How Much Soil for a 4x8 Raised Bed (and the Cheapest Mix)
A 4x8 raised bed filled 10 inches deep holds 26.7 cubic feet of soil — 18 bags at the 1.5-cubic-foot size most garden centers stock, 27 bags at the 1-cubic-foot size, or just shy of one cubic yard in bulk. That settles the volume question. The money question is sneakier, and it ambushed me: my first two beds cost almost as much to fill as they did to build. Below is the math at 6, 10, and 12 inches, followed by the layered fill that brought my second pair of beds in at less than half the cost.
Step 1: Get the Volume Right
Length times width times depth, all in feet. The footprint of a 4x8 bed is 32 square feet, so depth is the only number that changes anything:
- 6 inches deep (0.5 ft): 32 × 0.5 = 16 cubic feet
- 10 inches deep (10/12 ft): 32 × 10/12 = 26.7 cubic feet
- 12 inches deep (1 ft): 32 × 1 = 32 cubic feet
Two measuring habits will save you a return trip. First, measure the inside of the frame — a bed built from 2-by lumber loses about 3 inches of length and width to the boards, though on a 4x8 the difference is small enough that I round it away. Second, measure to your fill line, not to the top rail. Almost everyone leaves an inch or two of lip so water and mulch stay inside the box. If your 12-inch-tall bed will be filled to 10 inches, buy soil for 10.
Step 2: Convert Cubic Feet to Bags or Yards
Bagged soil mostly comes in 1 and 1.5 cubic-foot sizes; the 2-cubic-foot bag is largely a mulch format. Read the label every time, because two pallets can sit side by side at nearly the same price with different volumes inside. Round up — nobody will sell you 17.8 bags.
| Depth | Volume | 1.5 cu ft bags | 1 cu ft bags | Bulk |
|---|---|---|---|---|
| 6 in. | 16 cu ft | 11 | 16 | 0.6 cu yd |
| 10 in. | 26.7 cu ft | 18 | 27 | ~1 cu yd |
| 12 in. | 32 cu ft | 22 | 32 | 1.2 cu yd |
That bulk column matters more than it looks. One cubic yard is 27 cubic feet, which means a single 4x8 bed at 10 inches is almost exactly one scoop from a landscape yard’s loader. For odd bed sizes or in-between depths, the mulch and soil calculator runs the same cubic-feet-to-bags conversion for any dimensions.
Step 3: Price Bags Against Bulk Before You Commit
Two springs ago I built my first pair of 4x8 beds, 10 inches deep, and filled both entirely with name-brand raised-bed mix. Two beds need 53.3 cubic feet, which came to 36 bags at $8.97 each — $322.92 before tax. The cedar boards and screws for both frames had run me about $350. I stood in the driveway doing that math on my phone and realized the dirt cost nearly as much as the structure holding it.
Per cubic foot, that bagged mix worked out to $5.98 ($8.97 ÷ 1.5). My local landscape yard sells screened topsoil at $34 a cubic yard and a compost-heavy garden blend at $48 — that’s $1.26 and $1.78 per cubic foot. Bulk material was less than a third the price of bags, and I had paid the bag premium on every single cubic foot.
Bulk has two catches: weight and the delivery fee. Topsoil runs about 1 to 1.1 tons per cubic yard — 2,000 pounds and change — so hauling a yard yourself means a real trailer, not the trunk of a sedan. Compost is lighter at roughly 0.9 ton per yard. My yard charges $55 to deliver, which is why bulk only starts winning once you’re filling real volume. For one shallow bed, buy bags and be done in a single cart trip. For two or more beds at 10 inches or deeper, the delivery fee dissolves into the savings.
Step 4: Layer the Fill — Cheap Base, Good Mix Where You Plant
My first fill had a second mistake hiding in it, and it was vertical: I put $5.98-per-cubic-foot mix in the bottom of a 10-inch bed where nothing but the deepest roots would ever reach. The fix is layering — plain, cheap fill across the bottom and the quality mix in the top 6 inches, where the actual planting happens.
Run it bag-only first, per bed at 10 inches:
- Bottom 4 inches: 32 × 4/12 = 10.7 cubic feet → 11 one-cubic-foot bags of plain topsoil at $2.50 = $27.50
- Top 6 inches: 16 cubic feet → 11 bags of 1.5-cubic-foot raised-bed mix at $8.97 = $98.67
- Layered total: $126.17, against $161.46 for the same bed filled wall-to-wall with premium mix (18 bags × $8.97). That’s $35.29 back per bed, about 22%, with no change in the layer you plant into.
Bulk layering is where it gets good. Last spring I built a second pair of beds, same 4x8 at 10 inches. This time I ordered one cubic yard of topsoil ($34) and 1.25 yards of the garden blend ($60), delivered together for $55 — $149 for both beds, or $74.50 each. The topsoil handled the bottom 4 inches of both beds (21.3 cubic feet) with five-odd cubic feet left over for low spots in the lawn, and the blend filled the top 6 inches (32 cubic feet) with a couple of cubic feet to spare. Under half of what the first pair cost, and I can’t tell the cheap beds from the expensive ones.
One boundary worth stating plainly: this is volume-and-dollars math. What the top layer should be made of — how much compost versus topsoil versus bagged mix, and which blend suits tomatoes over carrots — is a gardening judgment call, not something a quantity calculator should decide for you. If you’re weighing the materials themselves, the topsoil vs. compost vs. garden soil guide compares what’s actually in each product.
Buy a Couple of Bags Long
Fresh fill settles. Both pairs of my beds dropped about an inch after the first few good waterings, and an inch across a 4x8 footprint is 32 × 1/12 = 2.7 cubic feet — call it two 1.5-cubic-foot bags. I keep two bags of mix in the garage at fill time and top off once the soil has had a week to slump. If you’re ordering bulk, round up to the next quarter yard instead; the leftover always finds a job.
Run the volume for your own depth, price one bag of mix against one bulk yard from the nearest landscape supplier, and the cheapest path usually announces itself. Mine was $322.92 the first time and $149 the second — same size beds, same growing layer on top, half the receipt.