not as a universal requirement. Root crops and large fruiting plants usually want more loosened volume and water buffering. Open-bottom beds can add depth if you loosen the native soil underneath. Closed-bottom planters need the full usable depth in the box. See the raised bed depth guide. Hugelkultur can replace 2–3 inches in a 12-inch box, or a thicker layer in a taller frame.
Is frame height the same as the soil I should buy?
No. Buy for the filled soil depth. A 24-inch accessibility frame does not have to be packed with mix to the top. Open-bottom beds can connect to loosened native soil; closed-bottom and elevated planters depend entirely on what you put in the container.
What is hugelkultur, and how much money does it save?
Hugelkultur is a woody layer under the mix — twigs and leaves in a 12-inch bed, logs only if the frame is deeper. A 4×8×12-inch bed with 3 inches of packed brush skips about 6 bags (around $48 if bags cost $8). A 24-inch bed can skip about 21 bags (around $168). Turn on the hugelkultur checkbox and enter your bag price.
Sources and methodology
- Raised bed gardens — University of Minnesota Extension
- How to use compost in gardens and landscapes — Oregon State University Extension
- Building a Raised Bed on a Budget — University of Georgia Extension