Raised Beds
How much soil for a 4×8 raised bed?
A 4×8 raised bed at 12-inch depth takes 32 cubic feet of soil, or about 0.91 cubic metres. That works out to 22 bags at the standard 1.5 cu ft bag size, and budget $160–$260 if you are buying premium bagged mix.
6 inches deep → 16 cu ft (11 bags @ 1.5 cu ft): herbs, lettuce, radishes only
8 inches deep → 21.3 cu ft (15 bags): most leafy greens, beans, peas
10 inches deep → 26.7 cu ft (18 bags): comfortable for most vegetables
12 inches deep → 32 cu ft (22 bags): tomatoes, peppers, root crops
18 inches deep → 48 cu ft (32 bags): carrots, parsnips, deep-rooted crops
The formula
Raised-bed soil volume is the simplest possible calculation: length × width × depth, with depth converted to feet.
For a 4×8 bed at 12 inches: 4 × 8 × 12 / 12 = 32 cu ft. For metric users: multiply cu ft by 0.0283 to get cubic metres, or skip ahead to the metric table below.
Why depth matters more than you think
Most published "raised bed depth" guidance says 6 inches is enough for lettuce. Technically true. Practically wrong, for three reasons:
- Settling. Fresh-filled soil settles 10–20% over the first growing season. A 6-inch fill becomes 5 inches. A 12-inch fill becomes 10 inches.
- Root depth. Tomato roots want 18 inches. Carrots want 12 inches. Even lettuce develops a fibrous mat that benefits from 8+ inches of friable soil to spread out laterally.
- Water buffer. Shallow beds dry out fast. A 6-inch bed in July needs daily watering. A 12-inch bed handles 2–3 days between waterings even in heat.
Our recommendation: 12 inches minimum for any bed that will hold tomatoes, peppers, root crops, or perennials. 8 inches is acceptable for leafy-greens-only beds. 6 inches is herb-and-radish territory.
The full volume table
| Depth | Cubic feet | Cubic yards | Cubic metres | 50L bags |
|---|---|---|---|---|
| 6 in (15 cm) | 16.0 | 0.59 | 0.45 | 10 |
| 8 in (20 cm) | 21.3 | 0.79 | 0.60 | 13 |
| 10 in (25 cm) | 26.7 | 0.99 | 0.76 | 16 |
| 12 in (30 cm) | 32.0 | 1.19 | 0.91 | 19 |
| 15 in (38 cm) | 40.0 | 1.48 | 1.13 | 23 |
| 18 in (45 cm) | 48.0 | 1.78 | 1.36 | 28 |
| 24 in (60 cm) | 64.0 | 2.37 | 1.81 | 37 |
Note: 50L bag count rounded up. Always round bag counts up; you cannot buy 21.3 bags.
Bag-size conversion
Bagged garden soil and compost in the US come in 1, 1.5, and 2 cubic-foot bags. In the UK and EU, soil typically ships in 40L, 50L, and 75L sacks. The math:
| Bag size | Cu ft | For 32 cu ft bed (4×8 at 12") |
|---|---|---|
| 1 cu ft (small) | 1.0 | 32 bags |
| 1.5 cu ft (standard US) | 1.5 | 22 bags |
| 2 cu ft (large US) | 2.0 | 16 bags |
| 40L sack | 1.41 | 23 bags |
| 50L sack | 1.77 | 19 bags |
| 75L sack | 2.65 | 13 bags |
Always round up. Buying 21 bags when you need 21.3 is a Saturday morning lost driving back to the hardware store.
The settling buffer (the buffer most calculators forget)
Brand-new compost, peat moss, and aged manure all contain trapped air. Once watered, walked on, and frost-cycled through a winter, the material consolidates. Cooperative-extension publications cite typical first-year settling at 10–20%, with 15% being a good planning number.
What this means in practice: order 15% extra on your initial fill. For a 32 cu ft bed, that is 36.8 cu ft. Round up to 37 cu ft, or 25 bags at 1.5 cu ft.
Alternatively, fill to within an inch of the rim on day one and plan to top up with 2–3 bags of compost the following spring. Both approaches work.
Soil mix recipes: what to put in the bed
Classic 60/30/10 (the workhorse)
- 60% screened topsoil
- 30% finished compost (mix of sources is better than one)
- 10% perlite or coarse sand
Buy the topsoil bulk if your bed is large (over 1 cubic yard); it is dramatically cheaper than bagged. The compost should be a mix: cow, mushroom, leaf mould. Avoid 100% steer manure (too high in salts).
Mel's Mix (Square Foot Gardening)
- 1/3 finished compost (ideally 5 different sources blended)
- 1/3 peat moss or coco coir
- 1/3 coarse vermiculite
Mel Bartholomew's original mix is excellent for high-intensity production but expensive; vermiculite is the cost driver. We recommend Mel's Mix for the top 8 inches and the classic 60/30/10 for the bottom 4 inches in a deep bed.
Budget mix (when bagged is the only option)
- 40% bagged garden soil
- 40% bagged compost or composted manure
- 20% bagged "raised bed mix" or top layer
Bagged "topsoil" alone is usually too dense and clay-heavy. Cutting it with compost and a lighter mix produces a workable bed even from the hardware-store aisle.
What it actually costs
Pricing varies massively by region, season, and whether you buy bulk or bagged. Approximate 2026 USA market figures:
| Source | Cost per cu ft | 4×8 bed at 12" (32 cu ft) |
|---|---|---|
| Bulk delivery (1 cu yd minimum) | $1.50–$3.00 | $50–$95 |
| Bulk pickup (you haul) | $1.00–$2.00 | $32–$65 |
| Bagged "garden soil" | $3.50–$5.00 | $110–$160 |
| Bagged "raised bed mix" | $5.00–$8.00 | $160–$260 |
| Mel's Mix from components (bagged) | $6.00–$10.00 | $190–$320 |
If you have more than one bed, bulk delivery pays off almost immediately. The break-even is roughly 1 cubic yard (27 cu ft), about one 4×8 bed at 10 inches deep. Two beds and bulk is dramatically cheaper.
Multiple beds? L-shaped? Circular?
The Soil Calculator handles rectangle, circle, and L-shape beds, multiple bed groups, custom soil-mix percentages, settling buffer, and per-component cost in your currency. Free to use.
Open the calculator →Three mistakes worth avoiding
Buying "topsoil" without seeing it
Topsoil is a totally unregulated term. The bag in front of you might be screened loam from a mature field, or it might be subsoil scraped off a construction site and sieved. Smell it, squeeze it, look for visible roots and organic matter. A good topsoil holds its shape briefly when squeezed and crumbles easily.
Filling the whole bed with bagged "raised bed mix"
Raised bed mix is great for the top 8 inches where the seeds, transplants, and shallow roots live. Filling the entire 12 inches with it is expensive and unnecessary. Use cheaper bulk soil or topsoil for the bottom layer.
Skipping the carbon layer
If your bed is over 16 inches deep, line the bottom 4–6 inches with rough organic carbon (small branches, wood chips, partially-decomposed leaves) before filling with soil. This "hugelkultur lite" approach reduces total soil needed by 20–30% and improves moisture retention over the next 2–3 years. It does mean a small extra settling allowance.
Sources: Oregon State University Extension (Building raised beds), University of Maryland Extension (Soils for Container and Raised Bed Vegetable Gardens, HG 600), Cornell Cooperative Extension (Soil Basics for Raised Beds), Mel Bartholomew (All New Square Foot Gardening, 3rd Edition).
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