How Much Soil Do You Need for a Raised Bed?

Key points

Length x width x depth (all in feet) = cubic feet. Divide by 27 for cubic yards.

Buy 10 to 20 percent extra; fresh soil and compost settle in the first season.

Once you need about a cubic yard or more, bulk delivery is usually cheaper than bags.

Twelve inches of good soil is enough for most vegetables.

Convert every measurement to feet first, including depth. That is where most soil math goes wrong: a 10-inch deep bed is 0.83 feet, not 10.

How do you calculate raised bed soil?

Multiply length x width x depth, all in feet, and the answer is the cubic feet of soil you need. To get cubic yards, which is how bulk soil is sold, divide cubic feet by 27.

For a 4 x 8 foot bed filled 12 inches deep: 4 x 8 x 1 = 32 cubic feet. 32 / 27 = 1.19 cubic yards.

Bed size	Depth	Cubic feet	Cubic yards	2 cu ft bags
4 x 4 ft	12 in	16	0.6	8
4 x 8 ft	12 in	32	1.2	16
4 x 8 ft	18 in	48	1.8	24
3 x 6 ft	10 in	15	0.6	8
4 x 12 ft	24 in	96	3.6	48

Bag counts are rounded up and don't include extra for settling. Bagged soil is commonly sold in 1, 1.5 and 2 cubic foot sizes, so check the label before you count.

How much extra should you buy?

Plan on 10 to 20 percent more than the math says. A bed filled to the rim in spring often sits an inch or two lower by midsummer as the soil settles and compost breaks down. Top it up each year with a couple of inches of compost.

Bags or bulk: which is cheaper?

Bulk soil is usually cheaper per cubic yard once you need about one yard or more, even after a delivery fee. Bags make sense for one or two small beds, for places a truck can't reach, or when you want a specific mix in a small amount.

When buying bulk, ask the supplier what's in their "garden mix" or "raised bed mix." Quality varies a lot, and some blends are mostly fine sand or wood fines.

What soil mix should go in a vegetable bed?

A reliable mix is roughly 60 percent screened topsoil and 40 percent compost by volume. That holds moisture, drains well and feeds plants through the first season. Pure compost is too rich and settles heavily; pure topsoil from a yard is often too dense once it's confined in a box.

Avoid bagged "potting mix" for whole beds. It's designed for containers, costs more, and can dry out fast in a large bed.

How deep does a raised bed need to be?

Eight to 12 inches of good soil grows most vegetables, especially if the bed sits on loosened ground that roots can push into. Go 18 inches or deeper for long carrots and parsnips, when the bed sits on pavement or poor compacted soil, or if you want less bending.

For tall beds, many gardeners fill the bottom third with untreated logs, branches or leaves to cut soil costs. It works, but expect extra settling as that material breaks down.

Should you test the soil?

If you're filling with bulk soil or reusing soil from your yard, a basic soil test from your county extension office is cheap and tells you pH and nutrient levels. In older neighborhoods, ask for a lead test before growing food in native soil.

Next step: measure each bed, run the numbers above, and call two local suppliers for a per-yard price and delivery fee before you buy bags.

Questions people ask

How many bags of soil for a 4x8 raised bed?

At 12 inches deep, a 4x8 bed holds 32 cubic feet: sixteen 2-cubic-foot bags or about twenty-two 1.5-cubic-foot bags, plus a few extra for settling.

Can I put grass or weeds under a raised bed?

Yes. Cut the grass short and lay cardboard over it before filling. The cardboard smothers the grass and breaks down within a season.

Do I need to replace raised bed soil every year?

No. Add 1 to 2 inches of compost each year to replace what settles and feeds the plants. Full replacement is only needed for disease problems or contamination.

Is a cubic yard of soil a lot?

One cubic yard is 27 cubic feet, about enough to fill a 4x8 bed 10 inches deep. It weighs roughly a ton when moist, so plan how you'll move it.