What this tool does and when to use it
This calculator turns the length, width and depth of a raised bed into the soil it holds, in cubic feet and cubic yards or in liters, then adds 10% for settling and shows how many 1, 1.5 or 2 cubic foot bags (or 40 and 50 liter bags) to buy and how much topsoil, compost and aeration goes into the mix. Use it before you go to the garden center or order a bulk delivery, and for each bed when you plan several.
How much soil does a raised bed need?
Multiply the length, width and depth of the bed in feet to get cubic feet. A classic 8 by 4 foot bed filled 12 inches deep holds 32 cubic feet, a little over a cubic yard. The calculator adds 10 percent because loose soil settles over the first few weeks and after watering, so you buy about 35 cubic feet, which is 18 bags of 2 cubic feet. That surprises most people standing in the garden center.
Raised bed soil volume chart
| Bed size | 6 in deep | 12 in deep | 18 in deep |
|---|---|---|---|
| 4 x 4 ft | 8 cu ft | 16 cu ft | 24 cu ft |
| 4 x 8 ft | 16 cu ft | 32 cu ft | 48 cu ft |
| 3 x 6 ft | 9 cu ft | 18 cu ft | 27 cu ft |
| 2 x 8 ft | 8 cu ft | 16 cu ft | 24 cu ft |
How many bags of soil for a raised bed?
These are real runs of the calculator above, each filled 12 inches deep unless noted, with 10% added for settling. Bags are rounded up.
| Bed size | Bed holds | Soil to buy | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|---|
| 4 x 2 ft | 8 cu ft | 8.8 cu ft | 9 | 6 | 5 |
| 4 x 4 ft | 16 cu ft | 17.6 cu ft | 18 | 12 | 9 |
| 6 x 3 ft | 18 cu ft | 19.8 cu ft | 20 | 14 | 10 |
| 4 x 8 ft, 6 in deep | 16 cu ft | 17.6 cu ft | 18 | 12 | 9 |
| 4 x 8 ft, 10 in deep | 26.7 cu ft | 29.3 cu ft | 30 | 20 | 15 |
| 4 x 8 ft | 32 cu ft | 35.2 cu ft | 36 | 24 | 18 |
| 4 x 10 ft | 40 cu ft | 44 cu ft | 44 | 30 | 22 |
| 4 x 8 ft, 18 in deep | 48 cu ft | 52.8 cu ft | 53 | 36 | 27 |
| 4 x 12 ft | 48 cu ft | 52.8 cu ft | 53 | 36 | 27 |
A 4x2x1 bed and an 8x4x1 bed are the same as the 4 x 2 and 4 x 8 rows. Once you need more than about a cubic yard (27 cubic feet), a bulk delivery of raised bed mix usually costs less than bags.
How many liters of soil for a raised bed?
In metric, multiply the length and width in meters by the depth in centimeters, then multiply by 10 to get liters. Real runs at 30 cm deep, with 10% added:
| Bed size | Bed holds | Soil to buy | 40 L bags | 50 L bags |
|---|---|---|---|---|
| 1.2 x 1.2 m | 0.43 m³ | 475 L | 12 | 10 |
| 2.4 x 1.2 m | 0.86 m³ | 950 L | 24 | 20 |
The best soil mix for raised beds
Garden soil straight from the ground is too heavy for a raised bed, and pure compost is too rich and settles a lot. A good mix drains freely but holds moisture and feeds plants all season.
- Classic mix: about 60 percent screened topsoil, 30 percent compost and 10 percent perlite, pumice or fine bark for air. Affordable and easy to find in bulk.
- No topsoil mix: equal parts compost, coco coir or peat moss, and coarse vermiculite. Light and fluffy, popular for square foot gardening, but more expensive to fill deep beds.
Topping up each year
Raised beds lose an inch or two of soil each season as organic matter breaks down. Top them up with 1 to 2 inches of compost every spring, and the soil keeps getting better. For layout ideas, see our small garden and backyard ideas, plan the planting with the plant spacing calculator and finish the paths with the mulch calculator.
