Calculator
Raised Bed Soil Calculator
Enter the inside length, width, and filled height of your bed. The calculator converts mixed units, multiplies by the number of beds, and estimates bag counts from common bag sizes. Optionally, it splits that volume into compost, coco peat or peat moss, perlite, and other ingredients using common mix ratios. Turn on hugelkultur to replace the bottom with twigs, leaves, or logs — a 12-inch bed can take about 3 inches of packed brush.
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
Backyard blend
- Screened topsoil or garden soil16 cu ft
- Finished compost9.6 cu ft
- Coco peat (coir) or peat moss3.2 cu ft
- Horticultural perlite3.2 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, split the volume above into compost, coco peat or peat moss, and perlite. These ratios are starting points — not a single correct recipe.
A practical mix for most vegetable raised beds: mineral soil for bulk, compost for fertility, coco peat or peat moss for moisture, and perlite for air and drainage.
| Ingredient | Share | Cubic feet | Cubic yards | 2 cu ft bags |
|---|---|---|---|---|
| Screened topsoil or garden soil | 50% | 16 | 0.593 | 8 |
| Finished compost | 30% | 9.6 | 0.356 | 5 |
| Coco peat (coir) or peat mossUse one or the other, not both at 10% each | 10% | 3.2 | 0.119 | 2 |
| Horticultural perliteCoarse perlite, not craft-store fine dust | 10% | 3.2 | 0.119 | 2 |
- Perlite is the white volcanic glass that keeps a mix from packing. It improves drainage and air around roots. Vermiculite holds more water. Use horticultural coarse perlite, not fine craft-store grades.
- Coco peat is coconut coir pith. It plays the same role as peat moss: hold water and keep the mix light. Use one of them in that slot, not a double layer of both unless you are deliberately making a wetter mix.
- Compressed coir bricks and peat bales expand when wet. Buy by the hydrated / loose volume on the label, not the dry brick size. The 2 cu ft bag column is only a planning estimate.
- These ratios are starting points. Climate, drainage, and compost quality matter more than hitting 30.0% exactly. See best soil mix for raised beds.
How this calculation works
Volume (cu ft) = length (ft) × width (ft) × height (ft) × number of beds. Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ bag size, rounded up. Mix recipes multiply that total volume by each ingredient’s share (for example 50% topsoil, 30% compost, 10% coco peat or peat moss, 10% perlite). 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
4 ft × 8 ft × 1 ft = 32 cubic feet, which is about 1.19 cubic yards. 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 backyard blend, that 32 cu ft is about 16 cu ft topsoil, 9.6 cu ft compost, 3.2 cu ft coco peat or peat moss, and 3.2 cu ft perlite. 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?
Plan on 5–10% extra. Soil settles after watering, especially mixes with a lot of compost. You can top off later instead of over-ordering bulk soil.
Is bagged soil the same as bulk soil?
Not always. Bagged “raised bed mix,” garden soil, and topsoil have different textures and compost content. Bulk soil is usually cheaper once you need more than about 1 cubic yard. Compare ingredients, not just the bag name.
What soil mix should I use?
There is no single correct blend. A common backyard starting point is about 50% screened topsoil, 30% compost, 10% coco peat (coir) or peat moss, and 10% horticultural perlite. A soilless mix is often equal parts compost, coir, and perlite. Classic Mel’s Mix uses vermiculite instead of perlite and holds more water. The calculator splits your bed volume into those recipes.
Perlite or vermiculite?
Perlite (sometimes misspelled pearlite) is the white, popcorn-like volcanic glass. It adds air and drainage. Vermiculite holds more water. For most backyard raised beds, perlite is the easier hardware-store choice. Do not use playground sand as a substitute.
Coco peat or peat moss?
They fill the same job: hold water and keep the mix light. Coco peat is coconut coir. Peat moss comes from bogs and is a debated ingredient. Use one in that slot. Perlite is separate — it is for air, not a substitute for coir.
How deep should I fill the bed?
Most mixed vegetable beds work well at 10–12 inches of soil. Root crops and tomatoes are happier closer to 12–18 inches. See the raised bed depth guide for crop-by-crop notes. Hugelkultur can replace 2–3 inches in a 12-inch box, or a much thicker layer in a taller frame.
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.