How Much Soil Do I Need for a Raised Garden Bed?
Quick answer: length × width × depth in feet gives cubic feet — a 4×8 bed at 12 inches deep needs 32 cubic feet: about 22 bags of 1.5 cu ft soil or 1.2 cubic yards of bulk mix. Most beds want 8–12 inches of soil, not a full fill to the rim.
- The volume formula
- Depth: how much to fill
- Bags vs bulk soil
- The 60/30/10 mix
- Worked example: 4×8 bed
- Common mistakes
- FAQ
The volume formula
Raised bed soil is pure volume math: length × width × depth, all in feet. A 4 ft × 8 ft bed filled 1 ft deep = 4 × 8 × 1 = 32 cubic feet. Convert to cubic yards by dividing by 27 (a yard of bulk soil), or to bags by dividing by the bag size (1.5 or 2 cu ft).
Depth: how much to fill
Fill to 8–12 inches for vegetables — most roots need 8 inches, tomatoes and root crops want 12. Shallow crops and herbs do fine at 6 inches. Soil settles 1–2 inches after the first watering, so fill slightly above your target depth. There's no need to fill the whole frame with soil unless the bed is only 12 inches tall anyway.
Bags vs bulk soil
Bagged soil (1.5 or 2 cu ft) is convenient for small beds — a 32 cu ft bed needs about 22 bags of 1.5 cu ft, which is a lot of hauling. Bulk delivery by the cubic yard is cheaper and easier for anything over ~1 cubic yard (27 cu ft): one yard fills a 4×8 bed to 12 inches and then some. Garden centers deliver by the yard with a minimum order.
| Bed size | Depth | Cubic feet | 1.5 cu ft bags | Cubic yards |
|---|---|---|---|---|
| 4×4 ft | 12 in | 16 | 11 | 0.6 |
| 4×8 ft | 12 in | 32 | 22 | 1.2 |
| 4×8 ft | 6 in | 16 | 11 | 0.6 |
| 3×10 ft | 12 in | 30 | 20 | 1.1 |
| 8×8 ft | 12 in | 64 | 43 | 2.4 |
The 60/30/10 mix
The standard raised bed recipe is 60% topsoil, 30% compost, and 10% aeration (perlite or coarse sand). For a 32 cu ft bed that's about 19 cu ft topsoil, 10 cu ft compost, and 3 cu ft perlite — or just buy pre-mixed raised-bed soil and skip the math. Avoid filling a bed with pure topsoil: it compacts and drains badly.
Worked example: a 4×8 bed at 12 inches
- Volume = 4 × 8 × 1 = 32 cu ft
- ÷ 1.5 cu ft per bag = 21.3 → 22 bags
- ÷ 27 = 1.19 cubic yards of bulk soil
- Mix split: 19 cu ft topsoil + 10 cu ft compost + 3 cu ft perlite
Common mistakes
- Measuring outside dimensions. A 4×8 frame built from 2-inch lumber holds less inside — use the inside measurements for volume.
- Filling to the rim. Soil settles 1–2 inches, and planting depth matters. Fill to your crop's root depth, not the frame's height.
- Ordering bags for a big bed. 22+ bags is miserable hauling and costs more per cubic foot. Go bulk past ~1 cubic yard.
- Pure topsoil. It compacts and waterlogs. Stick to the 60/30/10 mix or a pre-blended raised bed soil.
FAQ
A 4×8 bed filled to 12 inches deep needs 32 cubic feet of soil — about 21–22 bags of 1.5 cu ft bagged mix, or 1.2 cubic yards of bulk soil. Fill to 6 inches and you need 16 cu ft instead.
Multiply length × width × depth in feet. A 4 ft × 8 ft bed at 1 ft deep = 32 cubic feet. Divide by 27 for cubic yards, or by the bag size for bag counts.
Divide the bed's cubic footage by the bag size: a 32 cu ft bed needs about 22 bags of 1.5 cu ft soil, 16 bags of 2 cu ft soil, or 8 bags of the big 3 cu ft bales.
Fill at least 8–12 inches deep for vegetables — most roots need 8 inches, tomatoes and root crops want 12. Beds 6 inches deep suit shallow crops and herbs.
The standard mix is about 60% topsoil, 30% compost, and 10% aeration (perlite or coarse sand). Pre-mixed raised-bed soil works too. Never fill a bed with pure topsoil.