How Much Soil for a Raised Bed?

A 4x8 bed swallows almost a full cubic yard at just 10 inches deep. Order by guess and you usually come up short on the first try.

The math is simple. What trips people up is the depth, the settling, and the fact that "soil" is really three ingredients in three different bag sizes. Get those right and you order once.

This guide sits under the bigger raised bed soil hub, which covers mix, depth, and cost end to end.

The formula, and the one division that matters

A raised bed is a box. So its volume is plain geometry: length times width times height.

The only catch is units. Length and width come in feet, but fill depth comes in inches. Convert the depth to feet first by dividing by 12.

cubic feet = length_ft x width_ft x (height_in / 12)
cubic yards = cubic feet / 27

Why both numbers? Bulk soil is sold by the cubic yard, bagged soil by the cubic foot.

A cubic yard is a block 3 feet on every side, which holds 27 cubic feet. That 27 is the whole conversion.

Bag math comes last. Divide each ingredient's cubic feet by its bag size and round up, because you cannot buy two-thirds of a bag.

A worked example: the 4x8 bed at 10 inches

Run the standard bed through it. A 4x8 footprint is 32 square feet.

At 10 inches deep, that is 32 x (10 / 12), which lands at 26.67 cubic feet, or 0.99 cubic yards. Call it a cubic yard.

Then add for settling. Fresh mix drops after the first few waterings, so the calculator adds a 10 percent buffer. That bumps the order to about 29.33 cubic feet.

Some gardeners go up to 15 percent on tall beds. But 10 percent matches what the tool plans and what most beds actually drop.

That gives you a total in cubic yards. It still does not tell you what to put in the cart, which is where most guides stop.

Your per-component shopping list

A raised bed is not one product. The default vegetable blend is 60 percent topsoil, 30 percent compost, and 10 percent aeration (perlite or vermiculite). Each comes in a different bag size.

So the real shopping list breaks the cubic feet into three stacks of bags. For the 4x8 bed at 10 inches, the 26.67 cubic feet splits like this.

So one 4x8 bed is 11 bags of topsoil, 8 bags of compost, and 2 bags of aeration. That is the list you carry to the store, not a lonely "1 cubic yard" you translate at the register.

The 60/30/10 split is a balanced starting mix. The Raised-Bed Soil Calculator rebuilds this list for any bed size and depth, including the ready-made 4x8 size page.

How deep does a raised bed need to be?

Depth sets your whole order, and it depends on what you grow.

Shallow-rooted crops like lettuce, beans, and other greens are happy in at least 8 inches of mix. Root crops and deeper feeders like carrots, tomatoes, and squash want 12 to 24 inches, per the University of Maryland Extension guide to growing vegetables in raised beds.

So a 10 to 12 inch bed is a sensible default for a mixed garden. It clears the 8-inch floor for greens and gives most root crops enough room. The University of Minnesota Extension raised-bed guide lands in the same range for general vegetable beds.

The mix: what actually goes in the box

The 60/30/10 default is the easy answer, but it is not the only one.

The other recipe you will see is Mel's Mix from square-foot gardening: equal thirds of compost, peat moss or coco coir, and coarse vermiculite. It drains fast and suits intensive planting. It also costs more, because it uses no native topsoil.

Both lean heavily on compost, for good reason. Extension mixes for raised vegetable beds often run as high as one part topsoil to one part compost by volume.

Want to size the compost on its own? The how much compost do I need guide walks that number. Whichever recipe you pick, the growing layer needs real soil and organic matter, not just bagged potting mix.

The cheapest honest way to fill a deep bed

Spend on the top foot and save on the bottom. A 24-inch bed holds more than twice the mix of a 10-inch one, and buying premium blend all the way down is where the cost runs away.

The fix is to fill only the top 10 to 12 inches with growing mix, since that is the root zone. Pack the bottom 6 to 12 inches with cheaper coarse fill.

Coarse woody material, branches, or clean fill dirt all work down low. Burying logs and branches is the hugelkultur approach, and it can genuinely cut the volume of purchased mix in a deep bed.

Extension views are mixed, so treat it as a budget option that settles over time and may need topping up, not a guaranteed shortcut. Down low you are buying volume, not nutrition.

The Topsoil Calculator sizes that bottom layer if you would rather pay for screened topsoil than scavenge wood.

It comes down to four numbers. A cubic yard for a 4x8 bed at 10 inches. Eleven bags of topsoil, eight of compost, two of aeration. Ten percent more for settling. Run those and you load the truck once.

Got your bed dimensions? Open the Raised-Bed Soil Calculator for your exact bag list.