How much soil a raised bed takes
Length × width × depth gives the volume, 10% covers settling, and the mix is split into topsoil and compost before either is counted in bags. A bed 4 feet by 8 feet, 12 inches deep, holds 4 × 8 × 1 = 32 cubic feet; with 10% that is 35.2 cubic feet. Mixed 2 parts topsoil to 1 part compost, that is 23.5 cubic feet of topsoil and 11.7 of compost: 16 bags of 1.5 cubic feet of topsoil and 6 bags of 2 cubic feet of compost, $167.56 at the reference prices. The two are separate rows on the shopping list, each with its own bag table, because they are separate products on the shelf.
The mix
University of Maryland Extension says to fill a bed set on compacted or clay soil with compost and purchased topsoil "in a 1:2 or 1:1 ratio" — one part compost to two of topsoil, or equal parts. The page opens on 2 parts topsoil to 1 of compost, the leaner of the two. The ratio is a box under Assumptions: type 1 for equal parts, 3 for a mix closer to plain soil. Each product's bags, weight and price follow it as you type.
On a hard surface — a patio, a driveway — Maryland's advice is different: a soilless growing mix and compost 1:1, with topsoil only in beds at least 16 inches deep, up to 20% of the volume. This page counts topsoil and compost; for a soilless mix, use the compost row for the compost and buy the growing mix by the same volume.
How deep
| Crop | Soil depth |
|---|---|
| Leafy greens, beans, cucumbers | 8 in at least |
| Peppers, tomatoes, squash | 12 to 24 in |
Those are Maryland's figures for a bed on a hard surface, where the bed is all the soil the roots have. The page warns below 8 inches.
Bags or a delivered load
Each product is bought its own way. Topsoil goes to a delivered load from about 5 cubic yards, compost from about 2, and below that each is bagged; the comparison under the shopping list is for the topsoil, and the compost row says on its own whether it is bags or a load. A few beds are bags; a whole garden of them is usually a load of each.
Settling
A new bed sinks as it is watered and the soil settles round the roots. We found no Extension figure for how much, so the page does not pretend to one: the waste box starts at 10%, covering spill and settling together, and is yours to raise if you want the bed full to the rim after the first month.
Several beds
Each bed is an area on this page with its own depth, and the breakdown says what each one takes; the split, the bags and the shopping list are for all of them together. What the soil weighs is worked out for each product — a cubic yard of topsoil is about 2,360 lb, of compost about 800 lb — and the bags for this bed already come to more than one pickup load. We are not connected with any store or supplier named here.