How to calculate soil for a raised bed
A raised bed is a box, so the soil it holds is its inside length times inside width times fill depth. Convert the depth to feet by dividing inches by 12, add a settling allowance because fresh soil packs down after the first few waterings, then divide by your bag size or by 27 for cubic yards.
With settling = cubic feet × (1 + settling % ÷ 100)
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ bag size (cu ft), rounded up
Each ingredient = total × its share of the mix
For a round bed or stock tank, the area is 3.1416 × (diameter ÷ 2)². A 4 ft round bed has about 12.6 sq ft of area.
Worked example: one 4 × 8 ft bed, filled 12 in. deep
- Area: 8 × 4 = 32 sq ft.
- Volume: 32 × 12 ÷ 12 = 32 cu ft.
- Add 10% for settling: 32 × 1.10 = 35.2 cu ft, or 35.2 ÷ 27 = 1.3 cu yd.
- All in 1.5 cu ft bags: 35.2 ÷ 1.5 = 23.5, so 24 bags.
- As a 60/40 mix: 35.2 × 0.6 = 21.1 cu ft of topsoil (15 bags of 1.5 cu ft) and 35.2 × 0.4 = 14.1 cu ft of compost (10 bags).
Splitting the mix rounds each ingredient up separately, which is why 15 + 10 bags is one more than 24. If you are building several beds of the same size, enter the number of beds instead of adding them up by hand.
How much soil do common raised beds hold?
These volumes are before any settling allowance. Add about 10–20% to the number you buy.
| Inside size | 6 in. deep | 8 in. deep | 12 in. deep | 18 in. deep | 12 in. deep in cu yd |
|---|---|---|---|---|---|
| 4 × 4 ft (16 sq ft) | 8 cu ft | 10.7 cu ft | 16 cu ft | 24 cu ft | 0.59 |
| 3 × 6 ft (18 sq ft) | 9 cu ft | 12 cu ft | 18 cu ft | 27 cu ft | 0.67 |
| 3 × 8 ft (24 sq ft) | 12 cu ft | 16 cu ft | 24 cu ft | 36 cu ft | 0.89 |
| 4 × 8 ft (32 sq ft) | 16 cu ft | 21.3 cu ft | 32 cu ft | 48 cu ft | 1.19 |
| 4 × 12 ft (48 sq ft) | 24 cu ft | 32 cu ft | 48 cu ft | 72 cu ft | 1.78 |
Keep in mind that lumber sizes are nominal. A frame built from 2 × 6 boards is about 5-1/2 in. tall per board, so two stacked boards make a bed roughly 11 in. deep, not 12. Measure what you actually built.
Which soil mix should you use?
Straight topsoil packs down in a raised bed, and straight compost can be too rich and settles a lot. Most gardeners use a blend. These are common starting points, not strict rules:
| Mix | What it is | Notes |
|---|---|---|
| 60/40 topsoil and compost | Screened topsoil blended with compost | Economical, holds moisture well, easy to buy in bulk |
| 50/50 topsoil and compost | Equal parts | Richer; a common bulk "garden mix" from landscape yards |
| Thirds (compost, peat or coir, vermiculite) | Soilless blend popularized by square foot gardening | Light and loose; the most expensive per cubic foot |
| Bagged raised bed mix | A premixed product sold for raised beds | Easiest for small beds; read the bag for what it contains |
Whatever you choose, ask a bulk supplier what is in their garden mix, and look for soil that is dark, crumbly and free of trash and weed roots. If you are unsure about your topsoil, a soil test through your county extension office is an inexpensive way to check pH and nutrients before you plant.
How deep should a raised bed be?
| Crops | Typical soil depth |
|---|---|
| Lettuce, salad greens, herbs, radishes | About 6–8 in. |
| Most vegetables (beans, peppers, cabbage family) | About 8–12 in. |
| Tomatoes, squash, carrots, potatoes | About 12 in. or more |
If the bed sits on open ground, roots can keep growing into the soil below, so a shallower bed often works if you loosen that ground first. On a patio, over compacted soil or over a weed barrier, plan for the full depth your crops need.
Tips for filling raised beds
- Bulk vs. bags. Above about one cubic yard, a bulk delivery is usually cheaper and far less lifting. One 4 × 8 bed at 12 in. needs about 24 bags of 1.5 cu ft, so two or three beds add up fast. Enter both prices to compare.
- Tall beds. For beds deeper than about 18 in., some gardeners fill the bottom with logs, branches or other bulky organic material to save soil. Enter that depth in the filler field. Avoid filling with plastic or treated wood scraps.
- Settling is normal. Overfill slightly, water well, and top up after a week or two. Each season, add an inch or two of compost to replace what breaks down.
- Next steps. Mulch the paths with the mulch calculator, plan an in-ground bed with the topsoil calculator, or price any bulk material by volume with the cubic yards calculator.
Frequently asked questions
How much soil do I need for a 4x8 raised bed?
A 4 × 8 ft bed filled 12 in. deep holds 32 cubic feet, about 1.19 cubic yards. With 10% for settling, plan on about 35 cubic feet: 24 bags of 1.5 cu ft or 18 bags of 2 cu ft. At 6 in. deep it holds 16 cubic feet before settling.
How many bags of soil do I need for a raised bed?
Multiply the inside length by the width (in feet) and by the fill depth in inches divided by 12 to get cubic feet. Add about 10–20% for settling, then divide by the bag size. A 4 × 4 ft bed 12 in. deep needs about 17.6 cubic feet with 10% settling, or 12 bags of 1.5 cu ft.
What is the best soil mix for raised beds?
A common starting point is about 60% screened topsoil and 40% compost, or a 50/50 blend. A soilless mix of equal parts compost, peat moss or coir, and vermiculite is lighter but costs more. Bagged raised bed mix is the simplest choice for one or two small beds.
How much is a cubic yard of soil in bags?
A cubic yard is 27 cubic feet, which is 18 bags of 1.5 cu ft, 13.5 bags of 2 cu ft, or 36 bags of 0.75 cu ft.
Can I fill the bottom of a raised bed with something other than soil?
In tall beds, many gardeners fill the bottom with logs, branches or leaves to save soil. That material breaks down over time and the surface sinks, so expect to top up. Keep at least the root depth your crops need, typically 8–12 in. or more, as real soil.
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