How much soil for a raised bed: the math
A raised bed is a rectangular box, so the volume is simply length × width × depth — the only trap is units. Measure length and width in feet, depth in inches, and convert: cubic feet = L(ft) × W(ft) × D(in) ÷ 12. From there, divide by 27 for cubic yards (bulk orders) or by the bag size for bag counts. The calculator handles any number of beds and sums them, because most gardens have more than one and ordering once is cheaper than ordering twice.
Soil for common raised bed sizes
Computed from the formula above, filled to the brim (round up at the store — soil settles):
| Bed size | 10″ deep | 12″ deep | 18″ deep |
|---|---|---|---|
| 4 × 8 ft | 26.7 ft³ · ~18 bags* | 32 ft³ · ~22 bags | 48 ft³ · 32 bags |
| 4 × 4 ft | 13.3 ft³ · 9 bags | 16 ft³ · ~11 bags | 24 ft³ · 16 bags |
| 3 × 6 ft | 15 ft³ · 10 bags | 18 ft³ · 12 bags | 27 ft³ · 18 bags |
| 2 × 8 ft | 13.3 ft³ · 9 bags | 16 ft³ · ~11 bags | 24 ft³ · 16 bags |
| 3 × 3 ft | 7.5 ft³ · 5 bags | 9 ft³ · 6 bags | 13.5 ft³ · 9 bags |
*Bags = 1.5 cu ft, rounded up. One cubic yard = 27 cubic feet = 18 such bags.
The 60/30/10 raised bed mix
Straight topsoil compacts and drains poorly in a contained bed, which is why most raised-bed guidance blends it. A widely used starting point is 60% topsoil, 30% compost, 10% aeration amendment (perlite, coarse bark fines, rice hulls or similar). Other popular recipes — equal thirds of topsoil, compost and a moisture-retaining amendment, for instance — are easy to dial in with the percentage fields above; the module simply splits your total volume by whatever ratio you set.
One practical note: compost continues to break down, accounting for a good share of first-season settling. Beds mixed with 30%+ compost commonly drop an inch or two by autumn — topping up annually is normal, not a mistake in the math.
Hügelkultur: filling deep beds for less
For beds 12 inches and deeper, filling the bottom with logs, branches and woody debris — hügelkultur — replaces a big share of purchased soil. The wood absorbs and re-releases moisture, feeds the bed as it decomposes, and costs nothing if it comes from your own yard. Tick the option above and set what share of the depth the wood layer occupies (30–50% is common); the calculator removes that share from the soil total. Expect extra settling as the wood breaks down over a few seasons, and keep the top 8–10 inches as actual soil mix for root depth.
Bags or bulk?
The crossover is volume. Bagged soil is convenient and consistent but expensive per unit: at typical pricing, a cubic yard bought in 1.5 cu ft bags (18 of them) costs several times a bulk-delivered yard. The rule of thumb most garden suppliers quote is that bulk wins from roughly one cubic yard up — below that, the bulk delivery fee eats the savings. Enter your local bag price and bulk per-yard price in the cost module and it computes both totals for your exact volume, including the delivery-fee reality check.
Don't forget settling
New soil mix settles 10–20% in the first weeks as it consolidates and compost decomposes. Filling to the brim today means an inch or two below the rim by mid-season. Either buy ~10% extra now, or plan to top up — both are standard practice. (The calculator gives geometric volume; the rounding-up on bags absorbs part of this automatically.)