Start with the volume, not the bag count
A raised bed is a box and its fill is length times width times depth. Do the depth in feet: a 4 by 8 frame twelve inches high, filled to ninety percent, holds 10.8 inches of soil, so it is 4 x 8 x 0.9, which is 28.8 cubic feet, or 1.07 cubic yards. Two of them is 57.6 cubic feet, 2.13 cubic yards, and at 1.5 cubic feet a bag that is 39 bags.
Thirty-nine bags is the number that changes the plan. It is most of a pickup load, it is two trips with a car, and bagged soil is routinely several times the delivered bulk price per cubic yard. The crossover is somewhere near one cubic yard: below it, bags are convenient and the premium is small in absolute terms; above it, ordering bulk and shovelling is both cheaper and less work than opening bag after bag. The same crossover shows up in the mulch calculator and for the same reason.
Why the blend has three parts
Filling a bed with straight topsoil gives you a bed that compacts, drains badly and sets hard on the surface. Filling it with straight compost gives you something that holds too much water, slumps by a third in the first year and can be too rich for seedlings. The standard answer is a blend, and the three jobs being divided up are structure, fertility and air.
| Component | What it is doing | What happens without it |
|---|---|---|
| Topsoil or garden soil | Mineral body, cation exchange, thermal mass | Slumps, dries out fast, nutrients wash through |
| Compost | Fertility, biology, water holding | Inert soil, poor structure, needs feeding constantly |
| Aeration — bark, perlite, pumice, grit | Pore space that survives watering | Compaction, waterlogging in the lower half |
The ratios in the selector are all defensible and none of them is a law. Roughly half mineral soil, roughly a third compost, and the rest coarse material is the shape most working mixes land in. The equal-thirds mix is a different lineage, built for shallow intensive beds and containers rather than deep ground beds, and it is lighter, more expensive and thirstier.
You do not have to buy the bottom half
Bed height is the input that drives the order, and beyond a certain depth the extra volume is not doing horticultural work. Most vegetables root in the top eight to twelve inches. A twenty-four inch frame built at that height for accessibility rather than for root depth can be filled with site soil, coarse woody material or turned sods in the lower portion and good mix in the top foot, which can halve what you buy. Whatever goes in the bottom will settle, so build it in expecting to top up.
The trade-off is drainage. A bed sitting on undisturbed clay with a solid bottom still has to shed water somewhere, and loose material below the root zone helps. What does not help is a layer of gravel at the bottom on the theory that it improves drainage — the interface between fine soil and coarse gravel holds water above it rather than releasing it.
What to do once it is filled
The first fill is not the last one. Compost-heavy mixes lose volume as the organic fraction mineralises, and a bed that was level with the rim in April is often two inches down by the following spring. Budget an annual top-up of compost rather than a rebuild, and check the depth against the frame each season. Feeding is a separate calculation from filling, and the fertilizer application calculator handles the rate side of it. For what actually goes in the bed once it is full, the vegetable garden starter guide covers spacing and timing.
Questions people ask
How much soil does a 4x8 raised bed need?
At twelve inches of frame filled to ninety percent, a 4 by 8 bed takes 28.8 cubic feet, which is 1.07 cubic yards, or 20 bags at 1.5 cubic feet each. At six inches deep it is half that. At twenty-four inches deep it is a little over two cubic yards for one bed, which is why deep beds are usually filled part way with cheaper material. The arithmetic is length times width times depth in feet, and dividing by 27 turns cubic feet into cubic yards.
Is bagged or bulk soil better value?
Bulk is cheaper per cubic yard by a wide margin almost everywhere, and the question is whether the volume justifies the delivery. Under about a cubic yard the bag premium is small in absolute dollars and you avoid a delivery fee and a pile on the driveway. Over a cubic yard the arithmetic turns hard: eighteen bags per yard means a two yard order is 36 bags to buy, load, unload and open. Bulk also lets you specify a blend by ratio rather than accepting what is in the bag, which matters more than the price for anything larger than a single bed.
Can I fill a raised bed with garden soil from the yard?
You can use it as the mineral component, but not on its own, and it depends what the yard soil is. Heavy clay dug out and dropped into a frame compacts into a brick, and it brings weed seed and whatever perennial roots were in it. Screened and blended with compost and something coarse it is perfectly good material, and it is free. The place it makes most sense is the lower portion of a deep bed, where the demands are lower and the volume is largest.
Why did my bed sink over the winter?
Because the organic fraction is decomposing, which is what it is supposed to do. Compost, bark and any woody material you buried lose volume as they break down, and a mix with a third or more compost commonly drops one to three inches in the first year. That is not a fault in the mix and it does not mean the soil has washed away. Top up with compost in spring, which restores the level and the fertility in one operation. A mix that does not settle at all is usually short on organic matter.
Does the bed need a liner or landscape fabric at the bottom?
Not for drainage, and often not at all. An open bottom lets roots go deeper than the frame and lets water leave, both of which are advantages. Fabric under the bed is worth considering where you are building over turf or over aggressive perennial weeds, and hardware cloth is worth it where burrowing rodents are the problem. Solid plastic on the bottom is the one to avoid: it turns the bed into a container without drainage holes.