How to calculate soil for a raised bed
Soil is sold by volume, so you need the bed’s volume in cubic feet. Multiply length by width in feet, then by the depth in inches divided by 12. A 4 × 8 ft bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet. Divide by 27 for cubic yards, the unit bulk suppliers use: 32 ÷ 27 = 1.19 cubic yards. Divide cubic feet by the size printed on the bag for a bag count, and always round up.
For a round bed or a stock-tank planter, the floor area is 3.14 × the radius squared, so a bed 4 ft across has about 12.6 square feet. An L-shaped bed is easiest as two rectangles: the long arm, plus whatever part of the short arm sticks out past it. The calculator does all three, adds as many beds as you like and remembers them in your browser.
Soil for common raised bed sizes
Volumes to the rim, before any extra for settling. Add 10–20% to what you buy.
| Bed | Cubic feet | Cubic yards | 1.5 cu ft bags | 2 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft, 12 in deep | 16 | 0.59 | 11 | 8 |
| 3 × 6 ft, 12 in deep | 18 | 0.67 | 12 | 9 |
| 2 × 8 ft, 12 in deep | 16 | 0.59 | 11 | 8 |
| 4 × 8 ft, 6 in deep | 16 | 0.59 | 11 | 8 |
| 4 × 8 ft, 11 in deep (two 2×6 boards) | 29.3 | 1.09 | 20 | 15 |
| 4 × 8 ft, 12 in deep | 32 | 1.19 | 22 | 16 |
| 4 × 8 ft, 18 in deep | 48 | 1.78 | 32 | 24 |
| 4 × 12 ft, 12 in deep | 48 | 1.78 | 32 | 24 |
| Round, 4 ft across, 12 in deep | 12.6 | 0.47 | 9 | 7 |
Why buy 10–20% extra
A freshly filled bed sinks. Air pockets collapse the first few times you water, and compost and other organic matter keep breaking down through the season. Beds filled level with the top are often an inch or two lower by midsummer. Mounding the soil a little above the rim when you fill makes up for it.
How much to add depends on the mix. Blends heavy in topsoil settle least, so 10% is usually plenty. Compost-rich mixes and Mel’s Mix, which is mostly organic matter and peat, sink more, so allow 15–20%. Leftover soil is never wasted: keep it in the bags for topping up or potting.
Choosing a soil mix
Good raised bed soil drains freely, holds moisture, stays loose enough for roots and carrots, and is rich in organic matter. Three common ways to get there are built into the calculator. None of them is the one right answer.
Topsoil and compost, about 60/40
The usual choice for big beds or many beds, because both are cheap in bulk. Use screened topsoil and well-finished compost. Many landscape yards sell a ready-blended “garden mix” or “raised bed blend”; ask what is in it and in what proportion. Some gardeners go 50/50 for richer soil. It holds water better than lighter mixes and settles the least.
Mel’s Mix: equal thirds
Mel Bartholomew’s square foot gardening method fills shallow beds with a soilless mix of one-third compost, one-third peat moss and one-third coarse vermiculite, measured by volume. He recommended blending compost from several different sources. It is light, weed-free and easy to plant into, which is why it suits a 6-inch-deep square foot garden on top of poor ground. The downsides are cost, a mix that dries out faster in hot weather, and peat itself: it is harvested from slow-growing bogs, and many gardeners swap in coconut coir, which is used the same way.
Bagged raised bed mix
For one or two small beds, bags of ready-made raised bed soil are the simplest option: no mixing, nothing to shovel off a driveway. Compare the price per cubic foot, not per bag, since bag sizes vary. Bagged potting mix is a different product made for containers; it works in a bed but costs far more at this scale.
Bags or bulk?
One cubic yard is 27 cubic feet: 18 bags of 1.5 cu ft or 27 bags of 1 cu ft. Once you need more than a yard or so, bulk delivery is usually cheaper and much less work to carry, even with a delivery fee. Most suppliers sell in half or whole yards, which is why the calculator rounds bulk orders up to the next half yard. Enter both a bag price and a yard price to compare them for your own beds.
When you buy bulk compost or soil with manure in it, ask the supplier whether it has been checked for persistent herbicides. Some broadleaf weed killers used on hay and pasture survive composting and can twist and stunt tomatoes, beans and peppers.
Filling tall beds without buying a mountain of soil
Vegetable roots do most of their work in the top 12 inches or so. In a bed 24 inches or taller you can fill the bottom with something cheaper: rough fill soil from your own garden, or old logs and branches with the gaps packed with soil, then top it with at least 10–12 inches of your good mix. Woody fill settles more as it rots, so expect to top up more for the first couple of years. Set the calculator’s depth to the good-soil layer only.
Before filling a bed on lawn, cut the grass short and lay plain cardboard over it, overlapping the edges, to smother it. Over clay, loosen the ground with a fork so water and roots can move down.
Next steps
Once the bed is full, lay it out square by square in the garden planner, check how many of each crop fit with the square foot gardening chart and find out when to sow with the planting calendar for your ZIP code.