🌱How to Fill a Raised Bed Without Overpaying for Soil

By the CalculatorMulch editorial desk

How much soil does my bed actually need?

Length × width × depth in feet, divided by 27 for cubic yards. Here are the sizes people actually build, with bag counts rounded up.

The depth column carries all the weight here. Doubling a 4 × 8 bed from 12 to 24 inches doubles the fill from 32 to 64 cubic feet and roughly doubles the cost — for a bed that grows almost exactly the same vegetables, since most annual crops work the top 8 to 12 inches. Twelve inches over reasonable native soil is the value sweet spot. Deeper beds earn their keep on top of paving, hardpan or contaminated ground, where the bed is the soil profile. Your own dimensions go into the raised bed soil calculator.

What should the mix be?

Not one thing. Every workable raised-bed mix balances three jobs: mineral structure that does not collapse, organic matter that feeds, and porosity that lets water through.

General-purpose blend (recommended default) — 50 percent screened topsoil, 30 percent compost, 20 percent potting mix. For a 4 × 8 ft bed at 12 inches:

Mel’s Mix (square-foot gardening) — equal parts compost, peat moss or coconut coir, and coarse vermiculite. The same bed needs 10.7 cubic feet of each. It is a soil-less mix, so the usual “compost at 25 to 30 percent” ceiling does not apply: with no mineral soil present, the compost is not competing with anything for pore space, and the vermiculite is what keeps it open. Mixing the two philosophies — half topsoil plus a third compost — is how beds end up dense and waterlogged.

Potting mix is the one component sold in quarts rather than cubic feet, which makes the arithmetic annoying. One cubic foot is 25.71 dry quarts:

That conversion is also built into the potting soil calculator for containers.

How much extra do I order for settling?

Ten to fifteen percent. A bed filled to 32 cubic feet settles to roughly 28 after the first thorough watering, and organic-rich mixes keep dropping over the first season as the compost fraction breaks down.

So the real order for a 4 × 8 at 12 inches is about 36 cubic feet, not 32. This is the single most common shortfall in the whole subject, partly because it is quietly convenient for suppliers not to mention it. Note that the allowance applies to compost and screened soils — not to sand or gravel, which are already at their packed volume and settle by a couple of percent at most.

Where does bulk start beating bags?

Earlier than people expect, and the arithmetic is worth doing before a second trolley of bags.

Solved properly, bagged blend at $3.60 a cubic foot is $97.20 per cubic yard, against $45 in bulk. The $52.20 saved per yard pays off a $65 delivery at 1.25 cubic yards — one 4 × 8 bed at 12 inches with the settling allowance included. Two beds and bulk is clearly ahead; three beds and it is not close, at roughly $245 delivered against $389 in bags.

The practical brake is minimum orders. Most yards will not tip less than a cubic yard, and some set the floor at two, so a single 4 × 4 bed remains a bagged job whatever the per-yard maths says. The premium bagged mix row is there as a warning: it costs nearly twice what a self-blended equivalent does, and its main advantage is that somebody else did the mixing.

What can go in the bottom of a deep bed?

On beds 18 inches or deeper, the bottom third can be coarse organic material you already have: small branches, twiggy prunings, autumn leaves, spent straw, grass clippings in thin layers. For a 4 × 8 ft bed at 18 inches that is the bottom 6 inches — 16 of 48 cubic feet, a third of the purchase, worth around $58 at bagged prices.

Three conditions make it work rather than backfire:

• Depth. Keep at least 12 inches of real growing mix above the fill. Below that you are stealing root zone rather than saving money.
• Coarse, not compacted. Branches and prunings hold structure. A layer of packed grass clippings turns anaerobic and sour.
• Expect to top up. The fill decomposes and drops 2 to 4 inches over the first year. That is the mechanism working, not a failure, but budget a few bags next spring.

Do not use logs on a 12-inch bed, and do not put landscape fabric between the fill and the soil below — it blocks the worm traffic and drainage that make the whole arrangement work. For what goes into the growing layer, topsoil vs garden soil vs compost sorts out which product is which, and the compost calculator sizes the organic fraction.